Compilation and evaluation of a manualfor experimental nuclear engineering
Item Type text; Thesis-Reproduction (electronic)
Authors Goldstein, Jack, 1932-
Publisher The University of Arizona.
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Download date 26/06/2018 00:54:16
Link to Item http://hdl.handle.net/10150/551828
COMPILATION AND EVALUATION OF A MANUAL FOREXPERIMENTAL NUCLEAR ENGINEERING
byJack Goldstein
A Thesis Submitted to the Faculty of theDEPARTMENT OF NUCLEAR ENGINEERING
In Partial Fulfillment of the Requirements For the Degree ofMASTER OF SCIENCE
In the Graduate CollegeTHE UNIVERSITY OF ARIZONA
1 9 6 6
STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Libraryo
Brief quotations from Volume I of this thesis are allowable without special permission, provided that accurate acknowledgment of source is madeo Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the authoro
Brief quotations from Volume XI of this thesis are allowable without special permission, provided that accurate ncknowledgmont of source is made• Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the copyright holder.
SIGNED:
APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below:
Professor of Nuclear Engineering
ACKNOWLEDGMENTS
The author would like to express his gratitude and sincere appreciation to Dro Roy G. Post, without whose guidance and assistance this work would not have been possible. The help of many members of the Department of Nuclear Engineering, particularly Dr. Lynn Eo Weaver, Dr. Monte V. Davis, Dro Robert L. Seale, and Dr© Morton E. Wacks, is gratefully acknowledged* Appreciation is extended to the United States Army for the opportunity to complete this work. Finally, the author would like to recognize the invaluable contributions rendered by his wife and family. Their patience and understanding materially aided in the completion of this work0
iii
TABLE OF CONTENTS
LIST OF FIGURES ............... . . . . . . . . . . xLIST OF TABLES o . o o . . . . . * o . « o o . o o xiiABSTRACT o . o . o . . o o . * o * o o . o o . . « xxxx
VOLUME ICHAPTER
1. INTRODUCTION . . . . . . . . . . . . . . . . 12. EVALUATION OF BASIC RADIATION DETECTION
EXPERIMENTS . . . . . . . . . . . . . . . . . 4Experiment 1--Geiger-Muller and Proportional
Counters . . . . . . . . . . .Experiment 2— Portable Surveying Instruments Experiment ^--BF^ Neutron Detector ........Experiment 4— Semiconductor Detectors . . . . 10Experiment 6— Scintillation Spectrometry • • 12Experiment l4— Autoradiography . . . . . . . 14
3. EVALUATION OF ACTIVATION ANALYSIS ANDCOUNTING STATISTICS EXPERIMENTS . . . . . . . 1?Experiment ^--Counting Statistics . . . . . . 17Experiment 7--Flux Mapping by Foil Activation 19Experiment 8--Analysis of Mixtures of
Radioisotopes . . . . . . . . . . . . . . 21Experiment 9--Activation of Copper and Half-
Life Determinations..................... 224. EVALUATION OF EXPERIMENTS APPLYING
RADIOISOTOPE TECHNIQUES . . . . . . . . . 24Experiment 10--Removal Cross Sections . . . . 24Experiment 11--Chemical Separation ............ 26Experiment 12--Absorption of Beta Particles * 28Experiment 13--Decontamination of Surfaces . 30
Page
iv
vD *s
l ►£•
V
5. SUMMARY AND RECOMMENDATIONS . . ............. 32
Summary ' # © © © * © « * © * * * © * o o « * * 3 2Recommend ations # © © © © * © # © © © © © © * 3 3
APPENDIX A: ERRATA SHEET TO VOLUME II . © © . © . 35APPENDIX B: REVISED PROCEDURE FOR EXPERIMENT 11 . 38REFERENCES ........... © .......... » © . . . » . . 40
VOLUME IIINTRODUCTION................. © .......... © 1
PART ICHAPTER
1. NUCLEAR RADIATION AND ITS INTERACTIONS . . . 5Introduction . .Charged Particles Gamma Rays © © .Neutrons . © © ........ © . . . . © . . . . 10Radioactive Decay 10
2. RADIATION SAFETY ............................ 12Introduction............................. 12Biological Effects © © ........ © .......... 12Radiation Units as Applied to Dosimetry . . . 15Allowable Dose and Dose Rates » . . . . © © o 19Radiation Protection . . . « . . . . . . © 21Radiation Monitoring Instruments . . . © . © 22Rules for Laboratory Operation . . « . © . . 23Decontamination . . . o . . . . . . . . © . © 26
3© ANALYSIS OF ERRORS » . . . . . . . . . . . © 2?Introduction . . . . . © © * © © # . . # . © 27Definitions . # © . . . . . © . © . © . © © © 27Classification of Errors . . . . © . . . © . 28Errors from Radiation Detection Equipment . © 29Errors from Radiation Measurements . . . © . 30
TABLE OF CONTENTS--ContinuedPage
~v
lUl vi
viTABLE OF CONTENTS— Continued
4. COUNTING STATISTICS . . . . . . . . . . . . o 31Page
Introduction . . . . . . . . . 31The Binominal Distribution ............... 32The Poisson Distribution . . . . . . . . . . 34The Normal Distribution . 36Standard Deviations............ 37Propagation of Errors . . . . . . . . . . . . 40
5. REPORT PRESENTATION . . . . . . . . . . . . . 43PART II
EXPERIMENT 1: GEIGER-MULLER AND PROPORTIONALCOUNTERS . . . . . . . . . . . .......... . 51Purpose .......................Theory . . . . . . . . . . . .Apparatus .....................Procedure * ........ ..Results and Presentation of Data Questions and Problems . . . . Selected References . . . . . .
515154555859 59
EXPERIMENT 2: PORTABLE SURVEYING INSTRUMENTS . . . 60Purp o s e . . . . . . . . . . . .Theory . . . . . . . . . . . .Apparatus . ............. ..Procedure . . .................Results and Presentation of Data Questions and Problems . . . . Selected References ........ .
606063636566 66
EXPERIMENT 3*. BF^ NEUTRON DETECTOR 68Purpose .. ................. . . . . . .Theory . . . . . . . . . . . . . . . . .Apparatus . . . . . ........ . ........Procedure . . . . . . . . . . . . . . . .Results and Presentation of Data . . . .Questions and Problems . . . . . . . . .Selected References . . . . . . . . . . .
68687070727273
EXPERIMENT 4: SEMICONDUCTOR DETECTORS 74Purpose 74
viiTABLE OF CONTENTS— Continued
Theory App nratus ProcedureResults and Presentation of Data Questions and Problems . 0 0 . Selected References . . . o .
EXPERIMENT 5: COUNTING STATISTICSPurpose ........ 0 0 . 0 . . .Theory • © • e # « • • o 0 • •ApparatusProcedure . . . © « . . . . .Results and Presentation of Data Questions and Problems = . . .Selected References = . © . . .
EXPERIMENT 6 : SCINTILLATION SPECTROMETRYP u r p o s e o < « « * o * » o » . © < Theory © © • • © © © © • • • • <Apparatus . © ........ .. « © © ,Procedure © © © • © • © • o o © < Results and Presentation of Data Questions and Problems . . © . , Selected Referencos . . © . © . ,
EXPERIMENT ?: FLUX MAPPING BY FOIL ACTIVATIONPurpos c * » © # © # » © # © # # « Theory • • • • © © © © © © © • « Apparatus © • © • © © • • © • © < Procedure © , . . © . . . © © © , Results and Presentation of Data Questions and Problems © © © . « Selected References . . . © © © ,
EXPERIMENT 8: ANALYSIS OF MIXTURES OFRADIOISOTOPES . © .......... ..Purpose © • • # # 0 * © * * © © "T h e o r y .......... © . . . . © .Apparatus • © • © • • • © • • • <Procedure ........ © * . * * © ,Results and Presentation of Data Questions and Problems . . . © .Selected References . . . . . .
Page747677 79798081818181818284848585859192 92 9494959595100100101101102
103103103104 104 106 107 107
viiiTABLE OF CONTENTS— Continued
PageEXPERIMENT 9: ACTIVATION OF COPPER AND HALF-LIFE
DETERMINATIONS . . . . . • ° • • ° • • 108P m r p O S © e o o e e o o e o o • 0 © . 0 e O O 108Theory . . . . . . 108Apparatus o o # * . . . # IlkProcedure 0 * 0 . o 0 . o IlkResults and Presentation of Data o • • 0 ♦ 115Questions and Problems • © . o o . • • o o 115Selected References • © • © c . o 116
EXPERIMENT 10: REMOVAL CROSS SECTIONS 117Purpose O O e O O 117Theory # » # * # * * # © » 0 • © • © • 117Apparatus . . . . . . . . . © © • © • . 119Procedure . . . . . . . . 119Results and Presentation of Data • • o o o • 121Questions .md Problems . • • o • o . • 121Selected References . . . . • • ° ° • • 121
EXPERIMENT 11: CHEMICAL SEPARATION „ . 122Purpose ................. .. O O 0 O 122T h e o r y .......... © . © © • • © • © 122Apparatus ............... © * * # © • © • o 123Procedure o o « .......... O 0 • 12kResults and Presentation of Data • • o e • 125Questions and Problems © • 125Selected References © • • © • • o ° • 126
EXPERIMENT 12: ABSORPTION OF BETA PARTICLES ° • • 127PurpOSe o o o o o e o e e o * * © o e o . 127Theory • • • © © © • © • • • • • • o . o © 127Apparatus © © • © • • © • o © O • o • o • . 129Procedure © . . . o o . . 0 . 0 O 130Results and Presentation of Data O O e O 130Questions and Problems © © © © • o e O 0 # 131Selected References • © • , • 131
EXPERIMENT 13: DECONTAMINATION OF SURFACES • • • 132Purpose . . o o . . . c . . • o • 0 0 o 132Theory . . . . . . o o . . © o • • e • O . o 132Apparatus o o o o . o . o . o 13kProcedure o . o o . . . 0 . 0 . 0 • . o 0 • . 135
ixTABLE OF CONTENTS— Continued
PageResults and Presentation of Data # • # • # < > 135Questions and Problems IjGSelected References . o . e o . . # * o o o . 136
EXPERIMENT ik: AUTORADIOGRAPHY . . . 0 0 0 0 . . . 137Purpose . . . . . . o . *Theory o # * o * o * # # # Apparatus . . . . . 0 0 .Procedure . < , 0 . 0 . . 0 0Results and Presentation of Questions and Problems o o Selected References • © o •
. c ............ 137• • • © • • * • • 137o e e o o e o e e 1 3 ^
• • • o o e e e e 1 3 ^
Data • © © © • © 139e o e o e e o e e 1 3 9
• o o o o e e o o 1 3 9
LIST OF FIGURES
VOLUME I2-1 Illustration of Dead Time, Recovery Time,
and Resolving Time in a Typical GM Tub e 6
2-2 Spectrum Response to Window Width Setting . 154-1 Revised Lead Brick Arrangement . . . . . . 27
VOLUME II1-1 Diagram of Photoelectric Effect .......... 81-2 Diagram of Compton Event . . . . . . . . . 91-3 Diagram of Pair Production . • ............. 94-1 Plotting of Standard Deviations . . . . . o 4l
El-1 Ions Collected Versus Applied Voltage . . . 52El-2 Setup for GM Counter . . . . ............. 5&El-3 Setup for Proportional Counter . . . . . . . 57E2-1 "Good Geometry" Experiment . . 0 0 0 . . 0 . 62E2-2 "Poor Geometry" Experiment .......... . . . 62E2-3 GM and Cutio Pie Survey Meters . . . . . . . 64E2-4 Open End Cover . . . . . . . . . . . o . 64E3-I Setup for BF^ Detector 0 0 0 0 . . • • . . . 71E4—1 Setup for Semiconductor Detector .......... 78E5-1 Sample Recording of Ten Second Counts . . . 82E6-1 Diagram of Scintillation Detector o . o . . 87
Figure Page
x
LIST OF FIGURES— Continued
E6-2 Cobalt 60 Gamma Spectrum . . . . . . . 90E6-3 Scintillation Detector with Recording
Spectrometer 93E8-1 Scintillation Detector with Multichannel
Analyzer 105E9-1 Semi-Log Plot for Half-Life Determinations • 110E9-2 Pcierls' Method for Half-Life Determinations 110E9-3 Two Point Method for Half-Life Determina
tions o . . o ............. . . . o s . 113E10-1 Experimental Arrangement for Removal
Cross Sections . 120E12-1 Pure Beta Absorption Curves . . . . . o * 128E12-2 Feather Plot o o o o . o o . o o . . . . . . 129
xi
Page
LIST OF TABLES
3- 1
4- 1
4-2
4-3
1-12—12-22-34-14-24-34-44-5
TableVOLUME I
Comparison of Observed and Expected Values of 1 1 O ) Pu « • 0 0 0 0 * 0
Comparison of Observed and Expected Removal Cross Sections • * o * o o o * o « » * o
Comparison of Observed and Expected Beta Ranges 0 * * 0 0 0 0
Comparison of Observed and Expected Beta Energies o o o * # * * o * * # o o o * *
VOLUME IIProperties of Charged Particles . . . . .RBE and Types of Radiation • * • • • • • • •Neutron Flux Dose Equivalents .Maximum Allowable Quarterly Dose Rates o o • Calculations for Equation 4-8 . . o o . e . o Forms of the Standard Deviation o . . . . . .Counting Series . . . . . . . . . . .Counts from Decaying Source . . . . . . .Error Propagation...............*
18
26
29
29
61718
19 37 373940 42
Page
xii
ABSTRACT
A laboratory manual for an introductory course in experimental nuclear engineering was written and evaluated for the Nuclear Engineering Departmentt University of Arizona. The manual was designed to acquaint students with the basic understanding necessary for experimental research in nuclear engineering.
Evaluation of this manual is based in part on its use in a course entitled "Experimental Nuclear Engineering I," offered by the Nuclear Engineering Department, University of Arizona during the fall semester 1965. Observations of a series of fourteen experiments and of the laboratory reports that were submitted upon the completion of each experiment were madeo Analysis of the experimental results achieved shows that this manual achieves the objectives for which it was designedo
xiii
CHAPTER 1
INTRODUCTION
There have been many textbooks written in the field of experimental nuclear engineering. Such books as "A Manual of Experiments in Reactor Physics" by Frank A. Valente, "Radioisotope Techniques" by Ralph To Overman andII. Mo Clark, "Experimental Reactor Analysis and Radiation Measurements" by Donald D . Glower, and "Nuclear Reactor Experiments" by Barton J. Hoag all have added greatly to nuclear engineering as a field of engineering education. Effective textbooks are available in this field, however, the need for a well-organized introductory laboratory manual, coordinated with appropriate text material and containing a wide selection of experiments has not been met o
Future advancements in the field of nuclear engineering depend greatly upon experimental research. If future individual research is to be effectively conducted, it is essential that the student understand the basic principles of experimental measurements in nuclear engineering.
If an experiment is to be effective, it is imperative that the person conducting the research insure
1
2that the experiment will measure the quantity that has been specified, that the conduct of the experiment will not influence the results, that the experiment will be conducted with sufficient accuracy and sensitivity, and that the results of the experiment can be interpreted. This manual, Volume XI of the thesis, provides this understanding if the experiments presented are successfully completed and adequately analyzedo
Volume IX is written in two parts« Part I contains a brief review of the fundamental aspects of modern physics important in nuclear engineering experimentation, a review of the fundamentals of radiation safety, a discussion pertaining to analysis of experimental errors, and a format for report presentation. The experiments in Part II are designed to teach the student basic types and characteristics of radiation, methods of radiation detection, characteristics and operation of principal radiation detection equipment, identification of radioisotopes, radiation safety procedures, and the use of radioisotopes to perform specific laboratory experiments.
The evaluation, Volume I of this thesis, is intended to determine the effectiveness of the manual in meeting the outlined objectives*
An evaluation of any text or manual cannot be accomplished satisfactorily until it is used in teaching. Twelve students, two second year graduate, 8 first year
3graduate, and 2 senior undergraduate, enrolled in a course using this manual during the fall semester 1965* Results of the experiments conducted by these twelve students, combined with the twelve sets of fourteen laboratory experiments, provide insight into the effectiveness of the manual in meeting the objectives®
Each chapter in Volume I contains an evaluation of a particular group of experiments o Experiments 1, 2, 3, 4, 6, and l4 deal with basic radiation detection techniques and are evaluated in Chapter 2® Activation analysis, stripping techniques, and counting statistics are stressed in Experiments 5, 7, 8, and 9 and are evaluated in Chapter 3• Finally, Experiments 10, 11, 12, and 13, dealing with applications of previously mastered techniques of radioisotope measurement and handling, are evaluated in Chapter4®
In all fields of engineering, particularly in nuclear engineering, where so much vital research and development remains to be done, it is extremely important that a student be competent in methods of experimentalresearch
CHAPTER 2
EVALUATION OF BASIC RADIATION DETECTION EXPERIMENTS
EXPERIMENT 1— Geiger-Muller and Proportional CountersExperiment 1 was designed to acquaint the student
with the characteristics, the operation, and the uses of two basic radiation detectors, the Gciger-Muller and proportional counters. Additionally, emphasis was placed on methods for determining the resolving time of the Geiger-Muller counter.
The most important aspect of any initial nuclear engineering laboratory experiment is to insure that there is a basic understanding of the types of radiation being measured and how these measurements are accomplished © This is achieved by use of specific questions as part of the laboratory report. Correct answers to these questions depend upon an understanding of the theory of the experiment . Specifically, these questions require analysis of the type of gas used in the experiment, a discussion of background radiation, quenching, and chamber flushing, and an analysis of the advantages of the particular detectors used in the experiment. Another important aspect of any initial laboratory experiment is to familiarize the students with the basic electrical equipment to be used
5throughout the course. The students connected all electrical equipment used in this experiment from two photographic-type block diagramso These diagrams proved to be valuable training aids and there were few problems encountered in initial equipment setup»
An error in terminology resulted in a great deal of confusion in this experiment <, The term "dead time" was used to describe the minimum time interval between events which can be registered in a counterc Actually a more common term used in the literature to describe this event is "resolving time." Confusion on this point can be corrected easily if the "dead time" referred to throughout Experiment 1 is changed in all places to "resolving timeo"
Additionally, a paragraph should be added to the theory section of Experiment 1 to define accurately dead time and recovery time as well as resolving time• Dead time (t^) is the time, following an event in the tube, in which the tube can give no pulse or has no response to an ionizing event o The time required for the complete recovery of the pulse size after the end of the dead time interval is known as the recovery time (t^).
If the counting system in use has a very sensitive voltage amplifier, the resolving time (R) for the system approaches t^. For a less sensitive tube, the resolving time (R) lies between t . and t , + t .
6Inclusion of Figure 2-1 to Experiment 1 will
further clarify these points c
Maximum Resolving Time Minimum
Resolving Time Recovery 1 Dead Time (t ,) (t )
0 100 200 300 4oo 500 GooTime (p. sec)
Figure 2*1. Illustration of Dead Time, Recovery Time, and Resolving Time in a Typical GM Tube
This figure is redrawn from page 126 of Nuclear Radiation Detection by Price (See Reference 12).
The plateaus for the GM and proportional counters agree closely with those given in Price (See Reference 12). An average resolving time of 425 microseconds was found in this experimento This value will normally be between 300 and 700 microseconds depending on the sensitivity of the voltage amplifiero
EXPERIMENT 2--Portable Surveying InstrumentsThis experiment illustrated the use of portable
radiation surveying instruments for area surveys. Additionally, the attenuation of gamma photons and the effects of geometry on radiation detection were considered.
Use of the portable detectors was adequately illustrated by having each student prepare a laboratory diagram and conduct a survey of the laboratory using a portable GM counter o Sources of known intensity were placed throughout the laboratory to check on the accuracy of the survey conducted. This emphasized the necessity of careful monitoring in an area of suspected radiation. Correct procedures to follow in the use of portable survey instruments were stressed throughout the laboratory period and summarized in questions 1, 2, and 3 on page 66 in the laboratory manual© These questions basically determine whether or not the students are aware of the advantages and disadvantages of certain portable detectors and emphasize the correct procedure used in operating these detectors ©The fact that all students performed exceptionally well in the laboratory and answered questions 1, 2, and 3 correctly in their reports was evidence that these key points were clearly understood.
Early in this introductory course it is necessary to understand the concept of attenuation of photons from a well-collimated beam of monoenergetic photons and the
8factors that describe the geometry under which an experiment was conductedo A design that allows virtually no Compton scattered photons or annihilation photons, if pair production is involved, to reach the detector is one type of experimental arrangemento If, however, a cylindrical shell is placed around the source and the detector is left unshielded, then the experimental arrangement would allow Compton scattered photons or annihilation photons to reach the detectoro In this type of arrangement the intensity at the detector is greater than the uncollided intensity and the ratio of the observed to the uncollided intensity is sometimes called a build-up factor.
Build-up factors determined in this experiment were compared at p^x = 1 with those values listed in Goldstein (See Reference 7)•
With a sot of iron absorbers the average percentage error between the value of p^x determined experimentally and that given in Goldstein (See Reference 7) was 4.3 percent.
With a set of aluminum absorbers the percentage error could not be found at U x = 1 since, due to a mis- calculation in designing the aluminum absorbers, there was not an aluminum absorber available of sufficient thickness to give Pqx = 1o This would have required an absorber thickness of 2<>38 inches when in fact the maximum thickness available was 0*884 inches * Therefore, a comparison
9with Goldstein (See Reference 7) was taken at = 0.372and the average percentage error was found to be 6.25 percento
EXPERIMENT 3--BF. Neutron Detector
The purpose of Experiment 3 was to illustrate the basic principles involved in neutron detection. Additionally, an operating plateau for the boron trifluoride (BF^) neutron detector was determined.
This experiment was relatively simple and was performed with a minimum amount of difficulty. The operating plateau of the modified BF^ was found within 5 percent of the plateau given in Price (See Reference 12).
There was, however, some confusion in the answers given to questions 1 and 2 on page 12 of the laboratory manual. Three of the twelve students interpreted question 1 as requiring only the energy of the fast neutrons. Six of the twelve students only gave the absorption cross section of cadmium for thermal neutrons. In fact, questions 1 and 2 required that the total neutron energy spectrum for Pu-De neutrons be plotted as well as the absorption cross section of cadmium for this spectrum. Future emphasis should be placed on these points.
This experiment should be expanded to include, not only a BF^ neutron detector but also, a fission counter. Fission counters are frequently used as neutron detectors
10in experiments involving nuclear reactors. The large energy released per reaction makes it possible to discriminate against much larger fluxes of gamma rays than with detectors employing the (n, oc) or similar reactions o This latter property makes the fission chambers particularly useful for the measurement of the small neutron fluxes which are present in the start-up and shutdown of a reactor. These neutron fluxes are accompanied by large gamma ray fluxes so that discrimination becomes very important. For this type of application BF^ detectors are unsuitable. Since a subsequent laboratory course will have several reactor experiments that will use fission counters t it is important to acquaint the student with the characteristics and method of operation of this type of neutron detector.
In this experiment, after the data has been taken with a DF^, the BF^ should be replaced with a fission counter and a plateau curve determined„
In addition to adding the above data to the theory section of Experiment 3» the requirement to determine the sensitivity of the fission counter should be added as question 9 on page 73 of the manual.
EXPERIMENT 4— Semiconductor DetectorsSemiconductor detectors have many advantages in
experimental nuclear engineering as radiation-particle
detectors. They arc small, rugged, fast, simple, and do not require high voltage supplies, cooling, windows, or flow of gasseso Because of their increasing importance, an introductory experimental nuclear engineering course should contain at least one experiment in the theory and use of semiconductor detectors.
The students were required to use a photographic- type block diagram to make all electrical connections and once again this proved to be a satisfactory teaching aid.
One major problem occurred in this experiment due to the energies of the alpha particles emitted from the available source. The alpha emitting source was Cm with alpha energies of 5.76 and $.80 Mev. The range of 5-6 Mev alpha particles in air at 25°C and 760 mm of Hg is approximately six centimeters. In this experiment the distance from the source to the detector was much less than six centimeters and consequently there was very little attentuation of the alpha particles by any amount of air in the chamber. Therefore, the variation in counts recorded as a function of applied pressure stems mostly from statistical fluctuations.
This problem could have been corrected in one of three ways» Another alpha source with a reduced alpha energy could have been used. If another source were not available, the size of the vacuum chamber could be increased. Instead of either of these two corrections, a sheet of
11
paper could have been placed between the alpha source and the detector <,
There were a number of questions during the laboratory period concerning the effect of the bias voltage adjustment. The results and presentation of data section should be expanded to include a graph of counts recorded versus bias voltage applied. After this curve has been plotted its physical significance should be discussed starting from the zero voltage level and continuing to the high bias voltage region.
EXPERIMENT 6— Scintillation SpectrometryThis experiment was designed to study scintillation
spectrometry by analyzing known and unknown gamma ray spectra.
Use of the C o ^ source as the calibration spectrumled to some difficulty in plotting a calibration curve ofenergy versus base line. The two photopeaks obtained forC o ^ at 1o17 and 1.33 Mev are so close together that abetter procedure would have been to use both Co^® and
137as calibration spectra = Ce J 1 has a photopeak at 0067 Mev. By using this value in conjunction with the C o ^ values of lol7 and 1.33 a wider range and more accurate calibration curve could have been obtained.
The discussion given on page 89 and 91 of the laboratory manual of pair production resulted in some
12
initial confusion. It should be emphasized that several things can happen to the annihilation photons from the positron that results from a pair-production event. Both photons can escape the crystal creating a pair production peak 1.02 Mev below the photopeak. One photon can escape the crystal and the other undergo a photoelectric capture, in which case the pair production peak would appear 0»511 Mev below the photopeak. If both photons are absorbed in the crystal this energy would be included in the photopeak„
The illustrative C o ^ spectrum shown on page 90 of the laboratory manual should be modified to show the "sum" peak of 2.50 Mev. This can easily be accomplished by use of a 1024 channel multichannel analyzer and an x-y recordero
There were some problems encountered in this experiment with the window width setting on the recording spectrometer and with the subsequent answer to question 1 on page 94 of the laboratory manual« This question requires a discussion of the manner in which the window width setting on the radiation analyzer of the recording spectrometer would affect experimental results. The theory section of the experiment should be expanded to include a short discussion on the necessity for correct window width setting. For examplet in the C o ^ spectrum there are photopeaks at 1.17 and 1.33 Mev• If the window width setting were too large these peaks would tend to converge
13
and ultimately become indistinguishable. On the other hand, if the window width setting were reduced, continually finer resolution would be obtained. This is desirable, up to a point. If the resolution is allowed to become too fine the character of the vertical plot would be lost. The optimum setting would, therefore, be one which allowed a clear resolution of the peaks in a given spectrum, but does not resolve the energies into such small bands that the number of counts per channel becomes insignificant.
This point can be illustrated by means of the graph shown in Figure 2-2. Suppose that one had a total number of 100 counts recorded for an energy band of from 0 to 1.0 Mev. If the window width were sot to discriminate only every 0.5 Mev, the results would be as shown by A in Figure 2-2. Here the peaks tend to converge and ultimately become indistinguishable. If the resolution is too fine, and instead of discriminating every 0.5 Mev, it is done every 0.005 Mev, the results would be as shown by D in Figure 2-2.
EXPERIMENT l4— AutoradiographyPrinciples of autoradiography, the determination
of the distribution of the radioactivity in a specimen by use of photographic emulsions were emphasized in this experiment.
16No difficulties were encountered and the experiment
served to illustrate the many uses of this type of radiation detectoro
The second step of the procedure given on page 138 of the manual indicates that the aluminum-indium packets used in this experiment should be irradiated to an intensity of approximately 5 mr/hr. This level of radiation is higher than that required for the experiment and is above the allowable radiation limits for a laboratory similar to the one used for the conduct of the experiment. The radiation level of the packets should not be higher than 2.5 mr/hro
CHAPTER 3
.......
EVALUATION OF ACTIVATION ANALYSIS AND COUNTING STATISTICS EXPERIMENTS
EXPERIMENT 5— Counting StatisticsThis experiment was very closely coordinated with
the counting statistics information presented in Chapter 4 of the laboratory manual. In Chapter 4 the need for statistical analysis in processes associated with counting, the types of probability distributions, and the methods of using these distributions in laboratory procedure are briefly describedo
The close association between the "Results and Presentation of Data" section, of this experiment and the information in Chapter 4 presented some initial problems in the preparation of the laboratory reports• These problems can be reduced if there is a slight rearrangement of the numbering sequence in this section of the experiment o The third requirement, a plot of P^ (probability that a mean value m is missed by an amount u), experimental and theoretical, versus x, should not come until after step 14 in the sequence» It is not until step l4 that the experimental values for P^ are first calculated.
Questions 8, 12, and 15 of the "Results and Presentation of Data" section require a comparison between
17
18experimental and theoretical values for t (average time for a number of counts), 0 (standard deviation), and (probability that a mean value m is missed by an amount u)o
The results expected for t, O', and are 0.218, 105» and 0.62 respectively. These results are obtained from Chapter 4 of Part I in the laboratory manual. A chi squared analysis can be used to compare observed and expected results for P^ and t. An F test can be used to compare observed and expected results for O' • Table 3-1 lists observed and expected results with the value of chi squared for Py and t as well as the value of F for O'•
Table 3-1Comparison of Observed and Expected
Values for t,0» and P^
Item Expected Observed X F
t 0.218 0.236 0.0040.242
O' 105 99 <190p 0 oG2 0.69 0.014u 0 0 6 8
Therefore, based upon a chi squared analysis, thevalues observed for t and Pu can be expected to be withinthe statistical distribution 95 and 88 percent of the time
- ' '' :
19respectively. Since F is less than 1, the values obtained for (j are within the same population as the expected reading.
Experimental errors could be reduced if more samples were taken. However, this is not recommended since the length of this experiment is already excessive and the results obtained are sufficiently accurate to illustrate the desired points.
EXPERIMENT 7— Flux Mapping by Foil ActivationActivation of foils is a basic method by which many
nuclear engineering experiments are conducted. These experiments include, among others, flux mapping and cross section analysis. This method is based on the fact that certain stable isotopes, upon capturing a neutron, are transformed into radioactive isotopes. This experiment was designed to illustrate foil activation by measuring the flux in a neutron howitzer.
Some initial confusion existed with reference to the proper type of foil to select for a given experiment. The theory section of this experiment should be expanded to include the necessary considerations in selecting the correct foil for activation. These considerations include the estimated neutron energy level of interest, the magnitude of the neutron flux density to be measured, the total exposure required before foil activity can be
20measured, the accuracy required in the final result, and the availability of foil materials. This analysis will result in the selection of a foil which is available, which has the appropriate activation cross section and half-life, and which has these parameters known to the desired accuracye It should be realized that many activation cross sections are unknown, particularly for neutrons in an energy range other than thermal•
Question 3 on page 102 of the laboratory manual pertaining to the cadmium ratio resulted in some difficulty. Four of twelve students stated that the cadmium ratio should be lower at the edge of the howitzer than at the sourceo Actually, the cadmium ratio, defined as the ratio of the activity of a foil exposed to the neutron flux to the activity of an identical foil surrounded with cadmium and exposed to the same flux, is higher at the edge than at the source• This cadmium ratio actually approaches infinity as all the neutrons approach thermalization. Cadmium cut-off occurs at 0.4 ev. In other words, the cadmium essentially stops all neutrons below its cut-off energy and is nearly transparent to neutrons of greater energy. A cadmium ratio of 10 and 100, therefore, represents a thermal flux equal to 90 and 99 percent of the total flux, respectively.
EXPERIMENT 8--Analysis of Mixtures of RadioisotopesThis experiment was designed to familiarize the
students with a frequently used experimental technique known as stripping. Stripping is a method of separating the spectrum of a single radioactive isotope from the complex spectrum of a mixture of radioisotopes.
The experiment was conducted with a 2$6 channel analyzer and a 1-3/4" x 1-3/4" Nal crystal "well" for the sample. The experiment demonstrated this technique and satisfactory data were obtained. However, experimental results can be improved if a 3" x 3" Nal crystal is used.A larger crystal will aid in reducing the Compton effect in proportion to the photoelectric peak by increasing the proportion of quanta of the primary energies to be completely converted to' light in the crystal«
Step 6 in the "Results and Presentation of Data" section of this experiment required a comparison between the pure sodium spectrum that was plotted in step 3 and the results obtained when, in step 3, chlorine was stripped from a sodium chloride spectrum. These two spectra agree within 5 percento Errors resulted from the difficulty encountered in making an accurate determination of the exact time at which the stripping of the chlorine from the sodium chloride spectrum should terminate. These errors can be reduced if, during the stripping procedure, the most energetic photoelectric peak is observed. When the
21
22chlorine is stripped from this peak the procedure should be terminated. Another method that can be used is to strip for a time that is proportional to the mass ratios of the samples irradiated under identical conditions.
EXPERIMENT 9--Activation of Copper and Half-Life Determinations
Methods of determining half-lives of radioisotopes is an important aspect of an introductory course in experimental nuclear engineering. Often in the conduct of experimental research it will be helpful quickly and accurately to analyze the buildup and decay of a particular radioisotope and to determine the half-life of that isotope
This experiment was designed to illustrate these very important concepts by analyzing a buildup curve of C u ^ and decay curves of C u ^ and C u ^ . The half-life of C u ^ was to be determined by four different methods©Copper foils were irradiated in a neutron howitzer using two one-curie Pu-Be sources©
Limited experimental results were obtained due to the relatively small activity above background that was observed. This lack of activity resulted from two major causes © Cu^^ has a 4.3 barn activation cross section and Cu^ * has a 12.8 hour half-life. C u ^ has a 2©1 barn activation cross section and C u ^ has a 5*1 minute half- life© The relatively small neutron flux, 3.2 x 10^
23neutrons por square centimeter per secondy combined with the small activation cross sections associated with C u ^ and Cu^^t resulted in an insufficient neutron flux to give activity that was significantly above background.
Despite these difficulties, this experiment can be quite valuable if, instead of using the neutron howitzer for foil activation, a nuclear reactor is used. This will insure that a sufficient flux is available to activate the foils to a significant level of activity above background.
CHAPTER 4
EVALUATION OF EXPERIMENTS APPLYING RADIOISOTOPE TECHNIQUES
EXPERIMENT 10— Removal Cross SectionsThe objective of this experiment was to illustrate
the shielding behavior of water and water-metal combinations for neutrons from a fast neutron plutonium-beryllium sourceo This is accomplished by an analysis of the removal cross sections for water, iron, and aluminum.
An error in equation E10-2 of the laboratory manual resulted in initial problems in data analysis. This equation is derived incorrectly from equation E10-1. The correct derivation of an expression for the removal cross section of water-metal combinations is shown belowo
where:
I(x)
I = o
F<V
td ■t =W
-T.rtd (4-1)
the dose incident on the shield
= the observed attenuation of the dose in athickness t of water alone w
the total thickness of the metal slabsthe total thickness of the water between thesource and the detector
l . macroscopic removal cross section24
and- Z r tF(t ) = e w
w w (4-2)
Therefore,
" Z r tI (x ) = I \ eo
w w -Er*d (4-3)
where: £ = removal cross section of water only
I(x) ~ -zr *w -Zr*(jw(4-4)
= ln ITxTj ‘ r-wtw (4-5)
and\TUT, -Ir \
(4-6)
Using equation 4-6, derived above, in place of equation E10-2 in the manual, yielded removal cross sections that agree fairly well with those presented on page 188 of Valente (See Reference 13) and shown in Table 4-1.
Therefore, based upon a chi squared analysis, the value of removal cross sections determined for water, iron, and aluminum can be expected to be within the statistical distribution 90, 96, and 92 percent of the time, respectively =
26Table 4-1
Comparison of Observed and Expected Removal Cross Sections
Item Expected Observed 2%
Water 0.150 cm™1
-1
0.1880.162
-1cm-1cm-1
0.019
Iron 0.170 cm 0.1900.185
cm-1cm
0=003
Aluminum 0,080 cm™1 0.1050.092
—1cm-1cm
0.010
When placing the metal slabs over the source, a slight change in slab geometry would have affected the results achieved* This possible error can be corrected by construction of two channels perpendicular to the bottom of the tub into which the metal slabs would fit tightly.
To prevent streaming, the arrangement of the lead bricks at the bottom of the tub should be changed as shown in Figure 4-1.
EXPERIMENT 11— Chemical SeparationThis experiment was designed to demonstrate solvent
extraction, a useful method of chemical separation of radiotracers and of the use of tracers in chemical analysis. Ions of copper and iron were initially dissolved in
27
Figure *t.l o Revised Lead Brick Arrangement
sulfuric acid and iso-butyl alcohol was used as the extracting solution.
Some difficulties were encountered in obtaining correct extractions with the procedure outlined on page 124 of the laboratory manual. The selection of H^SO^ as the acidifying agent may have been a poor choice since according to Morrison and Freiser (See Reference 10) the sulfate tends to interfere with the extraction process.The mutual solubility of iso-butyl alcohol and water was too high for a good system.
A revised procedure for this experiment is given in Appendix B to this evaluation.
The use of ASgO^ in addition to CuSO^ and Fe(NH^)^ (S04)2 should aid in illustrating the basic chemistry. For
example, extraction of As(III) with ethyl ether should reach 68 percent while extraction of As(III) with benzene should reach 9^ percent. On the other hand, extraction of As(V) with ethyl ether should reach only 2-4 percent.
EXPERIMENT 12--Absorption of Beta ParticlesOne of the identifying characteristics of beta
radiation is its range and one of the more widely used experimental methods to determine this range was developed by Feather (See Reference 4). Feather's method compares the absorption curve of the particles whose range is to be determined with the absorption curve of a well-known standard.
This experiment uses Feather's method to determine60 234the range of the beta particles from Co and Pa when
210compared to Bi . The energies of the beta particles are determined from the relationships of Katz and Penfold (See Reference 9)• These relationships are presented on page 127 of Volume II of this thesis.
60 O O ZiExperimentally determined ranges for Co and Pa Jas well as expected ranges are shown in Table 4-2©
Therefore, based upon a chi squared analysis, the 60 234ranges observed for Co and Pa can be expected to be
within the statistical distribution 75 and 1 percent of the time, respectively•
28
29Table 4-2
Comparison of Observed and Expected Beta Ranges
Isotope Observed Expected 2X
O 0 0 82.7 mg/cm^ 280 mg/cm 0.1080.9 mg/cm^
Pa234 1015 mg/cm2 21105 mg/cm 8.7OIO65 mg/cm1'
(Experimentally determined energies for Co ;o ^and234Pa J as well as expected energies are shown in Table 4-30
Table 4-3Comparison of Observed and Expected Beta Energies
Isotope Observed Expected 2X
Co60 0.315 Mov 0.310 Mev 0.000090.312 MevPa234 2.11 Mev 2 <>32 Mev 0.0312.15 Mev
Therefore, based upon a chi squared analysis, the6o 234energies observed for Co and Pa can be expected to be
within the statistical distribution 99 and 85 percent of the time, respectivelye
30234As can be seen the results obtained for Pa were
not as accurate as those obtained for C o ^ . This was dueto the excessive extrapolation required on the Feather plot
234for Pa J . This can be remedied if a beta emitter with an234energy closer to Pa were used as the standard. Use of
32 as the standard would give additional Feather plot data points and serve to reduce the excessive extrapolation required.
A point of confusion stemmed from stop 3 in the "Results and Presentation of Data" section. This step
6o 234required that the absorption curves of Co and Pa be210normalized to the initial point of Bi • It was not clear
from the wording that the requirement was to normalize eachcurve separatelyo Then the initial points of the normalizedcurves should be made to coincide with the initial point of
210the Bi curve. A rewording of this step will avoid some possible confusiono
EXPERIMENT 13— Decontamination of SurfacesContamination results from a transfer of material
that would often be inconsequential except for its radioactivity• Such things as loss of a gas, evaporation of liquid, liquid transfer, manipulation of a solid, and absorption on surfaces all may lead to contamination•
The purpose of this experiment was to study thecorrect procedures used in the decontamination of surfaces
31It is important in a course of this type that the students thoroughly understand correct decontamination procedures o Future individual experimental research may be seriously affected by improper application of these principles.
To illustrate decontamination procedure, surfaces of glass, glazed and unglazed brick, painted and unpainted wood, asphalt tile, sheet iron, stainless steel, linoleum and plastic were tagged with radioisotopes of Na, Fe, and Cu. The students were then required to use the procedures outlined in Table 4-4 of Overman and Clark (See Reference 11) to decontaminate these surfaces.
This experiment was quite successful and illustrated the importance associated with a knowledge of correct decontamination procedures.
CHAPTER 5
SUMMARY AND RECOMMENDATIONS
SummaryThe field of nuclear engineering is developing
rapidlyo Paralleling this development, there has been a corresponding growth in the amounts and level of research being conducted in the fieldo
The manual presented in Volume II was designed to provide a beginning nuclear engineering student with a scries of basic experiments© These experiments, if successfully completed and adequately reported, would serve as a base upon which effective, individual research could be conducted© They would also serve to illustrate some of the basic concepts necessary to a successful mastery of this highly complex and widely diversified field of study.
Based upon the results of a series of fourteen experiments conducted by each of twelve students, it is apparent that this manual does achieve the objectives for which it was designed. The effectiveness of the manual in meeting these objectives can be further enhanced if the corrections as indicated in Chapters 2, 3, and 4 in Volume I are adopted ©
32
33Recommendations
1. The manual presented in Volume XI should be revised as indicated in Chapters 2, 3» and 4 and Appendices A and B of Volume X.
2o Part I of the manual should be expanded to include a chapter on radiochemistry.
3 o Part II of the manual should be expanded to include a wider choice of experiments. Some suggested experiments are:a. The use of liquid scintillation detectors, b o Determination of neutron age.c. Evaluation of Compton scattering cross-sections od. A study of isotopes present in the air by a
method of activation analysis.4o Finally^ it is recommended that experiments using
a subcritical assembly be included in this course and that these experiments be added to the manual presented in Volume II. These experiments should include:a. Determination of cadmium ratios, b o Measurement of the diffusion coefficient•Co Determination of geometric buckling, d o Measurement of the thermal utilization factoro
e. Determination of reflector savings.
34Inclusion of these experiments will make the manual useful for both beginning and advanced laboratorycourses
APPENDIX A
ERRATA SHEET TO VOLUME II
lo Page 2, line 23 as reads, "radiation monitoring, instruments" should road, "radiation monitoring instruments."
2o Page 8, line 6 as reads, "photoelectric is shown" should read, "photoelectric effect is shown*"
3 • Page 28, line 19 as reads, "dead-time" should read, "resolving time,"
4. Page 36, line 4 as reads, "P^ = 0 .210" should read,"Pj = 0 =210."
5• Page 4l, equation 4-13 as reads, "C^" should read,"CT ="
6= Page 54, lines 31 11, 12, l4, and l6 as read, "dead- time" should read, "resolving time."
7* Page 38, line 23 as reads, "dead-time" should read, "resolving time."
8. Page 59, line 1 as reads, "dead-time" should read, "resolving time."
9. Page 63, line 15 as reads, "Aluminum, iron, and lead" should read, "Aluminum and iron."
10. Page 65, line 13 as reads, "Repeat steps 7 and 8" should read, "Repeat step 7•"
35
11. Page 65> eliminate line l4.12. Page 651 line l6 as reads, "step 6" should read,
"step 5 e1113• Page 65j line 22 as reads, "steps 7-10" should read,
"steps 6-8."l40 Page 76, line 2 as reads, "reactions" should read,
"reaction."15• Page 76, eliminate line 10ol6o Page 771 eliminate lines 21 and 22.17• Page 79» eliminate lines 1 and 2.18. Page 79, line 4 as reads, "steps 10 through 13" should
read, "steps 10 and 11."19• Page 791 line 8 as reads, "steps 10 through 13" should
read, "steps 10 and 11."20. Page 791 eliminate lines l6 and 17«21. Page 83, eliminate line 4 and renumber steps 4-13 as
36
3-12.22. Page 83, line l4 as reads, "Problem 3" should read,
"Problem 6."23 o Page 84, line 3 change step l4 to step 13 and then
add step 3 from page 83 as step l4 on page 84.24. Page 84, eliminate lines 12 and 13.25o Page 971 line 8 as reads, "A /A , " should read,S S-tfl *
26« Page 113> Figure E9-3• There should be added and tg on the curve at midway points of the increments o
27• Page 113» line 11 as reads, "Chapter 2" should read, "Chapter 1."
28. Page 118, equation E10-2 as reads,
37
- E^wat ^wat-met
should read,
" Eid . E, tTTxTi rw w
29. Page 121, last line as reads, "Valenti" should read,"Valente 11
APPENDIX B
REVISED PROCEDURE FOR EXPERIMENT 11
Procedure1. Irradiate 55 milligrams of As^O^, Oc2 gram of Fe
(NII ) (SO^) » and 1 gram of CuSO^ to provide 20 Ic of arsenic, 14 p-c of iron, and 25 ^c of copper.
2. Dissolve the As^O^ with 20 ml of 2 N NaOII. Insure that no powder remains.
3o Add 25 ml of 12 N MCI to the As^O^-NaOH solution.4. Into each of 4 graduated cylinders, marked 1-4,
transfer 1 ml of As^O^ solution.5 ° Add 5 drops of H^O to cylinders 1 and 3.6. Add 5 drops of II O to cylinders 2 and 4.7* Fill cylinders 1-4 to 2 ml with 12 N HC1.8 . Add 2 ml of benzene to cylinders 1 and 2.9• Add 2 ml of ethyl ether to cylinders 3 and 4.10. Agitate cylinders 1-4 several times for 20 seconds
each.11. Remove 1 ml each of the aqueous and organic solution.
Mark the vials containing the aqueous solution A-D and the vials containing the organic solution E-H.
12. Count each vial and record results.13• Wash all glassware.
14. Mix the CuSO/t with 20 ml of 12 N HC1.15« Into each of 2 graduated cylinderst marked 1 and 2,
transfer 1 ml of CuSO^ solution.16. Add 5 drops of 10 percent hydroxylamine hydrochloride
to cylinder lo17• Add 5 drops of H^O^ to cylinder 2 o18. Fill cylinders 1 and 2 to 2 ml with 12 N HC1.19o Add 2 ml of methyl amyl ketone to cylinders 1 and 2.20. Agitate cylinders 1 and 2 several times for 20
seconds«21. Remove 1 ml each of the aqueous and organic solution®
Mark the vials containing the aqueous solution A and D and the vials containing the organic solution C andD.
22® Count each vial.23• Wash all glassware.24® Repeat steps 14-23 with Fe(NH^)^SO^)2 in place of
CuSO^e
39
REFERENCES
1. Dlatz, Hanson, Introduction to Radiological HealthtMcGraw-Hill book Company Inc., New York, 1964, Chapter 7 °
2. Dearnaley, G., and D . Co Northrop $ S emiconductorCounters for Nuclear Radiations * John Wiley Inc*, New York, 1963•
3 o Evans, Ro D *, The Atomic Nucleus, McGraw-Hill Book Company Inc o, New York, 1955•
kc Feather, N ., Proceedings of the Cambridge Philosophical Society, Vol» 34, p. 599 (193#)*
5• Flagg, John F ., Chemical Processing of Reactor Fuels, Academic Press, New York and London, 1961, Chapter4*
6. Glower, Donald D*, Experimental Reactor Analysis and Radiation Measurements, McGraw-Hill Book Company Inc *, New York, 1965•
7* Goldstein, Herbert, Fundamental Aspects of Reactor Shielding, Addison-Wesley Publishing Company, Reading, Massachusetts, 1959, pp• 367-369 •
8. Hoag, J . Barton, Nuclear Reactor Experiments, D . VanNostrand Company Inco, Princeton, New Jersey, 1958.
9* Katz, Lo and A. S. Ponfold, Reviews of Modern Physics, Vol. 24, p„ 28 (1952).10* Morrison, G . II. and II. Freiser, Solvent Extraction in
Analytical Chemistry, John Wiley Inc*, New York, 1957*
11• Overman, Ralph To, and H * Mo Clark, Radioisotope Techniques, McGraw-Hill Book Company Inc., New York, 1964.
12. Price, William Jo, Nuclear Radiation Detection, 2nded., McGraw-Hill Book Company Inc., New York, 1964.
13* Valente, Frank A., A_ Manual of Experiments in Reactor Physics, The Macmillan Company, New York, 1963•
40
Volk, William, Applied Statistics for Engineers, McGraw-Hill Book Company Inc., New York, 1958•
Yagoda, Herman, Radioactive Measurements with Nuclear Emulsions, John Wiley Inc., New York, 1949°
EXPERIMENTAL
NUCLEAR
ENGINEERING
BY
JACK GOLDSTEIN
ANDKENNETH D. KEARNS
NUCLEAR ENGINEERING DEPARTMENT COLLEGE OF ENGINEERING UNIVERSITY OF ARIZONA TUCSON, ARIZONA
EXPERIMENTAL NUCLEAR ENGINEERING
By
Jack Goldstein and Kenneth D. Kearns
Copyright © 1965 by Jack Goldstein and Kenneth D. Kearns
ACKNOWLEDGMENT
The authors would like to express their sincere appreciation to
Dr. Roy G. Post, without whose guidance and assistance this manual would
not have been possible. The help of many members of the Department of
Nuclear Engineering, particularly Dr. Lynn E. Weaver, Dr. Monte V. Davis
and Dr. Morton E. Weeks, is gratefully acknowledged. Finally, apprecia
tion is due Miss Alice Garcia and Mrs. Alice McCormick for the typing of
this manual. Their attention to detail greatly assisted in the comple
tion of the manuscript.
JG
KDK
ill
TABLE OF CONTENTS
Introduction ......................................................... 1
Part I. Introductory Chapters
Chapter 1. Nuclear Radiation and its Interactions ............ 5
Chapter 2. Radiation Safety ..................................... 12
Chapter 3. Analysis of Errors .................................. 27
Chapter 4. Counting Statistics................................... 31
Chapter 5. Report Presentation ................................... 43
Part II. Laboratory Experiments
Experiment 1. Geiger-Muller and Proportional Counters . . . . 51
Experiment 2. Portable Surveying Instruments and GeometricalEffects on Radiation ......................... 60
Experiment 3. BF^ Neutron D e t e c t o r .............................. 68
Experiment 4. Semiconductor Detectors .......................... 74
Experiment 5. Counting Statistics................ 81
Experiment 6. Scintillation Spectrometry ..................... 85
Experiment 7. Flux Mapping by Foil Activation ................ 95
Experiment 8. Analysis of Mixtures of Radioisotopes byStripping........... 103
Experiment 9. Activation of Copper and Half-lifeDeterminations ................................. 108
Experiment 10. Removal Cross Sections .......................... 117
Experiment 11. Chemical Separation .............................. 122
iv
Experiment 12. Absorption of Beta Particles .......... . . . . 129
Experiment 13. Decontamination of Surface . . . . . . . . . . 132
Experiment 14. Autoradiography .................................. 137
v
INTRODUCTION
Experimental research is the foundation for all fields of sci
ence and engineering. In the past, this research has led to important
new discoveries which have markedly influenced our way of life. In the
future, experimental research will continue to open new vistas of knowl
edge and understanding that will influence the course of world events.
The importance of experimentation to new scientific discoveries
necessitates the teaching of proper experimental techniques. These tech
niques include proper design and conduct of the experiment as well as ac
curate evaluation and presentation of results.
The design of an experiment should be carefully planned to in
sure that the experiment will measure the quantity that has been speci
fied. If prior planning is not sufficient, a great amount of time, efr
fort, and money will be wasted to obtain results which may be accurate
but worthless. Each experiment conducted must have a specific objec
tive. The rotating of dials or the random mixing of chemicals in the
laboratory does not constitute experimental research.
All the parameters that enter into an experiment must be care
fully considered to insure that the manner in which the experiment is
conducted will not influence results. If an item of equipment to be
used in an experiment has design limitations, these limitations must be
known, and compensated for, if the results obtained are to be accurate.
Extreme care must be taken to insure proper selection of equipment in
1
order that measurements will be taken with sufficient accuracy and sen
sitivity.
Once an experiment has been properly designed and conducted,
the results obtained must be objectively analyzed. Experimental re
search that begins with a strongly biased, preconceived notion of the
outcome is not only useless, but dishonest.
Years of excellent experimental design, testing and evaluation
may be forever buried in obscurity if the results obtained are not
clearly and concisely presented. Experimental results must be presented
in a manner to make all aspects of the experiment clear to the person*
interested in the area under study.
The purpose of this manual is to introduce proper techniques of
nuclear engineering experimentation.
Although the completion of a course in modern physics is as
sumed, the introductory chapters in Part I contain a brief review of
some of the fundamental aspects of modern physics important in nuclear
engineering experimentation. Basic radiation types, their interactions
with matter, and radioactive decay are summarized briefly.
Since safe procedures are so important in handling the radio
active materials which will be used in all experiments, Part I also con
tains a discussion of radiation safety. Particular emphasis is placed
on dosimetry, biological effects of radiation, allowable dose and dose
rates, radiation monitoring, instruments, procedures for handling radio
active isotopes, and decontamination procedures.
2
3The introductory chapters also contain a discussion of possible
errors encountered in experimental research. Such errors as general un
certainties in measurement, limitation of detection equipment, uncertain
ties peculiar to radiation measurements, and statistical errors are con
sidered.
A chapter devoted to report presentation concludes Part I of
this manual. There is no standardized method for preparing a laboratory
report. However, this chapter, in addition to discussing report writing
techniques, outlines the format which will be used in reporting the ex
periments in this manual.
Part II is devoted to the individual experiments which will be
conducted during this course. All of the experiments are designed to .
provide the student with direct personal experience with proper experi
mental techniques related to nuclear engineering. The first experiments
concentrate on the use of radiation detection equipment and have limited
experimental objectives. The latter experiments concentrate on more ad
vanced techniques and are designed to provide basic knowledge which can
be used in individual research.
CHAPTER 1
NUCLEAR RADIATION AND ITS INTERACTIONS
Introduction
This chapter is designed to provide a brief description of nu
clear radiation and the manner in which this radiation interacts with
matter. It is not intended to serve as a complete work on these topics,
but rather a means of assisting in the better understanding of the ex
periments in this manual. Therefore, not every particle nor every inter
action will be discussed, but only those necessary for proper under
standing of this course. It is assumed that the student will have com
pleted a course in modern physics and had, or will currently be enrolled
in, an introductory course in nuclear physics. The treatment of the ma
terial will be such that it will augment the theory sections of each of
the individual experiments.
There are three types of radiation that are of primary interest
in this course. They are charged particles, gamma or x-radiation, and
neutrons.
Charged Particles
The charged particles of interest and some of their properties
are given in Table 1-1.
In passing through matter, the alpha particle is not deflected
to a great extent by any single atom, but undergoes almost continuous
5
6
slowing down, dissipating its energy by removal of some 30,000 electrons
from atoms to form ion pairs. This large number of events, governed
Table 1-1. Charged Particles
Name Symbol Mass (amu) Charge
alpha Ct 4.003873 +2
beta 0.000549 -1
positron P+ 0.000549 +1
only by the energy of the alpha particle and the ionization energy of the
medium, means that the alpha particle is stopped at a well-defined dis
tance, called its range. Even though there is only a slight deflection
from a single interaction, the interactions are so numerous that the
range of alpha particles is relatively short. For example, an alpha
particle can be absorbed in a thick sheet of paper, a thin sheet of alu
minum, or a few centimeters of air.
The beta particle is an electron emitted from the nucleus of an
atom. The ionization process initiated by a beta particle is similar to
that of an alpha particle. The beta particle experiences more of a de
flection from a single interaction than does an alpha particle, but the
interactions are not so numerous. Because the mass of the beta particle
is relatively small, it undergoes acceleration and deceleration in the
vicinity of an atom. This acceleration and deceleration causes the beta
particle to give up some of its energy in the form of electromagnetic
radiation called bremsstrahlung.
7
One of the significant characteristics of the spontaneous beta
disintegration of a nucleus is the continuous energy spectrum of the
emitted particles. Each spectrum has a definite upper energy limit,
which is characteristic of the particular beta emitting nuclide.
The range of a beta particle is defined as the minimum thick
ness of material necessary to absorb the maximum energy beta particle of
the spectrum. Since the beta particle does experience more deflection
than the alpha particle, its range is not nearly so well-defined.
The positron is a particle equal in mass to the electron and
having an equal but opposite charge. The positron's interactions with
matter are discussed in the next section.
Gamma Rays
Gamma rays are electromagnetic radiation emitted by a nucleus
in an excited state. Each gamma emitting nuclide produces radiation of
one or more discrete energies corresponding to differences in nuclear
energy levels. The energy range of gamma rays is from about 0.01 to 10
Mev.These gamma energies may interact with matter by any of three
ways: the photoelectric effect, Compton effect, and pair production.
Each photon, interacting by one of these effects, will release an or
bital electron or an electron-positron pair and a lower energy photon.
In the photoelectric effect, the entire energy of the gamma
photon is transferred to an orbital electron. A portion of this energy
is used in removing the electron from its orbit. The remainder is
8
transferred to the electron as kinetic energy. Whenever an electron Is
removed from Its orbit, another orbital electron will fall into the va
cancy, and an x-ray will sometimes be emitted. The energy of the x-ray
is equal to the binding energy of the electron. The only difference be
tween gamma rays and x-rays is that the latter are produced outside the
nucleus. A schematic representation of the photoelectric is shown in
Figure 1-1.
IncidentGamma Ray
Figure 1-1. Photoelectric Effect
In a Compton event the photon strikes an orbital electron, re
leasing it from the atom. In this case only a part of the photon energy
is transferred to the electron. The remaining energy appears in the
form of a secondary photon of reduced energy. The proportion of the
initial photon energy converted to kinetic energy of the electron and
the proportion given to the secondary photon is dependent on the
’scattering* angle. Figure 1-2 gives a schematic representation of a
Compton event.
9
Figure 1-2, Compton Event
Pair production takes place in the coulomb field of the nu
cleus. It occurs when a gamma photon of 1.02 Mev or higher energy is
converted into an electron and a positron. Every positron ultimately
dies in an encounter with an electron, in which about 20^0* of electro-
magnetic radiation is emitted. The electron and positron are annihi
lated producing two photons of 0.511 Mev each. These photons are called
IncidentGamma Ray
0.511 Mev
Figure 1-3. Pair Production
10
annihilation radiation. Figure 1-3 gives a schematic representation of
pair production.
Neutrons
The neutron is a neutral particle with a mass nearly equal to
that of a hydrogen nucleus. Because they are uncharged, the interac
tions between neutrons and matter are entirely dependent upon the nu
clear forces that exist between a neutron and the nucleus of an atom.
Since neutrons are uncharged, they cause only negligible amounts of
ionization. Therefore, neutrons travel relatively long distances in
matter when compared with charged particles. When neutrons interact
with the nucleus of an atom, the interaction may be by elastic scat
tering, inelastic scattering, or capture.
If elastic scattering occurs, the neutron is deflected by the
nucleus with a resultant loss of kinetic energy by the neutron and gain
of kinetic energy by the nucleus. If inelastic scattering occurs, the
neutron enters the nucleus, and a neutron emerges at a different energy.
In a capture reaction, the nucleus captures the neutron and often emits
another type of radiation.
Radioactive Decay
Certain elements have radioactive isotopes which undergo spon
taneous disintegration resulting in the emission of alpha and beta par
ticles and the formation of a different isotope. The rate of decay of
an isotope at any time is proportional to the number of atoms present at
that time. If N0 is the number of atoms initially present, then the
number of atoms present (N) at any time (t) is given by
N = N0e"Xt (1-1)
where X represents the decay constant of the isotope under considera
tion.
The half-life an isotope is defined as the time in
terval over which the chance of survival of an atom is exactly one-half.
Therefore, from equation 1-1,
11
Tl/2 = (ln 2)/x (1-2)
The mean or average life (?) is given by the sum of the times
of existence of all the atoms divided by the initial number. It can be
shown that
T = 1/X (1-3)
A means of identifying an unknown isotope is by determining the
half-life or the mean life. This process will be illustrated in the ex
periments .
CHAPTER 2
RADIATION SAFETY
Introduction
Since the beginning of the Manhattan Project in 1942, work with
nuclear radiation has involved ever increasing numbers of people.
Today, the uses of radiation have created a new industrial field. In
1942 it was realized that large numbers of relatively untrained people
would be processing enormous quantities of very active isotopes and that
adequate protective measures would have to be taken. A great amount of
research was initiated, and has continued, to determine possible harmful
effects of radiation as well as means to be used to minimize these ef
fects. This chapter will outline the common types of radiation, quanti
tative measures of radiation doses, possible biological effects of ra
diation, allowable dose rates, radiation monitoring instruments, radia
tion protection, rules for laboratory operation, and decontamination
procedures. This chapter is not intended to serve as a complete work
in the field of radiation safety, but rather as a guide to proper con
duct in the laboratory.
Biological Effects of Radiation
Biological damage results from the destruction of cells by the
extremely concentrated or localized -deposition of energy by particulate
and electromagnetic radiation in passing through matter. In nuclear
12
13laboratory work, this damage is due basically to three types of radia
tion: charged particles, x- or gamma radiation, and neutrons. These
types of radiation and their interactions with matter were discussed in
Chapter 1.
Although the effects of ionizing radiation on living organisms
are not completely understood, certain facts appear to be valid. Expo
sure of living cells to radiation usually causes an undesireable change.
The amount of change in a cell is somewhat proportional to the radiation
exposure. However, at extremely low levels, such as background, this is
not always valid. Some cells, once subjected to ionizing radiation, can
be replaced by normal cells while some others cannot. Some cells, such
as those of the lymph nodes, are more susceptible to ionization than
others. For this reason, certain parts of the body, such as the hands,
are able to absorb much larger doses of radiation without harmful ef
fects.
As well as producing an undesireable change in living cells,
radiation can sometimes produce desireable changes. Although it is
known to produce cancer, radiation is also used as a treatment for
cancer. Since cancer cells grow more rapidly than healthy cells, they
are more sensitive to radiation. Therefore, radiation has a preferen
tial effect on cancer cells over healthy cells.
As has been discussed, the body has some recovery capability
after radiation damage, dependent on the speed of replacement of certain
body cells. For this reason, large doses incurred over short periods of
time (acute exposure) are more harmful than small doses incurred .
14
frequently (chronic exposure). If very large doses are received, large
enough numbers of cells may be affected to preclude any recovery.
Exposure to radiation can be incurred either externally or in
ternally. Although the biological effects are the same in either case,
an internal exposure is potentially much more damaging. This increased
damage results from the fact that the irradiation is continuous until it
is eliminated from the body. Also, it is difficult to increase the
body's normal elimination rate. Radiation, taken internally, is diffi
cult to assess quantitatively.
Radioactive materials can enter the body by ingestion, inhala
tion, or by absorption in the skin. In considering internal exposure,
the biological half-life of the radioactive material is useful. The
biological half-life represents the time it takes for the body, by na
tural elimination processes, to decrease the amount of the radioactive
material present by one-half. Usually it is necessary in estimating po
tential damage to the body to consider not only the biological half-
life, but a combination of the radioactive and biological half-lives.
This combination is known as effective half-life (T) and is given by
T - %Tb + Tr (3-1)
It is easily seen from equation 3-1 that if a material has a relatively
long radioactive half-life, the importance of the biological half-life
is greatly increased.
The factors which determine the extent of radiation damage to
the body are total dose, dose rate, type of radiation, energy of radia
tion, type of tissue involved, volume of tissue affected, and part of
body affected.
The above discussion is -intended to show the need of res
pect for radiation and adequate protection to prevent damage by radia
tion. The remaining portions of this chapter will elaborate on neces
sary protective measures.
Radiation Units As Applied to Dosimetry
It would be most ideal to have a unit.of radiation-effect mea
surement for any type of material that was the same as the reading on a
survey meter for any type of radiation. No such ideal unit exists; in
stead, there is in present use a wide variety of terms to explain the
effects of ionizing radiation. Increased knowledge in the field of ra
diation safety has led to the gradual development of more general radia
tion units.
Originally the most widely known and important radioactive ma
terial was radium. All other radioactive materials were measured by the
amount of radium present in the compound. However, when the method of
measuring the disintegration rate of a radioisotope was developed, it
was necessary to develop a unit for materials other than radium. The
'rutherford' (10^ disintegrations/second) was proposed in honor of the
prominent nuclear physicist. Lord Rutherford, but this unit has found
little usage. The 'curie', named in honor of Marie Curie, expresses the
15
rate as
16
amount of radioactive material which disintegrates at the st
one gram of radium.
Curie: 3.7 X 10 disintegrations per second,
Radiation units are used to indicate the amount of radiation
being emitted from a source or the amount of radiation being absorbed by
a certain material in a specific location. The first generally accepted
unit of radiation measurement was the roentgen.
Roentgen (r): that amount of gamma or x-radiation that willproduce one electrostatic unit (esu) of charge in one cubic centimeter of dry air at standard temperature and pressure.
It should be noted that this radiation unit deals only with gamma or x-
rays and for absorption in air. It therefore has limited usage.
As understanding developed in studying the effects of radia
tion, the need for quantitative measurement of the energy absorbed in
biological tissue and in materials other than air became apparent. It
was also necessary to define the absorption of energy for types of ra
diation other than gamma or x-rays. This need led to the development of
a unit known as the roentgen equivalent physical (rep).
Rep: the amount of radiation from any radiation source equivalent in energy dissipation to 1 r of high-voltage x-rays.
However, it was realized that the rep also had certain disadvantages.
For the same source of radiation, its absorption in various materials
varied according to the physical characteristics of the material.
17
Due to the shortcomings of the rep, it was desireable to have a
unit that would be applicable to all types of materials. In 1953 the
rad was adopted by the International Commission of Radiological
Protection.
Rad: a measure of the dose of any ionizing radiation to bodytissue in terms of the energy absorbed per unit mass of tissue. One rad is the dose corresponding to the absorption of 100 ergs per gram of tissue.
The rad gives an expression of the amount of energy dissipated which can
then be related to the biological effect produced if the type and energy
of radiation are known.
The relative biological effectiveness (RBE) is an expression of
the biological hazard of particular types of radiation.
RBE: the ratio of gamma or x-radiation dose to the dose required to produce the same biological effect as the radiation in question. Both doses are measured in rad or rep.
The values of RBE for different types of radiation are given in Table
2-1.
Table 2-1. Type of Radiation Versus RBE
Type RBEgamma 1x-ray 1beta 1alpha 10neutron 2.5 -
18
The R B E for neutrons varies from 2.5 to 10 depending on the neutron
energy. The Incident number of neutrons per square centimeter equiva
lent to one rem may be estimated from Table 2-2.
Table 2-2. Neutron Flux Dose Equivalents
N e u t r o n E n e r g y ( M e v )
Number of Neutrons Per cm2 Equivalent to a Dose of 1 rem
Thermal 0.0001 0.005 . 0.02 . 0.1 . . 0.5 . . 1.0 . .2.5 . . 5.0 . .7.5 . .10 . .
10-30 .
970 X 10* 720 X 10° 820 X 10° 400 X 10° 120 X 10° 43 X 10° 26 X 10° 29 X 10° 26 X 10° 24 X 10° 24 X 10° 14 X 10°
The most frequently used unit for expressing radiation effect
or biological dose is the roentgen equivalent mammal (rem).
Rem: a measure of the dose of any ionizing radiation to bodytissue in terms of its estimated biological effect relative to a dose of one roentgen of high energy x-radiation.
As has been previously discussed, when any radiation passes
through matter, it loses energy in the form of ion pairs. A means of
expressing this loss of energy is by the linear energy transfer (LET).
LET: an expression of the rate at which the energy that hasbeen absorbed is used up in the system along its path length. Normally expressed as ion density per unit length of path.
19
Allowable Dose and Dose Rates
Part 20 of Title 10, Code of Federal Regulations, prescribes
maximum radiation limits. In any case where doubt may exist as to an
allowed exposure, this document should be consulted.
During any calendar quarter the whole body dose to which a
person may be exposed should not exceed 1-1/4 rems. Once this quarterly
dose level is reached, it is necessary to wait 13 weeks before an addi
tional radiation exposure. It should be noted that the 1-1/4 rem per
quarter figure refers to the whole body dose. Quarterly allowable doses
for other parts of the body are given in Table 2-3.
Table 2-3. Maximum Allowable Quarterly Dose Rates
Allowable Quarterly DoseBody Part __________ (rem)
Whole body, head and trunk . . . . . . . . 1-1/4S k u l l ......................................... 1-1/4Eye lens ....................................... 1-1/4Pelvis .................. . . . . . . . . . 1-1/4G o n a d s ................................. 1-1/4H e a r t ..................................... 5L i v e r ......................................... 5Stomach ...................................... 5Intestines . . . . . . . . . ............... 5Bladder ....................................... 5Lungs ..........................................5Spleen ........................................ 5P a n c r e a s ................................ 5K i d n e y s .......................... 5Bone structure .............. . . . . . . . 7S k i n ........................................... 7-1/2T h y r o i d ....................................... 7-1/2Hands ........................................ 18-3/4Ankles ....................................... 18-3/4F e e t .......................................... 18-3/4
Under unusual circumstances, a quarterly allowable dose of 3
rem may be permitted. During an emergency, a one-time exposure of 25
rem may be incurred. Part 20 of Title 10 gives necessary information
relating to these exposures. It should be noted that all radiation
doses are compatible with radiation occurring from natural sources.
These sources include natural background (from cosmic radiation and na
turally occurring terrestrial radioactive materials), displaced terres
trial radioactive materials (such as in building or paving materials),
and man-made radiation sources (such as fallout deposits from nucleai
weapons or nearby radiation sources in storage).
At any time, the dose to the whole body, when added to the ac
cumulated dose to the whole body, should not exceed 5(N - 18) rems,
where N is the age of the individual at his last birthday. This rule
considers that under the age of eighteen the body tissues, still in a
developmental stage, are more susceptible to radiation damage.
In many instances it will be necessary to compute the lengths
of time an individual can work within a certain proximity of a source.
The intensity of radiation from a source generally decreases with the
distance from a source. For a point source, this decrease is inversely
proportional to the square of the distance from the source. .Normally
a source can be considered as a 1point* if measurements are made one
foot or more from a source whose largest dimension is not more than four
inches. The sources used in this course will normally be considered as
point. An example will serve to illustrate the calculations.
20
21
Example 2-1: The background radiation in the laboratory is 1
mrem/hr. A source emittihg 1.5 Hev gammas is brought into the lab
oratory. The radiation dose reading one foot from the source is
0.5 rem/hr. How many hours per week, every week, could you work ati
a distance of 10 feet from the source without exceeding your allowed
dose?
If £3 is the source strength, and R is the distance from the
source, then
0.5 100 or Sx * 0.005 rem/hr.
Therefore at 10 feet from the source, you are receiving 0.005 rem/hr
plus the background radiation of 0.001 rem/hr or a total of 0.006
rem/hr. You are allowed a whole body dose of_1.25 rem/quarter.
Therefore, you can work
(1.25)/(0.006) * 208 hours in 13 weeks
(208)/(13) * 16 hours per week.
Radiation Protection
There are three basic ways in which an individual may protect
himself from excessive external radiation exposure. These are time,
distance, and shielding. If an individual is exposed to exceedingly
high radiation levels, then care must be taken to insure that the
22
exposure does not exceed allowable limits. Accurate knowledge of the
time and amount of exposure will aid in radiation protection. Gener-.
ally, the intensity from a radioactive source decreases with increased
distance from the source. If a source must be handled, it should be
kept as far away from the body as possible. Tongs should be used to in
crease the distance between the source and the body. If an individual
must work close to a source for a length of time that would make his ex
posure excessive, shielding from the source should be provided. Shield
ing can be accomplished by placing, between the person and the source, a
material which will attenuate the type of radiation being emitted to ac
ceptable levels.
In choosing a suitable shielding material, not only the ability
of the material to attenuate should be considered, but also its cost,
and chemical and physical characteristics. Most shields are constructed
basically of either lead, steel, or concrete. For effective neutron
shielding, some material with a low atomic mass number (such as hydro
genous materials) should be used along with a good neutron absorber,
such as boron*
Radiation Monitoring Instruments
To monitor radiation levels within a laboratory, certain ra
diation survey devices and instruments are used. These include, but are
not limited to, film badges, pocket dosimeters, Geiger-Muller survey
meters, and portable proportional counters (Cutie Pie)♦
23
The film badge consists of a small metal "or plastic holder con
taining one or two packets of film. One packet is sensitive to beta and
gamma radiation and the other is sensitive to neutrons. The radiation
absorbed causes a darkening of the film proportional to the amount of
radiation received.
The pocket dosimeter, a direct-reading quartz-fiber electro
scope, is useful for monitoring where high radiation levels are ex
pected. It has the advantage of direct reading and gives immediate in
formation on the radiation exposure. A variation of the pocket dosi
meter is the pocket chamber which is read on an auxiliary instrument.
These instruments are quite useful but should be used in conjunction
with a film badge which will give a permanent record of radiation expo
sure.
T w o p o r t a b l e i n s t r u m e n t s , t h e G M c o u n t e r a n d t h e C u t i e P i e , a r e
v a l u a b l e f o r l a b o r a t o r y r a d i a t i o n s u r v e y . T h e G M c o u n t e r i s r e l a t i v e l y
a c c u r a t e f o r d e t e c t i o n o f b e t a a n d g a m m a r a d i a t i o n u p t o 2 0 m r e m / h r .
For higher radiation fields, up to 2500 mrem/hr, a Cutie Pie is more
efficient. The Cutie Pie may also be used to detect the presence of,
but not monitor for, alpha particles. For extremely high radiation
levels, specialized monitoring instruments must be used.
Rules for Laboratory Operation
A laboratory containing radioactive materials must have rules
to prevent unnecessary exposure to radiation. These rules are designed
to prevent ingestion, to minimize general radiation levels, and to
24
i n s u r e t h e p r o p e r r e t e n t i o n o f r a d i o a c t i v e m a t e r i a l s . T h e s e r u l e s a r e
a p p l i c a t i o n s o f s o u n d p e r s o n a l h y g i e n e a n d g o o d c o m m o n s e n s e .
The rules for laboratory operation are in four categories:
general procedures, handling of radioisotopes, radioactive waste dis
posal, apd contamination.
A. General Procedurest
1« Nothing will be placed in the mouth. This prohibits smoking,
drinking, eating, and pipetting by mouth.
2. The laboratory must be kept clean and orderly at all times.
3. Personal belongings brought into the laboratory may be lost by
contamination or corrosive chemicals. " Such losses are the personal
risks of the student.
B. . Handling of Radioisotopes
1. Film badges will be worn whenever radioisotopes are, or may be,
present.
2. Pocket dosimeters will be worn whenever high radiation levels
are expected. The dosimeters afford immediate knowledge of the amount
of exposure.
3. Required radiation warning signs will be posted. (See Part 20
of Title 10, Code of Federal Regulations.)
4* All containers with radioactive materials will be clearly
tagged showing isotope, amount, and date.
5. All liquid radioisotopes must be stored in double containers.
Volatiles and dusts must be used in the hood only.
25
6. No area, without controlled access, may have a radiation level
above 5 mrem/hr. Suitable shielding and containment may be used to r e - '
duce the radiation to this level.
7. When there is a possibility of hand contamination, rubber
gloves should be worn. Any person with breaks in the skin must wear
rubber gloves.
8. Goggles, or other eye protection, must be worn when handling
hazardous chemicals.
9. At the completion of any experiment using radioisotopes, the
students, instructors, equipment, and room area must1 be surveyed for ra
diation.
C. Waste Disposal
1. Radioactive waste, certified by the instructor to read at or
below background level, may be disposed of by placing it in a drain or
normal refuse can.
2. Radioactive waste reading above background level mist be dis
posed of by the instructor (by controlled burial under the direction of
the University Health Physicist).
3. Liquid radioactive waste should be stored separately from solid
waste.
4. Radioactive wastes should be stored separately according to
half-life.
5. All radioactive wastes must be stored in containers which are
clearly marked.
26
D. Contamination
1. Any contamination of the body or clothing should be reported to
the laboratory instructor at once.
2. If apparatus in the laboratory is contaminated, decontamination
procedures will be undertaken to reduce the radiation level to back
ground.
3. Under no circumstances will contaminated apparatus be returned
to its shelf position.
Decontamination
Once a piece of equipment in the laboratory or portion of the lab
oratory itself become contaminated, proper decontamination procedures
must be undertaken. These decontamination procedures will vary de
pending upon the type, nature, and amount of the spill. There are var
ious methods available for decontaminating certain types of material.
Refer to Table 4-4 of Radioisotope Techniques by Overman and Clark for
specific methods of decontamination
CHAPTER 3
ANALYSIS OF ERRORS
Introduction
An experiment Is designed to answer one or more questions. It
must be designed to give these answers without influencing their out
come, and to give answers with sufficient accuracy to define the an
swers. In order for an experiment to accomplish all this, the experi
menter must have adequate knowledge of the uncertainties involved in
every phase of the experiment. He must know these uncertainties in
order to analyze the data taken and determine the reliability of the
answers.
It is the purpose of this chapter to present some of the uncer
tainties involved in nuclear engineering experimentation, and how uncer
tainties are analyzed. It is impossible to mention all the experimental
errors which might be found; however, this chapter will give a few exam
ple's. It is expected that detailed discussions of the errors associated
with the individual experiments will be given in the laboratory reports.
Definitions
An error in a recorded value is defined as the difference be
tween the recorded value and the true value. Generally, errors are ex
pressed as percentage errors or probable errors. These terms are
27
28
defined by the following equations. If x is a measured value of an
event, and T is the true value of that event, then
% error (x - T)(1Q0) T
(4-1)
Since the true value is almost never known, an expected value or best
estimate is used in its place. For example, if the average value is
taken as the best estimate to the true value, then
where
probable error in x (x - sHlOO)
x(4-2)
(4-3)
It should be noted that other means may be used to define the best esti
mate. In the laboratory reports, the percentage error in the measure
ment and the resulting percentage error in the final answer should both
be given.
Classification of Errors
Errors can be classified either as systematic errors or random
errors. Systematic errors are determinate errors which can be minimized
by applying known correction or calibration factors. As an example, the
efficiency of a radiation detector can be determined, and the proper
correction can be made to the recorded values. If the dead time (see
Experiment 1) of a counter is known, a suitable correction can be made
to give a more accurate count. It should be noted, however, that
29
although the correction is made, there is still some random error in
herent in the correction itself.
Random errors are those which are accidental or indeterminate.
They can be subclassified as either gross random errors or statistical
errors. Gross random errors result from such things as contamination or
impurities in materials, both of which may cause the complete failure of
an experiment.
Statistical errors are those which result from the statistical
nature of the event. For example, in measuring the length of an object,
the exact length cannot be found no matter how accurate the measuring
device may be. The chance of the measurement being too short is as
probable as the chance of it being too long. Certain statistical dis
tributions describe the frequency of occurrence of the long and short
measurements. These distributions and their application to nuclear
decay processes are discussed in Chapter 4.
O f t e n r a n d o m e r r o r s c a n b e r e d u c e d b y u s i n g t h e p r o p e r e q u i p
m e n t a n d t h e p r o p e r e x p e r i m e n t a l t e c h n i q u e . F o r e x a m p l e , i f a s a m p l e i s
c o n t a m i n a t e d , t h e s p e c t r u m ( s e e E x p e r i m e n t 6 ) c a n b e i n v e s t i g a t e d i n
o r d e r t o d e t e r m i n e w h i c h c o n t a m i n a n t s a r e p r e s e n t .
E r r o r s F r o m R a d i a t i o n D e t e c t i o n E q u i p m e n t
I m p r o p e r s e l e c t i o n o f e q u i p m e n t i s a c o m m o n m i s t a k e a l w a y s r e
s u l t i n g i n l a r g e e r r o r s . I n t h i s c o u r s e , h o w e v e r , t h e e q u i p m e n t s e l e c
t i o n h a s a l r e a d y b e e n m a d e f o r e a c h e x p e r i m e n t . S t i l l , i t i s i m p o r t a n t
30
that each piece of equipment is properly assembled and correctly used in
order to insure the success of the experiment.
Although the proper selection of the equipment is made, there
are still errors resulting from certain inherent factors in the radia
tion detection devices themselves. It is important that the student be
aware of these errors although he may have no control over them. Some
of these inherent errors are systematic and therefore can be corrected
to some degree. Others are completely random. Electronic noise, spu
rious counts, erratic performance of any piece of equipment, and age
variation of the equipment are all error producing factors over which
the student has no control. Care, however, must be taken to insure that
the equipment is correctly operated.
Errors From Radiation Measurements
These errors result from factors peculiar to radiation measure
ment. Examples of factors giving rise to this type of error are absorp
tion of radiation in detection chamber walls, self-absorption of radia
tion, scattering and backscattering of radiation, geometrical effects,
production of secondary radiation, and absorption of radiation in air.
Often these errors can be estimated or calculated, and the recorded
values can be adjusted accordingly.
CHAPTER 4
COUNTING STATISTICS
Introduction
This chapter is intended to describe briefly the need for sta
tistical analysis in processes associated with counting, the types of
probability distributions, and the methods of using these distributions
in laboratory procedure. This chapter is not intended to give detailed
derivations of the probability distributions, but rather to provide a
reference to the necessary equations as well as brief explanations and
examples of their use. It is expected that statistics, where appli
cable, will be used in preparing the laboratory reports. Many refer
ences which give the detailed theory of nuclear statistics are avail
able. In this chapter the concern is not the uncertainties from the de
tector, the associated equipment, or the laboratory technique, but only
the uncertainties resulting from the random nature of the relatively
small number of events in nuclear decay processes.
All nuclear and atomic events are random. However, when one
measures a macroscopic event such as the Faraday by plating silver, for20example, the large number of events, of the order of 10 , reduces the
influence of a single variation to an undetectable variation. In
counting, however, the much smaller numbers make each single variation
important.
31
32
Probability theory has a very important application in nuclear
processes because radioactive decay occurs in a random manner. Because
of this random manner, there are no true or correct values. To get the
best results, average or mean values are used. Along with this mean
value, a range around this value, within which a successive trial might
be expected to fall, is given.
A probability can be thought of as the ratio of the number of
expected successes to the number of trials. In general, we talk about
frequency distributions which describe the frequency of successes. The
following distributions have been found to be useful for nuclear pro
cesses .
The Binomial Distribution
The binomial distribution is a frequency distribution des
cribing yes-no type random events. It describes bivalued processes such
as flipping coins. By bivalued, it is meant that the events can take on
only two values.
If Px is the probability that an event will occur x number of
times, then
zlx xl(z-x): PX d - P)Z-X . (4-1)
where z is the number of trials, £ is the probability that the event
will occur, and (1-p) is the probability that the event will not occur.
In this case the two values are (1) that the event will occur and (2)
that the event will not occur. Because of the nature of this
33
distribution, it is obvious that x and z are confined to integers.
Equation 4-1 is the general term of the binomial expansion of (p + q)Z
where q = (1 - p ) . Therefore,
(P + q)Z - P; + + Pz.2 + ••• + P0 - (6-2)
The following example will illustrate one of the uses of the
binomial distribution.
Example 4-1: If five dice are thrown, what is the probability that
face 2 will come up on at least three dice?
The binomial distribution is applicable to this example because
throwing dice is a bivalued process in that the face 2 will either
come up or it will not. Because there are five dice, z ■* 5, and be
cause face 2 is one of six faces on a die, p * 1/6. Since we would
like the face 2 on at least three dice, we must sum the probabili
ties of it occurring on exactly three, exactly four, and exactly
five dice. Therefore,
5*. 250r3 " ( 1 / 6 ) ( 5 / 6 > ' - 7776- ’
P4 - T&r (I/O W - *
ps - A <i/6>5<5/6>° 7776
P - P3 + P4 + P5 7776
Therefore, if we rolled the five dice 7,776 times, we would expect
the face 2 to come up on at least three dice 276 times*
34
The binomial distribution can be used in considering the pro
bability of radioactive disintegration, because this also is a bivalned
process. (An atom either disintegrates or it does not.) If the total
number of atoms present is H0 , the probability of x number of these
atoms decaying in a time t_ is given by
No'x x'.CN -x) 1
-Xtxx, -Xt^N0-x(1 - e ) (e (4-3)
The following example will illustrate the use of Equation 4-3.
Example 4-2: If 10 Copper 66 atoms are present initially, what is
the probability of exactly 4 atoms decaying in four minutes? The
half-life of Copper 66 is 5.2 minutes.
t - (In 2)/T1/2 X t « 0.533
e-Xt - 0.588, (1 - e~Xt) - 0.412
NQ ” 10, x ■ 4
^4 = tTgT (0.412)4(0.588)*
P4 - 0.25
The Poisson Distribution
The Poisson frequency distribution describes all random pro
cesses whose probability of occurrence is small and constant. It is ap
plicable to nearly all observations made in nuclear physics. This dis
tribution is a limiting case of the binomial distribution with p « 1,
35
t h e m e a n v a l u e ( m ■ p z ) c o n s t a n t , a n d z v e r y l a r g e . T h e e q u a t i o n f o r
t h e P o i s s o n d i s t r i b u t i o n i s
Px 4e-mx ! (4-4)
Since x can only represent integers, a graph of the Poisson distribution
(Px versus x) can only be a histogram.
One very useful form of the Poisson distribution describes the
distribution in the size of time intervals between successive events of
a random process. In this case Equation 4-4 is written in terms of
time. The mean value, m, becomes equal to at where a is the average ac
tivity (counts/time) and _t is the time. The probability Px that x
counts will not occur in a time _t is given by
Px (4-5)
More specifically, if the counter is set to shut off at a fixed number
of counts £, the probability of obtaining £ counts in a time T or less
is given by
s-1P - 1 - £ P„ (4-6)1 x-0
The following example will illustrate the use of Equation 4-6.
Example 4-3: With an average counting rate of 0.8 counts/second,
what is the probability of obtaining 4 counts in 4.5 seconds or
less?
36
a “ 0.8/sec, t * 4.5 sec, s ■ 4,
at “ 3.6
PQ - 0.027, P1 - 0.093,
P 2 “ 0.175, P4 “ 0.210
?T - 1 - (0.027 + 0.093 + 0.175 + 0.210)
PT - 0.495
The Normal Distribution
The normal frequency distribution Is an approximation to the
binomial distribution when z is very large. In the normal distribution,
the observed variable (x) is not confined to integer values but can take
on any value from -oo to +oo. The probability that a value will lie be
tween x and x + dx is given by
(4-7)
where o’ is the standard deviation discussed in the next section of this
chapter. The probability that the mean value m is missed by an amount u
on any given trial is given by
(4-8)
Some useful calculations for Equation 4-8 are given in Table 4-1.
37Table 4-1. Calculations for Equation 4-8.
uAr 0 0.500 0.6745 1.000 1.645 2.000
*u 1 . 0.617 0.5000 0.317 0.100 0.455
From Table 4-1, it Is seen that If u “ o’, (1 - 0.317) * 0.68 or 68% of
the trials will fall within ±tr or the value of m, the true average.
Standard Deviations
For any frequency distribution, the standard deviation (cr) is
defined as the square root of the average value of the square of the in
dividual deviations from the true value. Therefore, for a large series
of n measurements of x,
2 1“n 2crz = (1/n) E (xi - m)z (4-9)i-1
where m is the true value of what is being measured. For the distribu
tions discussed previously, the standard deviations take the forms shown
in Table 4-2.
Table 4-2. Forms of the Standard Deviation
Distribution Standard Deviation
Binomial cr “ s M i - p )
Binomial (Form for Radioactive Decay) cr
Poisson or = x/nT
Poisson (Time Interval) <r « Vs/a
Normal cr “ cr
38
For most of the experiments in this manual, the form of the
standard deviation for radioactive disintegration will be required.
However, in many cases, X _ t will be much less than 1 thus making e
approximately equal to 1. In this case the standard deviation takes
the form of that for the Poisson distribution, <r ■ V m . This result is
expected because for the Poisson distribution p ■ (1 - e , and there
fore the condition that £ be much less than 1 is satisfied.
In general, we will not be interested in the standard deviation
of the number of counts but rather of count rates (counts/time). If m
is the true value of the number of counts, the count rate (R) is given
by
R = m/t (4-10)
and the standard deviation of the count rate is given by
o-R = </m/t (4-11)
Because it is never possible to find the true value, a method
of estimating the standard deviations is necessary. If the number of
counts is reasonably large, m can be replaced by the number of counts
(x) . The following example will help to illustrate the method of
finding the average value and the standard deviation.
Example 4-4: Suppose we take a series of ten one-minute counts and
obtain the results shown in Table 4-3.
39
Table 4-3. Counting Series
Trial Counts/minute
1 2202 2143 1984 2045 2216 2137 2188 2089 217
10 207Total 2120
The number of counts and its standard deviation are given by
x “ 2120, (rx -V2120 - 46
x = 2120 ± 46
The average count rate and its standard deviation are given by
' R - 2120/10 ■ 212 counts/minute
<rR - V2120/10 - 4 .6
R ■ 212 ± 4 . 6 counts/minute.
This means that the probability of the true average being between
218.6 and 207.4 is approximately 68%. Suppose we had taken ten ten-
minute counts and obtained a total of 21,200 counts. The average
counting rate would then be given by
40- _ 21200 ±/21200 R " 100
R ■ 212 db 1.5 counts/minute.
Therefore by taking longer counts, we have narrowed the expected
range of the true average.
Often In the experiments In this manual, such as for a rapidly
decaying source, it will not be possible to obtain more than one count.
In these cases we consider each separate count to be an average and find
the standard deviation by taking the square root of the number of
counts. As an example, consider the decaying counts given in Table 4-4.
In plotting data, it is desirable to show the standard deviation on the
graph in order to give a better idea of the uncertainties involved.
Using the data in Table 4-4, the method of plotting is shown in
Figure 4-1.
propagation of Errors
Table 4-4. Decaying Source
Trial Counts
12345
1000 ± 32 550 ± 23 350 ± 19 280 ± 17 160 ± 13
Often it is necessary to perform some mathematical operation on
numbers with standard deviations. In these cases, we are interested in
41
Figure 4-1. Plotting Standard Deviations
the standard deviation of the result. The following equations show the
propagation of standard deviations. If r is a function of the form
r - f O ^ r ^ r g , . . . ) (4-12)
and o^, eg,... are the standard deviations of r^, ^ ,... respectively,
then the general form of the standard deviation of r is given by
" k - ai + B . 0*2 • • •
12 (4-13)
Some examples of the use of Equation 4-13 are given in Table 4-5.
MP>
42
Operation
r ■ rl ± r2
r ” rlr2/r3
r - ar1 + b
Standard Deviation
Table 4-5. Error Propagation
(fR-aei
crR ” a(r/r1)<r1
trR = X r ^
CHAPTER 5REPORT PRESENTATION
The manner in which a report is presented will, to a great ex
tent, determine how readily experimental results are accepted. No re
sults are meaningful until they have been communicated to other persons.
Reports should be informative, accurate, concise, technically and gram
matically correct, readable, legible and efficiently organized. In
addition, reports should be complete enough to insure that they cover
all relevant aspects of the experiment. No necessary information should
be left to speculation or conjecture by the reader; however, unnecessary
and unrelated material should be carefully eliminated' from all reports.
The report must provide all of the information necessary for the reader
to understand and evaluate the experiment. Excess verbiage detracts and
may prevent acceptance.
Although there are many forms for laboratory reports, those
submitted in this course will conform to the minimum standards specified
in this chapter.
Laboratory reports will be required for each experiment con
ducted. Group discussions during the laboratory period are encouraged,
but the written reports must be individual efforts. It is expected that
the literature will be searched for further information relating to each
experiment. The reports will adhere to the following format.
43
44Title Page: Include the number and name of the course, number and
title of the experiment, name of student and partners,
date performed, and date submitted (see Example 5-1).
Abstract: Give an informative but brief digest of the report, con
veying all essential information. This section should be
complete even when separated from the rest of the report.
It should answer the question: "What is this report
about?"
Table of Contents: Indicate major subsections of the report showing
pages (see Example 5-2).
Description: Include experimental objectives and general experi
mental method. Do not give a step by step procedure.
Include any diagrams, not presented in the manual,
which will clarify the presentation of the report.
Recorded Data: Include in tabular form both the original and re
duced data, clearly labeled. ("Original1 does not
necessarily mean the original data sheet.)
Sample Calculations: Include typical examples of each type of non
trivial calculation involved in the experi
ment.
Results: Give results in both graphical and tabular form if this
enhances the clarity of the reports.
Discussion: Evaluate the results of the experiment. Discuss the
validity of the methods used and compare results with
theoretical values and/or other experimental values
45
when possible. Estimate the sources of possible errors
and their magnitude. If relevant, include the limita
tions of the experiment and how they may be removed.
Questions and Problems: Completely answer the questions and prob
lems indicated in each experiment.
References: List all references used in preparing the report.
Each graph must be neatly and accurately drawn and labeled.
As a minimum, all graphs will contain the title of the experiment, title
of the graph, date the experiment was performed, and the name of the
person who performed the experiment (see Example 5-3). All tables must
be labeled and presented in a manner to be easily understandable (see
Example 5-4).
NUCLEAR ENGINEERING 221
EXPERIMENTAL NUCLEAR ENGINEERING I
EXPERIMENT 1
GEIGER-MULLER AND PROPORTIONAL COUNTERS
By
John Doe
With
Jack Smith and Tom Brown
PERFORMED: OCTOBER 10, 1965
SUBMITTED: OCTOBER 24, 1965
(Example 5-1 Sample Title Page)
TABLE OF CONTENTS
DESCRIPTION OF EXPERIMENT......................... 1
RECORDED D A T A ...................................... 2
SAMPLE CALCULATIONS.................... ........... 5
RESULTS ............................................. 7
DISCUSSION ......................................... 9
QUESTIONS AND PROBLEMS ........................... 13
R E F E R E N C E S ......................................... 15
(Example 5-2. Sample Table of Contents)
(Example 5-3. Sample Graph)
EXPERIMENT 2Geometrical Effects On Radiation
Counts/minute vs BackingNovember 10, 1965
John Doe
Added Backing, mg/cm A1
4>co
49
Table 2-3. Maximum Allowable Quarterly Dose Rates
Allowable Quarterly DoseBody Part __________ (rem)
Whole body, head and trunk . . . . . . 1-1/4S k u l l ......................................... 1-1/4Eye l e n s .......................... 1-1/4P e l v i s ............................... 1-1/4Gonads .................................. 1-1/4H e a r t ................................ 5L i v e r ....................... ............... 5Stomach ..................... ............... 5Intestines .................................. 5B l a d d e r ................................ .. 5Lungs .............................. .. 5Spleen ..................... ................. 5Pancreas .................................... 5Kidneys . ..................... . . . . . . 5Bone structure . ............ ............. .. 7S k i n .............., ......................... 7-1/2T h y r o i d ........... 7-1/2H a n d s ........................................ 18-3/4Ankles .................................... • 18-3/4F e e t .......................................... 18-3/4
(Example 5-4. Sample Table)
EXPERIMENT 1
GEIGER-MULLER AND PROPORTIONAL COUNTERS
Purpose
The purpose of this experiment is to demonstrate the character
istics, the operation, and the uses of the Geiger-Muller and the propor
tional counters.
Theory
Both the Geiger-Muller (GM) counter and the proportional
counter operate on the principle of ionization of a gas. This principle
has been discussed in Chapter 1. These counters generally consist of an
anode wire enclosed by a cylinder filled with some organic gas. When a
charged particle enters the tube, it produces several ion pairs by an
ionization reaction. These ions are accelerated to their respective
electrodes by an applied electric field. They are discharged upon
reaching the electrodes, resulting in a current in the external circuit.
When the counter has detected ionizing radiation, the signal is
sent to the preamplifier (see Figure El-2). The preamplifier serves as
an impedance matching device between the detector and the non-overload
ing amplifier.
The non-overloading amplifier and pulse height selector is a
linear pulse amplifier. It is capable of fast recovery after overload
and caq amplify small pulses in the presence of overload pulses. The
51
52
incoming signal from the preamplifier is shaped by an input network,
part of which is the coarse gain control. The signal is amplified and
then passes a fine gain control• The pulse height selector sets a
threshold for the size of the pulses.
The pulse with the desired amplitude, after leaving the non
overloading amplifier, goes to a scaler-timer combination where the
counts are recorded.
The high voltage power supply furnishes the voltage between the
collector electrode and the cathode within the detector.
A graph of the number of ions collected versus the voltage ap
plied across the electrodes is useful in explaining the operation of
the two counters. This graph is given in Figure El-1. In region A the
relatively weak electric field between the electrodes allows some of the
Volts
Figure El-1. Ions Collected Versus Applied Voltage
ions produced to recombine before they can be collected. As the voltage
is increased, the number of recombinations become smaller because the
ions accelerate to the electrodes faster, and have less time for "
53
recombination♦ When region B is reached, the recombination losses are
negligible, and a plateau exists* In this region all of the ion pairs
produced are collected. As the voltage is increased to region C, the
primary ions produced by the outside radiation acquire enough energy
from the electric field to produce further ionization. This phenomenon
is known as gas multiplication or gas amplification. In this region the
number of ions collected is proportional to the initial ionization. It
is in this region that the proportional counter is operated. Region C
preserves the dependence of pulse size on the primary ionization thus
making it possible to distinguish among different types of radiation.
In region D the electrons from the primary ionization reactions are ac
celerated to the extent that the secondary ionizations are limited only
by the GM tube and the external circuit. Hence, any type of initial
ionization reaction will produce the same pulse size. In this case the
gas amplification is known as an avalanche of electrons. It is in this
region that the GM counter is operated; therefore, the GM counter pulse
size is independent of the initial ionizing reaction, thus of the nature
and energy of the radiation. Region E is the region of continuous dis
charge and should be avoided in order to prevent damage to the GM tube.
While the avalanche is proceeding, the electrons, due to their
greater mobility, travel much faster to the anode wire than the ions are
able to move away to the cathode cylinder. As the electrons proceed to
the anode, they cause more and more ionizing reactions. Most of these
reactions occur in the vicinity of the wire. Because of their greater
mobility, the electrons are collected by the wire before the ions have
54
moved appreciably away from it* During the period that the ions are
moving to the cathode, the counter is insensitive to any outside ion
izing radiation. This period is called the dead-time of the counter.
Upon striking the cathode, the ions may knock out other electrons thus
causing another avalanche. However, the probability of a sufficient
number of electrons being produced by this phenomenon is generally small
with organic, gas-filled tubes.
Since radiation enters the Geiger tube during the period of in
sensitivity, the number of actual recorded counts is inaccurate. There
fore, when any degree of accuracy is desired in the number of counts, a
positive dead-time correction is necessary*
The dead-time of most GH counters is usually on the order of
hundreds of microseconds. It should be noted that all radiation detec
tors have inherent dead-times.
In this experiment the correct operating plateau of the GM and
proportional counters will be determined. The dead-time of the GM
counter will also be determined. Future experiments will require the
use of this information to insure that the counters are operated cor
rectly.
Apparatus
1. Alpha source such as Polonium 210.
2. Beta sources such as Bismuth 210 and/or Cobalt 60.
3. Scaler, Baird Atomic, Model 134.
4. High voltage power supply, Baird Atomic, Model 319A.
555. Timer, Baird Atomic, Model 630.
6. Non-overloading amplifier, Baird Atomic, Model 215.
7. Fan unit, Baird Atomic, Model 1268.
8. Preamplifier, Baird Atomic, Model 231.
9. Required cables as shown in Figures El-2 and El-3.
10. Four pi windowless proportional chamber.
11. Geiger chamber, end window type.
12. Proportional gas, P-10, 90% Argon, 10% methane.
13. Geiger gas, 99.05% Helium, 0.95% isobutane.
Procedure
1. Assemble the equipment (with Geiger chamber) as shown in
Figure El-2.
2. Allow the Geiger gas to flow at a rapid rate (approximately 20
bubbles per second) through the chamber for approximately five to ten
minutes.
3. Place the Bismuth 210 beta source in the chamber near the
Geiger tube.
4. Regulate the flow of gas to approximately three bubbles per
second.
5. Slowly increase the high voltage until counts begin to appear
on the scaler.
8. Take a two-minute count.
7. Repeat step 6 at 50-volt increments. Caution: Avoid the re
gion of continuous discharge. (Do not exceed 1700 volts.)
56
Geiger Chamber
Input
Hi Out
J-102
Non-overloading Amplifier
PreamplifierHV
Fan
■ ■ ■
0-5000 VoltsHigh Voltage Power Supply
Figure El-2. Geiger-Muller Counter
57
Hi Out
J-102Input
a* ■* v v »Timer
iHfBilhiraB--- * V. "Scaler
I - *^ -i « • ■Non-overloading Amplifier
%■
Fan
HV
0-5000 VoltsHigh Voltage Power Supply
Figure El-3. Proportional Counter
58
8. Place the voltage setting approximately in the center of the
plateau region, place one of the beta sources in the chamber, and count
for two minutes.
9. Without moving the first source, place the second beta source
in the chamber and count the two sources together for two minutes.
10. Remove the first source and count the second source alone for
two minutes.
11. Obtain a background reading.
12. Repeat steps 8 through 11.
13. Remove the Geiger chamber and preamplifier and replace these
with the four pi windowless chamber as shown in Figure El-3.
14. Place the alpha and beta sources in the chamber.
15. Repeat step 2 with P-10 gas, and repeat steps 4 and 5.
16. When the counts begin to appear, take two-minute counts at 50-
volt increments until the alpha and beta plateaus are reached.
17. Remove source and obtain background readings at voltages cor
responding to the centers of the alpha and beta plateaus.
Results and Presentation of Data
1. Plot voltage versus counts recorded for the Geiger counter and
indicate the operational plateau.
2. Plot voltage versus counts recorded for the proportional
counter and indicate the alpha and heta plateaus.
3. Derive the equation for the dead-time of the GM counter. Use
this equation and the data recorded in steps 8 through 12 of the
procedure to calculate two values of the dead-time for the GM counter *
Find the percentage difference between these two values. Explain the
differences.
Questions and Problems
1. What constitutes background radiation?
2. Why are different gases used in the proportional counter and
the (31 counter?
3. W h y m u s t t h e c h a m b e r s b e c o n t i n u a l l y f l u s h e d ?
4. What causes continuous discharge?
5. What are the advantages and disadvantages of using the propor
tional counters?
6. Why is quenching necessary? Describe a method by which quench
ing is accomplished.
Selected References
Atomic Accessories, Inc., Instruction Manual, Four Pi Flow Counter Model FC110.
Hoag, J. Barton, ed., Nuclear Reactor Experiments. C. Van Nostrand Company, Inc., Princeton, New Jersey, 1958, Chapter 2.
M o n t g o m e r y , C. G . , a n d D. D. M o n t g o m e r y , T h e P h y s i c a l R e v i e w , V o l . 57, p. 1930, (1940).
Overman, Ralph T. and Herbert M. Clark, Radioisotope Techniques, McGraw- Hill Book Company, Inc., New York, 1960, Chapter 2.
Price, William, J., Nuclear Radiation Detection, McGraw-Hill Book Company, New York, 1958, Chapters 2, 5, and 6.
Stever, H. G., The Physical Review, Vol. 61, p. 38, (1942).
59
EXPERIMENT 2
PORTABLE SURVEYING INSTRUMENTS AND GEOMETRICAL EFFECTS ON RADIATION
Purpose
The purpose of this experiment is to demonstrate the character
istics, the operation, and the uses of portable surveying instruments
and to demonstrate the effects of geometry on radiation intensity.
Theory
In principle, it is a relatively simple matter to carry out the
survey of radioactive materials or to determine the presence of radia
tion fields. The radiation detection instruments now available provide
fairly accurate means by which this radiation is detected.
The detection of radiation, whether by portable or fixed
counter, is based on the fact that ,a charged particle or a photon pas
sing through matter leaves along its path a string of ionized or excited
atoms which can be detected and counted (see Chapter 1).
Portable counters find their principle use in area survey re
quirements . These requirements may include laboratory radiation checks,
civil defense radiation monitoring, and military applications for area
survey.
The Cutie Pie survey meter, which is a portable ionization
chamber, is designed to measure beta and gamma radiation of medium and
high intensity as well as to detect the presence of alpha radiation.
60
61
One outstanding feature of this meter is that it can be zeroed in an ex
isting radiation field. This counter is shown in Figure E2-3.
The portable G M counter, shown in Figure E2-3, is a transistor
ized survey meter designed to measure low intensity beta and gamma ra
diation. This counter has a tendency to saturate in a strong field,
and this saturation is seen as a sharp drop in meter reading as the
source is approached. Calibration in a radiation field cannot be accu
rately accomplished with this meter.
The portable radiation detectors provide relative radiation in
tensities and thus have specific application in those areas previously
mentioned. If these portable detectors are to be used to their maximum
efficiency, care in operation must be exercised. The proper instruction
manuals should be consulted prior to the operation of these instruments.
The passage of gamma radiation through matter is governed
mainly by the three interactions: photoelectric effect, Compton scat
tering, and pair production. These interactions have been discussed in
Chapter 1. The probability of a gamma photon traversing a given amount
of matter is the product of the survival probabilities for each type of
interaction. Therefore,
Ipri - Ioe”02ce"’rrxe"636 = I0e‘<ff + T + O x (E2-1)
where Ipr£ is the primary (unattenuated) radiation intensity at a dis
tance x into the matter, I0 is radiation intensity incident on the
matter, a-, t, and e are linear attenuation coefficients for the Compton,
62
photoelectric, and pair production interactions, respectively. The
total linear attenuation coefficient is then given by
» o- + T + e (E2-2)
The attenuation of a well-collimated beam of monoenergetic
gamma photons in a 'good geometry' experiment (see Figure E2-1) is des
cribed by Equation E2-1. 'Good geometry' implies that virtually no
Compton scattered photons or annihilation photons, if pair production
is involved, reach the detector. If, however, a cylindrical shell is
placed around the source and the detector is left unshielded, the ex
periment would have 'poor geometry* (see Figure E2-2).
Absorber
Figure E2-1. 'Good Geometry' Experiment
Absorber Source
Detector
Figure E2-2. 'Poor Geometry* Experiment
In the 'poor geometry' experiment, some of the Compton scattered and
annihilation photons are seen by the detector. Therefore, the intensity
at the detector is greater than the primary intensity. The ratio of the
63i n t e n s i t y s e e n b y t h e d e t e c t o r t o t h e p r i m a r y i n t e n s i t y i s k n o w n a s t h e
g a m m a r a y d o s e b u i l d - u p f a c t o r ( B ) .
B O b s e r v e d i n t e n s i t y P r i m a r y i n t e n s i t y
(B2-3)
A number of tables giving values for build-up factors are available.
One such table is given by Goldstein (see Selected References).
This experiment is designed to provide a familiarity with the
operation of some portable survey meters. In addition, use is made of
these meters to determine gamma ray dose build-up factors and to study
radiation intensity as a function of distance from a source.
A p p a r a t u s
1. Gamma source such as Cobalt 60 or Cesium 137.
2. Portable GM counter, Johnson, Model GSM-5.
3. Cutie Pie survey meter, Nuclear-Chicago, Model 2586.
4. Vernier caliper. Craftsman.
5. Aluminum, iron, and lead cylinder sets.
6 . M a s k i n g t a p e .
7. Lead bricks.
P r o c e d u r e
1. Make a diagram of the laboratory room.
2. Using the portable GM counter, survey the laboratory for radia
tion levels and record on the diagram.
3. With the lead bricks, build an open-end cover and with the
masking tape, mark off a line perpendicular to the open end and lines
64
0
p M
.
1
Figure E2-3. Geiger-Muller and Cutie Pie Survey Meters
Figure E2-4. Open End Cover with Markers
6545° on either side of the perpendicular (see Figure E2-4).
4. On the masking tape, mark spaces of four inches beginning at
the face of the cover. The total length should be 20 inches.
5. Place the Cobalt 60 source in the cover and record radiation
intensities with the Cutie Pie meter at each of the points marked in
step 4.
6. Place the Cutie Pie meter eight inches from the bare Cobalt 60
source and record the intensity.
7. Place the smallest cylinder of the aluminum set around the
source and record the intensity. Repeat for each cylinder in the set.
With the vernier caliper, measure and record the thickness of each cy
linder.
8. Repeat steps 7 and 8 with the iron set.
9. Repeat steps 7 and 8 with the lead set.
Results and Presentation of Data
1. Using the data obtained from step 6 of the procedure, plot ra
diation intensity versus distance from the source for each line along
which the measurements were made.
2. Draw a complete diagram of the laboratory room showing the ra
diation intensities measured during the survey. Be certain to indicate
hot spots.
3. Using the data obtained in steps 7 - 10 of the procedure, plot
intensity versus absorber thicknesses for each material on semi-log
66
graph paper. On the same graphs, plot the primary Intensity versus
thickness.
4. Considering the source to be a point, how should the radiation
intensity change with the distance from the source? Discuss how your
results compare with those expected.
5. From the graphs, find the build-up factors for |i0x ” 1. Compare
these build-up factors with those given in the appendices of Goldstein
(see references) by finding the percentage errors.
Questions and Problems
1. Which counter, the portable GM or the Cutie Pie, should be used
to survey for very low intensity beta particles? Why?
2. What are the advantages of the Cutie Pie over the portable GM
counter?
3. Discuss the proper procedure for placing the Cutie Pie into
operation.
4. Discuss the proper procedure for using the portable survey
meter to survey the laboratory.
5♦ Discuss the possible causes of the hot spots found in the lab
oratory .
Selected References
Evans, R. D . , The Atomic Nucleus, McGraw-Hill Book Company, Inc., New York, 1955, pp. 728-735.
Goldstein, Herbert, Fundamental Aspects of Reactor Shielding, Reading, Massachusetts, 1959, pp. 367-369.
67
Nuclear Chicago Corporation, 2500 Series Cutie Pie Survey Meters, Instruction Manual, February, 1963.
William B. Johnson & Associates, Inc., Instructions, Operation and Care of Model GSM-5 Survey Meter, Montville, New Jersey.
EXPERIMENT 3
b f 3 NEUTRON DETECTOR
Purpose
The purpose of this experiment is to demonstrate the character
istics, the operation, and the uses of the boron trifluoride (BF^)
thermal neutron counter as well as to determine the operating plateau of
the modified BFg counter.
Theory
In common with all neutral particles, neutrons can be detected
only by means of secondary charged particles which they generate in pas
sing through matter, or by secondary processes which produce ionizing
radiation (see Chapter 1), One of the means by which these secondaryj
particles are produced is nuclear disintegration. An example of this
nuclear disintegration is the alpha particle from the B^(n,oQ reaction:
5B10 + Qn1 — > 3Li7 + 2He4 . (E3-1)
The BFg, boron trifluoride, gas-filled detector was developed
to measure thermal neutron densities. This detector employs the reac
tion shown above to initiate the necessary gas ionization in the tube.
Subsequent multiplication of the ion pairs due to the high field gra
dient near the anode wire results in a pulse of electrons at the anode.
68
69
Because of the very significant drop in cross section for the B10(nsa) reaction with an increase in neutron energy, the sensitivity of
this counter is very small for fast neutrons. However, if the counter
is enclosed in a neutron moderator in such a way that the neutrons are
slowed down before entering the counter, then the counter can detect
fast neutrons.
The BF3 counter, when modified to detect fast neutrons, is
known as a long counter. The conventional BF3 counter is embedded in a
cylinder of polyethylene. Around the polyethylene is a layer of cadmium
which is a very good absorber of slow neutrons but a very poor absorber
of fast neutrons.
As neutrons enter the counter, they have energies extending
from thermal up to several million electron volts. The cadmium layer
with its high thermal neutron cross section absorbs the thermal neutrons
but passes the fast neutrons into the polyethylene. As these fast neu
trons travel through the layer of polyethylene, they are slowed to velo
cities within the range of the detector. The neutrons at higher ener
gies have a slightly better chance of detection than those at lower
energies, since the faster neutrons are slowed down at greater depths
in the polyethylene and have less chance of escaping before detection.
Since the BFg ionization chamber is in effect a proportional
counter, the function of the preamplifier, non-overloading amplifier,
scaler-timer, and high voltage power supply are the same as described in
Experiment 1.
70
In this experiment the operational plateau of the BFg counter
will be determined, Future experiments will require the use of the data
determined.
A p p a r a t u s
1. Neutron sources, Plutonium-Beryllium (Pu-Be) (2).
2. Scaler, Baird Atomic, Model 134.
3. Timer, Baird Atomic, Model 630.
4. High voltage power supply, Baird Atomic, Model 319A.
5. N o n - o v e r l o a d i n g a m p l i f i e r , B a i r d A t o m i c , M o d e l 215.
6. Fan unit, Baird Atomic, Model 1268.
7. Preamplifier, Baird Atomic, Model 231.
8. Required cables as shown in Figure E3-1.
9. Modified BFg counter, Reuter-Stokes, Model RSN-7A, El314.
10. Neutron howitzer.
11. Lead bricks.
12. 14 inch polyethylene plug, 4 inch polyethylene plug.
P r o c e d u r e
1. Assemble the equipment as shown in Figure R3-1.
2. Place a 14 inch polyethylene plug inside the neutron howitzer.
3. Place the two Pu-Be sources inside the howitzer on top of the
polyethylene plug.
4. Place the modified BF^ counter on top of the howitzer, sup
porting it with lead bricks.
71
S'•»
Timer
flHHBScaler
L i ,
0-5000 VoltsHigh Voltage Power Supply
Figure E3-1. BF^ Neutron Detector
72
5. Increase the voltage slowly until the threshold voltage is
reached.
6. Increase the voltage in 50-volt increments until the opera
tional plateau is reached. Record the number of counts at each voltage.
Caution: Do not exceed 2500 volts.
7. Reduce the voltage to threshold.
8. Place the 4 inch polyethylene plug on top of the sources and
repeat step 6.
Results and Presentation of Data
1. Plot counts recorded as a function of applied voltage, with
and without the 4 inch polyethylene plug inserted. Indicate the opera
tional plateau.
Questions and Problems
1. What are the energies of the neutrons emitted from the sources
used in this experiment?
2. What is the absorption cross section of cadmium for these neu
trons?
3. How are pulses resulting from reactions, other than those pro
duced by neutrons, kept from reaching the scaler?
4. What is the cable-attenuation of the cable from the modified
BFg counter to the preamplifier? How does this affect your recorded
data?
5. Compute the thermal neutron sensitivity of the BF3 counter used in this experiment. The filling gas is 96% enriched BF^ at a pressure
73
of 20 cm of mercury. Consider a Maxwell-Boltman distribution.
6. Discuss at least two other means by which neutrons may be de
tected.
7. What are the advantages of using the B^(n,a) reaction for slow neutron detection?
8 . What is the purpose of the holes in the front face of the poly
ethylene in the modified BFg counter?
Selected References
Allen, W. D . , Neutron Detection, George Newnes Limited, London, 1960, Chapter 2.
Price, William J., Nuclear Radiation Detection, 2nd ed., McGraw-Hill Book Company, 1964, Chapter 10.
Reuter-Stokes Electronic Components, Inc., Boron TrifluorideProportional Counter Bulletin, No. 106, Cleveland, Ohio.
Sharpe, Jack, Nuclear Radiation Detectors, 2nd ed., John Wiley and Sons, Inc., New York, 1964, Chapter 4.
EXPERIMENT 4
S E M I C O N D U C T O R D E T E C T O R S
P u r p o s e
T h e p u r p o s e o f t h i s e x p e r i m e n t i s t o d e m o n s t r a t e t h e c h a r a c t e r
i s t i c s a n d o p e r a t i o n o f s e m i c o n d u c t o r d e t e c t o r s b y a n a l y z i n g a l p h a a n d
b e t a s p e c t r u m s u s i n g t h i s d e t e c t o r .
T h e o r y
O n l y i n r e c e n t y e a r s h a s t h e i m p o r t a n c e o f s e m i c o n d u c t o r d e t e c
t o r s t o e x p e r i m e n t a l n u c l e a r e n g i n e e r i n g b e e n f u l l y r e a l i z e d . S o f a r ,
t h e m o s t e x t e n s i v e u s e o f s e m i c o n d u c t o r d e t e c t o r s h a s b e e n f o r p a r t i c l e
s p e c t r o m e t r y . S e m i c o n d u c t o r d e t e c t o r s c a n b e u s e d t o d e t e c t t h e s a m e
t y p e s o f r a d i a t i o n a s t h e i o n i z a t i o n c h a m b e r s p r e v i o u s l y d i s c u s s e d .
H o w e v e r , t h e y p o s s e s s c e r t a i n a d v a n t a g e s o v e r a n i o n i z a t i o n c h a m b e r .
T h e p r i n c i p l e a d v a n t a g e i s t h e i m p r o v e d e f f i c i e n c y r e a l i z e d b y t h e r a p i d
c o n v e r s i o n f r o m i n c i d e n t p a r t i c l e e n e r g y t o a d e t e c t a b l e s i g n a l . A n
o t h e r a d v a n t a g e i s t h e s m a l l n e s s o f s i z e o f t h e d e t e c t o r .
A l t h o u g h s e m i c o n d u c t o r d e t e c t o r s p o s s e s s t h e a d v a n t a g e s l i s t e d ,
c e r t a i n i n h e r e n t d i s a d v a n t a g e s m u s t a l s o b e c o n s i d e r e d . A m o n g t h e s e
d i s a d v a n t a g e s a r e a n i n a b i l i t y t o s t o p p a r t i c l e s o f r e l a t i v e l y l o n g
r a n g e a n d s u s c e p t i b i l i t y t o n u c l e a r r a d i a t i o n d a m a g e .
W h e n a s e m i c o n d u c t o r d e t e c t o r i s u s e d t o d e t e c t n u c l e a r p a r t i
c l e s , e a c h o n e t h a t i s d e t e c t e d c o n t r i b u t e s t o t h e d e t e r i o r a t i o n o f t h e
74
75
detector. This radiation damage decreases charge collection efficiency.
Care must be taken to insure that radiation damage is held to an abso
lute minimum. Unnecessary exposure to a particle emitter should be
avoided.
The semiconductor detector used in this experiment is a silicon
wafer. When a charged particle, such as an alpha or beta, enters this
silicon wafer, free electron-hole pairs are created. The rate of charge
carrier formation is nearly independent of particle energy and ioniza
tion density. The charge carriers are moved to their respective elec
trodes by an applied electric field. Upon reaching the electrodes, the
new charge in the circuit causes a drop in voltage across a resistor in
the external circuit.
After a particle has been detected, the resulting signal is
sent to a preamplifier which again acts as an impedance matching device
(see Figure E4-1). The signal then goes to a biased amplifier which
passes only certain amplitude pulses. Finally a scaler-timer combina
tion is used to record the pulses.
The bias supply and noise meter provide the necessary bias vol
tage for semiconduction detection and present a constant indication of
the noise level of the system. The ultra stable mercury pulse generator
is used as a laboratory standard to cross-calibrate pulse height analy
zers, discriminators, and amplifiers.
Semiconductor detectors count any charged particle incident on
the sensitive surface as long as the incident angle is such that the
particle will penetrate the silicon wafer appreciably. Neutrons may be
76
detected by placing an appropriate'foil in front of the detector and
counting the reactions products. A discussion of the use of the de
tector for gamma ray detection will be left to the student. (See the
questions and problems section of this experiment.)
This experiment will illustrate the use of a semiconductor de
tector as a standard laboratory system. The plot of an alpha spectrum
will indicate significant properties of this detector.
A p p a r a t u s
1. Alpha source such as Polonium 210.
2. Beta source such as Carbon 14.
3. Scaler-timer, RIDL, Model 49-25.
4. I n s t r u m e n t c a s e a n d p o w e r s u p p l y , R I D L , M o d e l 29-1 (2).
5. Semiconductor detector vacuum chamber, RIDL, Model 2-10.
6. Linear amplifier, RIDL, Model 30-21.
7. N u v i s t o r p r e a m p l i f i e r , RIDL, M o d e l 31-18.
8. Bias supply and noise meter, RIDL, Model 40-14.
9. Ultra stable mercury pulse generator, R I D L , Model 47-7.
10. Semiconductor detector, ORTEC, Model NDH 025 CO.
11. Multichannel analyser such as Technical Measurement
Corporation*s Model 220.
12. Pump, Cenco, Model 90510.
13. Motor, General Electric, Model 5KH33GG106F.
14. Gage, vacuum, Cenco, Model 94030.
15. Vacuum hose, six feet.
77
16. Cables as shown In Figure E4-1.
P r o c e d u r e
1. Assemble the equipment as shown in Figure E4-1.
2. Set the two way pneumatic valve on the vacuum chamber to VAC.
Note: The two way pneumatic valve marked ATM, VAC allows venting the
chamber without turning off the vacuum pump.
3. Turn the vacuum pump on and allow it to run for one minute be
fore taking any readings. NoteV The vacuum pump should be left running
throughout the entire experiment.
4. Take a two-minute background count.
5. Set the two way valve on the vacuum chamber to ATM.
6. Place the alpha source in the chamber on the fourth sample
holder groove from the top.
7. Set the two way pneumatic valve to VAC.
8 . A d j u s t t h e d e t e c t o r b i a s t o a l e v e l w h i c h a l l o w s m a x i m u m b i a s
c o n s i s t e n t w i t h o p t i m u m s i g n a l t o n o i s e r a t i o . N o t e : T h i s i n e f f e c t
d e t e r m i n e s t h e o p e r a t i n g p l a t e a u f o r t h i s d e t e c t o r .
9. Take a two-minute count with the chamber completely evacuated.
10. Increase the pressure in the chamber from 0 to 14.7 psi, taking
a one-minute count at each increase of 1 psi*
11. Remove the alpha source and repeat steps 4 through 10 using a
beta source.
12. Repeat steps 4 through 10 using an alpha source placed on the
bottom of the detector tray assembly.
78Input Pulser
Preamplifiero E
Linear Amplifier Mercury Pulse Generator
Bias Supply
Vacuum Chamber
Multichannel Analyzer
Scaler-TimerFigure E4-1. Semiconductor Detector
79
13. Repeat steps 4 through 10 using a beta source placed on the •
bottom of the detector tray assembly.
14. Record all counts as a function of pressure.
15. Monitor steps 10 through 13 with a multichannel analyzer by re
cording counts per minute as a function of channel number.
Results and Presentation of Data
1. Plot counts per minute as a function of pressure for each of
steps 10 through 13 of the procedure.
2. Plot counts per minute as a function of channel number for
steps 10 and 11 of the procedure.
3. Referring to the alpha spectrum, discuss the reason why the en
ergy of the alpha particle rises to a peak and then drops sharply.
4. In looking at the graph of alpha particles detected versus
pressure, it is seen that the number of counts per minute as a function
of vacuum chamber pressure decreases rapidly. Why does this happen?
5. In step 4 above, why is the decrease less pronounced for beta
particles?
Questions and Problems
1. Why is the semiconductor detector inefficient for use as a
gamma ray detector?
2. What is the approximate energy loss of a particle in the forma
tion of an ion pair in the semiconductor detector? How does this com
pare with the approximate energy loss in the formation of an ion pair
in the proportional counter?
3. Discuss the semiconductor detector's energy resolution (FWHM)
compared to a gas detector and a scintillation detector.
4. What are the purposes, characteristics, and operation of the
following items of equipment used in this experiment?
a. Ultra stable mercury pulse generator
b. Bias supply and noise meter
c. Linear amplifier
d. Vacuum chamber
S e l e c t e d R e f e r e n c e s
Deamaley, G., and D. C. Northrop, Semiconductor Counters for Nuclear Radiations, John Wiley, Inc., New York, 1963.
Goulding, Fred S., "Semiconductor Detectors— Their Properties and Applications” , Nucleonics, Vol. 22, No. 5, (May, 1964).
Oak Ridge Technical Enterprises Corporation, Instruction Manual for Surface Barrier Detectors, Oak Ridge, 1961.
Price, William J., Nuclear Radiation Detection, 2nd ed., McGraw-Hill Book Company, New York, 1964, Chapter 8 .
EXPERIMENT 5
C O U N T I N G S T A T I S T I C S
Purpose
The purpose of this experiment is to study the counting statis
tics discussed in Chapter 4 by directly applying the Poisson, the
Poisson time interval, and the normal distributions to experimental re
sults .
Theory
The theory supporting this experiment was discussed in Chapter
4, Counting Statistics. Additional information needed in this experi
ment will be brought out in the results section of the experiment.
Apparatus
1. GM chamber and associated equipment assembled as shown in
Figure El-2.
2. Beta source such as Bismuth 210.
3. Aluminum absorber set. Atomic Accessories, Inc.
Procedure
1. Using the beta source: and the procedure from Experiment 1 find
the plateau region of the GM counter.
2. With the aluminum absorbers, adjust the count rate to approxi
mately one count/second.
81
82
3. Take several background readings throughout the laboratory
period.
4. Record the times at which the counter reads counts for ap
proximately 200 trials.
5. Take a long count (10 minutes) to be used in determining the
average activity.
6. Take approximately 150 ten-second readings. Note: the best
method for taking these data is to number from zero to about twenty in a
column and record each trial with a mark by the correct number
<///// /)•7. Take another long count (10 minutes) to be averaged with the
count found in Step 5.
8 . With the absorbers, readjust the count rate to approximately
10,000 counts/minute.
9. Take a long count (5 minutes).
10. Take approximately 50 one-minute counts.
11. Take a long count (5 minutes).
Results and Presentation of D a t a
1. From the data taken in Step 6 of the procedure, make a table as
shown in Figure E5-1:
x l(x) Px L(x)
Figure E5-1. Table of Ten Second Counts
where x is the number of counts, l(x) is the number of trials in which
x counts were recorded, Px is given by Equation 4-4 (a is found from
Steps 5 and 7), and L(x) ■ nPx where n is the total number of trials.
2. Plot a histogram of l(x) and L(x) versus x.
3. Plot Pu , experimental and theoretical, versus x.
--Poisson Time Interval
4. From Steps 5 and 7 of the procedure, calculate the average ac
tivity.
5. Calculate at for each reading taken in Step 4.
6 . Find the fraction of values of at which are less than 2, 4,
6, 8, 10, and 12. (This gives the experimental values of P% in Equation
4-6.)
7. Calculate the theoretical values of Pg, P4 , Pg, Pg, F^q , and
using Equation 4-6 and compare these with the experimental values
found in Problem 3.
8 . Calculate the average t for 8 counts both experimentally,__ nt = x/( £ tj/n), and theoretically, t * s/a, and compare.1—1
9. Calculate the standard deviation in this average time.
--Poisson
10. Discuss the differences between l(x) and L(x) given above.
11. From Steps 9 and 11 of the procedure, determine :m.
12. Calculate the experimental standard deviation (Equation 4-9)
and the theoretical standard deviation (o'theo. = V™) • Compare these
values.
--Normal
13. Find the experimental values of Pu by counting the fraction of
83
84
trials (Step 10) which fall outside (a) m + 0.5creXp, (b) m ± 1 •0o'exp,
(c) m ± l.Sffgjjp, and (d) m ± 2 .0(rexp.
14. Calculate Pu (Equation 4-8 and Table 4-1) for the values shown
in a, b, c, and d above.
15. Discuss the differences in the curves of the experimental and
theoretical values of P^.
16. What differences in the experimental curve of Pu would there
have been if you had counted the fraction of trials below m - 0 .5
and above m + 0 ♦5crexp, etc., separately?
Questions and Problems
1. From Equations 4r7 arid 4-8 show that for “ “ 1, Pu * 0.317.2. From Equation 4-1 and the conditions of the Poisson distribu
tion, derive Equation 4-4.
Selected References
Evans, Robley D., The Atomic Nucleus. McGraw-Hill Book Company, Inc., New York, 1955, Chapters 26 and 27.
Friedlander, Gerhart, and J. W. Kennedy, Nuclear and Radiochemistry, John Wiley and Sons, Inc., New York, 1955, Chapter 9.
Volk, William, Applied Statistics for Engineers, McGraw-Hill Book Company, Inc., New York, 1958.
Wilks, S. S ., Elementary Statistical Analysis. Princeton University Press, Princeton, New Jersey, 1948.
Youden, W. J., Experimentation and Measurement, National Science Association, Washington, D. C., 1962.
EXPERIMENT 6
S C I N T I L L A T I O N S P E C T R O M E T R Y
P u r p o s e
T h e p u r p o s e o f t h i s e x p e r i m e n t i s t o s t u d y s c i n t i l l a t i o n s p e c
t r o m e t r y b y a n a l y z i n g k n o w n a n d u n k n o w n g a m m a r a y s p e c t r a .
T h e o r y
S c i n t i l l a t i o n d e t e c t i o n i s b a s e d o n t h e f a c t t h a t a c e r t a i n
c l a s s o f s u b s t a n c e s , u s u a l l y r e f e r r e d t o a s p h o s p h o r s , e m i t l i g h t a s a
r e s u l t o f i n t e r a c t i o n s w i t h r a d i a t i o n . T h i s l i g h t e m i s s i o n i s c a u s e d b y
t h e f l u o r e s c e n t n a t u r e o f t h e m a t e r i a l . T h e t e r m p h o s p h o r , a l t h o u g h
g e n e r a l l y u s e d , i s a m i s n o m e r b e c a u s e m o s t s c i n t i l l a t i o n c r y s t a l s u s e d
t o d a y a r e f l u o r e s c e n t , a n d n o t p h o s p h o r e s c e n t , m a t e r i a l s . W h e n a
c h a r g e d p a r t i c l e e n t e r s t h e f l u o r e s c e n t c r y s t a l ( t h e p h o s p h o r ) , i t g i v e s
u p i t s e n e r g y i n e x c i t i n g a n d i o n i z i n g t h e a t o m s o r m o l e c u l e s . T h e e x
c i t e d a t o m s t h e n r e t u r n t o t h e i r u n e x c i t e d s t a t e s b y g i v i n g u p t h e i r e x
c i t a t i o n e n e r g y a s p h o t o n s . T h e i n t e n s i t y o f t h e l i g h t e m i t t e d f r o m t h e
p h o s p h o r i s u s u a l l y d i r e c t l y p r o p o r t i o n a l t o t h e a m o u n t o f e n e r g y g i v e n
u p b y t h e i n c i d e n t p a r t i c l e . W h e n u n c h a r g e d g a m m a r a y s e n t e r t h e p h o s
p h o r , t h e l i g h t p h o t o n s a r e p r o d u c e d b y t h e s e c o n d a r y p a r t i c l e s r e ? *
s u i t i n g f r o m t h e i n t e r a c t i o n s o f g a m m a r a d i a t i o n w i t h m a t t e r . I n t h i s
c a s e , a l s o , t h e i n t e n s i t y o f t h e e m i t t e d l i g h t i s p r o p o r t i o n a l t o t h e
e n e r g y a b s o r b e d . S c i n t i l l a t i o n s p e c t r o m e t r y i s b a s e d o n t h i s
85
86
proportionality. Using scintillation spectrometry it is possible to
identify the energies of the incident radiation. In this experiment we
will be concerned with the operation of a scintillation detector and the
use of this detector in analyzing a gamma ray spectrum.
As was discussed in Chapter 1, gamma photons may interact with
matter by three processes: the photoelectric effect, Compton scat
tering, and pair production. Each photon, interacting by one of these
effects will eventually release an orbital electron of the crystal's
atoms. The electrons then undergo energy changes resulting in emission
of light.
By means of a light pipe and reflector, a large fraction of the
light photons are transmitted to the photocathode of photomultiplier
tube. To minimize losses it is necessary for the crystal to have a high
optical transparency. When the light photons strike the photocathode,
they produce photoelectrons by the photoelectric effect. These photo
electrons are then accelerated in an electrostatic field between the ca
thode and the first dynode (see Figure E6-1). This dynode is at a posi
tive potential relative to the cathode. Each accelerated photoelectron
possesses enough energy to knock out several electrons from the dynode.
There may be as many as ten secondary electrons for each initial elec
tron striking the dynode. This amplification is repeated at each dy
node, and the electron current is amplified as the electrons are acce
lerated from dynode to dynode. The output current at the anode may be
more than a million times as great as the current originally emitted
from the photocathode. The pulses, however, remain proportional to
87
Light LightReflector
+200 V +600 V
+100 V +300 V +500 V +700 V
FirstPhosphor Photocathode AnodeDynode
Figure E6-1. Scintillation Detector
photon intensity.
The pulses from the scintillation detector are sent to a linear
amplifier in the analyzer (see Figure E6-3). From the amplifier, the
pulses are sent to two discriminator circuits. One discriminator is set
to pass all pulses over a voltage determined by the base level control.
The other discriminator passes all pulses over a certain voltage deter
mined by the base level and window width controls. By comparing pulses,
an anticoincidence circuit allows only a narrow band of energy to pass
to the ratemeter. The width of this narrow band of energy is adjustable
from 0 to 10 volts by the window width control.
This narrow band of energy is processed through integrating
circuitry to a counter where it is displayed as counts. Simultaneously
a scan control panel is used to display the information on a recorder.
88
In the photoelectric effect, the entire energy of the gamma photon is transferred to an orbital electron where:
E = photoelectric energy hy = energy of the incident photon w * binding energy of the electron h * Planck’s constant v = frequency in cycles per second
hvx = energy of X-ray
then,
E - hv - w + hvx (E6-1)
hvx * w (E6-2)
E ■ hv (E6-3)
In analyzing a gamma ray spectrum, the most prominent peaks
below 1.5 Mev are usually due to the photoelectric effect. Note the
photoelectric peaks in Figure E6-2. The center of the photoelectric
peak is the photoelectric energy, and this energy is proportional to
the original gamma energy.
In a Compton event, the gamma photon is scattered or deflected
by an orbital electron. Compton electrons have a range of energies de
pendent on the scattering angle. These energies will be indicated as
a continuum increasing to a maximum below the photopeak where:
Ece " Compton edge, or maximum energy of the Compton electron.
then.
89
Ece hv1 + .511
2hv(B6-4)
The Compton continuum and peaks are shown in Figure E6-2.
With a crystal of normal size (1" to 3"), some gamma rays will
pass through the crystal, and a proportion of these will be reflected
into the crystal at reduced energies. These photons will be added to
others that are scattered near 180° within the crystal to produce a
hackscatter peak. This peak is shown in Figure E6-2. Therefore, where:
E^g = hackscatter peak energy
then.
Ebs h v _ 1 + 2hy
.511(E6-5)
Pair production occurs when a gamma photon of 1.02 Mev or
higher energy is converted into an electron and a positron. The elec
tron and positron are annihilated producing two photons with energies of
0.511 Mev. There is a high probability that the two scintillations will
be seen by the phototube as one flash of double intensity. When both
annihilation photons escape the crystal, a pair production peak will
occur at 1.02 Mev below the photopeak. Therefore, where:
then.
Ea *» resultant energy where both annihilation photons escape the crystal
Backscatter Peak
Photoelectric Peak Photoelectric Peak
Compton Edge
Compton Continuum
Figure E6-2. Cobalt 60 Gamma Ray Spectrum VOo
91
Ea ■ hv - 2m0c2 (E6-6)
If one of the annihilation photons strikes an electron within the re
solving time of the crystal, the equivalent photon energy will be added
to show a peak at 0.511 Mev below the photopeak. Therefore, where:
Ey - resultant energy where one annihilation photon escapes the crystal
then,
Eb = hv - myC2 (E6-7)
If both annihilation photons are absorbed in the crystal, the total en
ergy will show at the photopeak. These peaks are shown in Figure E6-2.
A spectrogram is a graphic record of the total of all effects
in the scintillation crystal indicated as counts per time for each en
ergy. Each photoelectric peak will have a corresponding Compton effect
pattern, and if the energy level is high enough, pair production peaks.
In this experiment unknown isotopes will be determined from
analysis of their gamma ray spectra. Gamma ray spectrometry has wide
application in experimental nuclear engineering and will be further il
lustrated in Experiment 8 .
Apparatus
1. Gamma source such Cobalt 60.
2. TWo unknown gamma sources.
3. Recording spectrometer. Nuclear-Chicago, Model 1820A.
4. Recorder, Bristol, Model 1FH560-51.
5. Scintillation detector. Nuclear-Chicago, Model 1421.
Procedure
1. Assemble the equipment as shown in Figure E6-3. Note: the in
structor will have allowed a one-hour warm-up for the radiation ana
lyzer.
2. Place the Cobalt 60 source in the scintillation detector.
3. Follow the instructions outlined in the instruction manual for
calibration and initial adjustments of the recording spectrometer.
4. Run a gamma spectrum with the Cobalt 60 source.
5. Run gamma spectrums for the two unknown sources.
Results and Presentation of Data
1. Plot the gamma spectrum from the Cobalt 60 source on semi-loga
rithmic graph paper. Identify the photoelectric peaks, pair production
peak, Compton edge, Compton continuum, backscatterrpeak, and annihila
tion radiation peaks.
2. Plot a calibration curve (energy versus base line) from the
above data on linear paper. Note: measure the base; line in half-inches
from the start of the tape.
3. Plot the two unknown gamma spectrums on semi-logarithmic graph
paper. Identify all peaks as in 1, above.
4. From the calibration curve, determine the gamma energies of the
unknown sources and from this identify these sources. Justify your de
cision.
92
94
5. With the Cobalt 60 source, at what energy would you expect the
Compton edge and backscatter peaks to occur?
Questions and Problems
1. Discuss the manner in which the window width setting on the ra
diation analyzer of the recording spectrometer would affect your result.
2. Given that a 1.2 Mev gamma photon enters a scintillationo
crystal, and the crystal gives off light at 4500 A, determine the number
of light photons per gamma photon if the system is 75% efficient.
3. At what energies are the following effects dominant: the pho
toelectric effect, the Compton effect, and pair production?
Selected References
Birks, J. B., Scintillation Counters, McGraw-Hill Book Company, Inc., New York, 1953.
Chase, Robert L . , Nuclear Pulse Spectrometry, McGraw-Hill Book Company, Inc., New York, 1961.
Nuclear-Chicago Corporation, Model 1820B Recording Spectrometer Instruction Manual, Des Plaines, Illinois, 1961.
Overman, Ralph T., and H. M. Clark, Radioisotope:Techniques, McGraw- Hill Book Company, Inc., New York, 1960, Chapter 2.
Price, William J., Nuclear Radiation Detection. 2nd ed., McGraw-Hill Book Company, Inc., New York, 1964, Chapter 7.
EXPERIMENT 7
FLUX MAPPING BY FOIL ACTIVATION
Purpose
The purpose of this experiment is to illustrate the method of
foil activation in determining a neutron flux distribution.
Theory
A standard method of measuring neutron flux is that of foil ac
tivation. This method Is based on the fact that certain stable iso
topes, upon capturing a neutron, are transformed into radioactive iso
topes. The resulting activity can be counted by standard methods and
related to the neutron flux.
If ora represents the microscopic absorption cross section, V
the volume, and N the number of nuclei per cubic centimeter of the
foil, then the activity (A) produced by the neutron capture in the
foil is given by
A - 0N<raV(l - e‘Xt) (E7-1)
where 0 is the neutron flux at the foil, \ is the decay constant of
the radioisotope resulting from the neutron capture, and t is the ex
posure time of the foil. If • p is the density and A the atomic
weight of the foil, N0 is Avogadro*s Number, then N is given by
95
96
(E7-2)
The product of the number of nuclei per cubic centimeter and the micro
scopic cross section is known as the macroscopic cross section (2).
S a e (E7-3)
Therefore Equation E7-1 becomes
A - 0 2 aV(l - e"Xt) (E7-4)
If the exposure time is long compared to the mean life of the
radioactive isotope, then
(E7-5)
where A g is the saturation activity. The saturation activity is the
maximum activity that a foil can attain from a given neutron flux.
Therefore, from equations E7-4 and E7-5,
As m —i
A ____■ e-Xt (E7-6)
It is evident that if A, X , and V are known, the flux can be
found from Equation E7-5.
By the use of cadmium sandwiches placed around the foils, it is
also possible to determine the thermal neutron flux. Recalling from
Experiment 3 that cadmium absorbs almost all thermal neutrons, it is
seen that only neutrons above about 0.4 ev will reach the foil in the sandwich. Clearly, if A g_ ^ represents the activity induced by thermal
neutrons ( 0 . 4 ev) and.As„fa8t: represents the fast neutron flux
( 0 . 4 ev), then
97
Dividing Equation E7-7
A s-th = Ag ™ ; A s-fast
by A s-fast 8ives
— - i + As -.th A s-fast A s-fast
(E7-7)
(E7-8)
A s/Ag_£a8t. is defined as the cadmium ratio (R^^) and is a measurement of
the degree of thermalization of neutrons.
In finding the thermal flux, the quantity A g / A g , ^ is of in
terest. This quantity is known as the cadmium ratio correction and is
defined by
kcr = 1 ' l/*cd
Therefore the thermal flux is given by
_ As-th kcrAs 0th ” Z a V 2 a V
(E7-9)
(E7-10)
In order to determine 0 ^ , the quantities which must be measured are A s
and Ag_j=ag f If Gy and Cc are the count rates observed for the bare and
cadmium covered foils respectively, we have
0% = KyAg(l - e-Xt), and (E7-11)
Cc = ^cA s-fast(^ ” e ) (E7-12)
98
where t is the irradiation time and Ky and Kp are constants of propor
tionality which account for the decay of the foils before and during
counting, as well as for various efficiency factors. For a foil, K is
given by
K « KjK^KQKpK^KgKgKf (E7-13)
where:
-XTKj = e = correction for decay between end of irradiation and beginning of count (T ■ elapsed time),
1 -K = — --- ■ = correction for decay during counting (t = dura-d tion of count),
Kq * correction to zero bias for the counting system,
Ke = correction for counter efficiency, including geometry factor (use 60% in this experiment),
K ° 1 - CTjj * correction for dead time of counter (C * count rate, td * dead time),
Kg = self-shielding factor.
Kg ■ backscatter correction factor,
Kf = flux depression factor for the foil.
Each of the above factors should be carefully considered; however, in
some cases, certain of these corrections are not required. Since the
foils are very thin, the self-shielding and backscatter corrections are
99
negligible (i.e., Kg - Kg ” 1). The factor Kq accounts for the loss of
pulses resulting from the bias discrimination against noise. The method
of finding Kq will be indicated in the procedure section of this experi
ment. Kg accounts for the flux depression of the neutron distribution
in the foil due to absorption of neutrons by the foil. For this experi
ment Kg is assumed to be 0.85.
From Equations E7-11 and E7-12 we have
A = <B 7 -U >
and
A =-fast ■ -jr ! _Cg-\t (E7-15)
Therefore from Equations E7-14 and E7-15 and from the definition of the
cadmium ratio given earlier, we have
KcCbRcd " <E7-16>
In order to obtain accurate results, it will also be necessary
to normalize the weights of the foils.
In this experiment a method of foil activation to determine
neutron flux will be used. The correction factors necessary to obtain
accurate fluxes will also be illustrated. Foil activation is a commonly
used technique in many areas of experimental nuclear research, and this
experiment will provide a basis for further individual work using this
method.
Apparatus100
1. GM counter and associated equipment as shown in Figure El-2.2. Neutron howitzer.
3. Polyethylene inserts marked 1 through 6 .
4. Polyethylene plugs (2), 4 inches and 14 inches.
5. Plutonium-Beryllium sources (2) .
6 . Indium foils marked 1 through 12.
7. Cadmium sandwiches (6).
8. Analytical balance, Mettler, Number 61569.
Procedure1. Assemble equipment as shown in Figure El-2.
2. Weigh the indium foils.
3. Tape the indium foils on the polyethylene inserts at the red
lines. Tape foil one to insert one, etc.
4. Place the inserts into the holes corresponding to the numbers
on the inserts.
5. Place the 4 inch polyethylene plug in the howitzer, place the
Pu-Be sources on top of this plug, and place the 14 inch plug on top of
the sources. Record the time of insertion of the sources.6 . Irradiate the foils for 5 hours.
7. Remove the foils simultaneously and record the time of removal.
8 . Allow the foils to decay for ten minutes prior to counting.
9. Using two of the foils, determine the dead-time of the GM
counter.
10. Using one of the foils, record counts versus bias voltage
101
setting (Pulse Height Selector) from 10 volts to 80 volts In Increments
of ten.
11. Count each of the foils for two minutes. Record the time at
the start of each count and the number of the foil being counted.
12. Place the remaining indium foils into the cadmium sandwiches.13. Repeat steps 3, 4, 5, 6, 7, 8, and 11.
Results and Presentation of Data
1. Plot counts versus bias voltage from data taken in Step 10 of
the procedure.
2. Plot thermal neutron flux versus distance from the source.
3. Plot RC(} and kcr versus distance from the source.
4. Plot the total neutron flux versus distance from the source.
5. Normalize the weights of the foils and calculate A s from
Equation E7-11 for each of the six positions. Calculate 0 for each
position using Equation E7-5. (Note: V * m/p).
6 . Calculate K c .
7. Calculate Rc<j» kcr, and 0 for each position.
8 . From the bias voltage curve, calculate Kq by taking the ratio
of the counts at the bias voltage used to the counts at zero bias vol
tage (extrapolated).
9. Calculate Ky from Equation E7-13.
Questions and Problems
1. Indicate the nuclear reaction involved in this experiment and
the appropriate decay scheme.
102
2. What properties of indium make it appropriate for use in foil
measurements? List several other foil materials and discuss the proper
ties which make them useful.
3. Should the cadmium ratio be higher or lower at the source rel
ative to its value at the edge? Why?
4. Derive the equation Kg = (1 - e~^'t) A t .
Selected References
Hoag, J. Barton, Nuclear Reactor Experiments. D. Van Nostrand Company, Inc., Princeton, New Jersey, 1958, pp. 10-14.
Hughes, Donald J., and R. B. Schwartz, "Neutron Cross Sections", U S A E C Report, BNL-325 (2nd ed.), 1958.
Price, William J ., Nuclear Radiation Detection, 2nd ed., McGraw-Hill Book Company, New York, 1964, pp. 336-342.
Valente, Prank A. (ed.), A Manual of Experiments in Reactor Physics, The Macmillan.Company, New York, 1963, pp. 58-70.
EXPERIMENT 8
ANALYSIS OF MIXTURES OF RADIOISOTOPES BY STRIPPING
Purpose
The purpose of this experiment is to demonstrate stripping, a
method of separating the spectrum of a single radioactive isotope from
the complex spectrum of a mixture of radioisotopes.
Theory
The methods of scintillation detection and analysis of gamma
ray spectra have been discussed in previous experiments. The knowledge
gained from these experiments will now be used to analyze mixtures of
radioisotopes. This type of analysis has valuable application in the
field of nuclear engineering as well as many other scientific disci
plines .
The type of analysis used in this experiment is stripping.
Stripping is accomplished by the subtraction of gamma ray spectra to ob
tain a desired result. This result may be the determination of a single
isotope from a mixture of isotopes, the determination of a pure isotope
without background, or the determination of impurities in a radioiso
tope.
A multichannel analyzer examines and sorts pulses into one of
its channels. This sorting or analysis is done on an amplitude basis.
Data display and various readout systems provide observation and
103
104
recording of the amplitude as data accumulates or after It has accumu
lated.
Input pulses are converted Into a digital number and stored in
a digital computer memory. Numbers from this memory unit can be dis
played or readout by the multichannel analyzer.
In this experiment we will strip the spectra of background and
Chlorine 38 (NH^Cl) from a N a ^ C l ^ spectrum. This will be accomplished
automatically by using a multichannel analyzer. Initially a spectrum of
Sodium 24 (NagCO^) should be analyzed to ascertain critical peaks of so
dium. Therefore, when the stripping procedure has been accomplished, it
will be possible to compare the results with the previously analyzed
Sodium 24 spectrum.
Apparatus
1. Radioactive samples of NaCl, NH^Cl, and Na^COg.
2. Cobalt 60 source for calibration.
3. Multichannel analyzer with digital recorder.
4. Scintillation detector, Baird Atomic, Model 810.
5. Non-overloading amplifier, Baird Atomic, Model 215.
6 . High voltage power supply, Baird Atomic, Model 319A.
7. Required cables as shown in Figure E8-1.
Procedure
1. Assemble the equipment as shown in Figure E8-1. (The in
structor will demonstrate the use of the multichannel analyzer.)
105
Amp In
a# * %# ' #$ % % %>__
• w
%# e e e
* e j *
• see
M M
Figure E8-1. Scintillation Detector with Multichannel Analyzer
106
2. Run a 10-minute Cobalt 60 spectrum and print tape.
3. Run a 10-minute background spectrum and print tape.244. Run a 10-minute Nag COg spectrum.
5. Electronically subtract background for 10 minutes and print
tape.
6 . Plot this spectrum.
7. Run a 10-minute N a ^ C l ^ spectrum and print tape.388 . Electronically subtract a NH^Cl spectrum until the spectrum
on the multichannel analyzer agrees with that plotted in 6 above and
print tape.
Results and Presentation of Data
1. Plot the Cobalt 60 spectrum and identify all significant peaks.
2. From 1, plot a calibration curve.
3. Plot the NagCOg spectrum. Identify significant peaks.
4. Plot the NaCl spectrum. Identify significant peaks.
5. Plot the resulting spectrum when the spectrum of NH^Cl is sub
tracted electronically from NaCl.
6 . Compare the data presented in 3 and 5 above. Discuss possible
differences between the spectra.
7. At what energies would you expect the photoelectric peaks of
N a ^ and Cl^® to appear?
8 . From the data determined in 7, determine at what energies you
expect the backscatter peaks, Compton edge, and annihilation peaks to
occur for N a ^ and C l ^ \
107
Questions and Problems
1. Discuss the method of operation of the multichannel analyzer
used in this experiment. Include as a minimum the operation of signifi
cant controls for spectrum analysis.
2. What isotopes of sodium and chlorine are irradiated to giveO Z O Q O Z O Q
Na and Cl ? What percentage of the activity of Na and Cl are by
gamma emission?
3* What type of crystal is used in this experiment? Why?
4. Draw the decay schemes of N a ^ and Cl^**.
Selected References
Overman, Ralph T. and H. H. Clark, Radioisotope Techniques. McGraw-Hill Book Company, Inc*, New York, 1960, Chapter 10.
EXPERIMENT 9ACTIVATION OF COPPER AND HALF-LIFE DETERMINATIONS
Purpose
The purpose of this experiment is to study the buildup and
decay of radioisotopes as well as elementary methods of determining
half-lives.
Theory
The relationship of the half-life of a radioisotope to nuclear
processes has been discussed in Chapter 2. The activity (disintegra
tions/time) of a foil at a particular exposure time, t is represented
by Equation E7-4.
A - 0 2 aV(l - e-Xt) (E9-1)
Since the count rate (N) is directly proportional to the activity (A),
the ratio of two count rates for two different exposure times can be
written as
Ni 1 - e *"1N2 1 - e-kt2
(E9-2)
From Equation E9-2 and a representative buildup curve it is possible to
transcendentally solve for X and from this determine the half-life of
the radioisotope.
108
109The half-life of a radioisotope can also be determined directly
from a decay curve. If a decay curve (counts versus time) represents
only a single radioisotope, the half-life can be determined in the fol
lowing manner:
1. Pick any count rate (N) on the decay curve,
2. Divide this by 2 (N/2),
3. The time from N to N/2 on the decay curverepresents the half-life.
However, a decay curve may represent the combined activity of
two or more radioisotopes. In this case it is necessary to separate the
complex curve into individual decay curves representative of each radio
isotope present. If, in a complex curve, the half-lives are signifi
cantly different, the activities are large enough, and the counting time
is long enough, the process of separating the curves is relatively .
simple. The activities of the radioisotopes with the shorter half-lives
will decay from the complex decay curve, eventually leaving only the
constant decay rate of the isotope with the longest half-life. For two
radioisotopes, the typical decay curve is given in Figure E9-1. The decay curve for the longest-lived isotope is then found by extrapo
lating from the constant decay portion of the total curve (see Figure
E9-1)♦ The decay curve for the shorter-lived radioisotope is found by
subtracting the long-lived decay curve from the complex curve (see
Figure E9-1). The separate half-lives are then easily found by the
method described above for a single isotope.
110
Figure E9-1. Semi-log Plot
Another standard method of experimentally determining half-
lives is Peierls1 method. In this method the number of counts (X) are
plotted as a function of time. Figure E9-2 shows a typical plot.
Figure E9-2. Peierls’ Method
Ill
It should be noted that Figure E9-2 does not show a buildup curve, but
rather a plot of the total counts (summed) versus time for a decaying
source. The abscissa is divided into n equal time intervals ( ATj_).
The total elapsed time is then n A T . If A represents the number of
atoms decaying in the i-th interval, then A N ^ atoms have an average
lifetime (S^) of
and for A Ng atoms.
- 1/2 A T
S2 = 1/2A T + A T « 3 / 2 A T
and so forth for S3 , 84,..., Sn . Furthermore,
A N i = A X £ - b A T
(E9-3)
(E9-4)
(E9-5)
where b is the count rate of background alone.
The total of the lifetimes of all the atoms which decay between
0 and n A T is given by
A N 2 U / 2 A T ) + A N 2(3/2AT) + A N 3(5/2AT) + ••• (E9-6)
During this time the total number of atoms whose individual lifetimes
are observed is given by
N - A N l + A N 2 + A H 3 + (E9-7)
Therefore, the average life (S) of all the observed atoms is given by
1X2
or
A Ni (1/2 A T ) + A N2(3/2 AT) + a N3(5/2a T) + ••• ANj + ANg + ANg + ••• (E9-8)
AN i + 3AN2 + 5AN3 + ••• - AT ANi + AN2 + AN3 + ••• 2 (E9-9)
The average life is related to mean life (t ) by the following equation:
S =L
nATtdN
1
n A T ,(N0/T)te /Tdt
/„n A TdN
L
n A T(N0/T)e"t/Tdt
(E9-10).
Completing the integration gives
S = T 1 -nATT
n A T / r , e - Iif T « t (E9-11)
In determining the half-life, S is found from Equation E9-9 and is
substituted into Equation E9-11. The mean life can be related to the
half-life (see Chapter 2).
The two-point method is still another means of calculating the
half-life. It uses the same type of plot shown in Figure E9-2. How
ever, in this case, the curve is divided into two equal intervals (see
Figure E9-3). The average activity at t^ and t2 (see Figure E9-3) is
given by A N ^ / A T and A N 2/ A T , respectively, where A N ^ and A N 2 are
given by Equation E9-5. In Chapter 2 we saw that the instantaneous ac
tivities at t^ and t2 can be related by the following equation:
113a2 ■ " tl) (E9-12)
If the relation between the Instantaneous activity and the average acti
vity Is not dependent on time, then
a2av tl) (E9-13)
Therefore,
and, therefore.
± 2 2 = ^ 1 e-X(t2 - tl)A T A T
T t2 “ tl
In ANja n 2
(E9-14)
(E9-15)
Figure E9-3. Two-Point Method
This experiment will amplify the discussion in Chapter 2 by ac-
tually determining the half-life of Cu by four different methods. In
addition to this, the methods previously explained in Experiment 7 will
114
be used to plot a buildup curve of C u ^ and decay curves of C u ^ and
Cu66.
Apparatus
1. GM chamber and associated equipment as shown In Figure El-2,
Experiment 1.
2. Copper foils marked 1 through 6.
3. Polyethylene insert number 1.
4. Neutron howitzer.
5. Plutonium-beryllium sources (3).
Procedure
1. Assemble the equipment as shown in Figure El-2, Experiment 1.
2. Remove the foil that the instructor has irradiated for 24 hours
and count the foil by recording the counts for a 25 second time period
out of every 30 seconds. Note: this allows five seconds to record and
reset for the next count.
3. Tape copper foil number 1 to the polyethylene insert.
4. Irradiate foil number 1 for one minute.
5. Successively tape foils numbered 2 through 5 to the polyeth
ylene insert and irradiate for 2, 5, and 10, and 20 minutes respec
tively.
6 . Count each foil for two minutes, being certain to keep the time
from the end of irradiation to the start of counting constant for all
foils.
7. After all foils have been counted for two minutes, count the
115
foil that was Irradiated for 20 minutes, recording successive times for
200 counts. Continue this until the decay rate approaches a constant
value.
Results and Presentation of Data661. Plot buildup curves (experimental and theoretical) for Cu .
2. Plot the decay curve for the foil irradiated 20 minutes.
3. Plot the decay curve for the foil irradiated 24 hours.
4. Plot the decay curve as shown in Figures E9-2 and E9-3 for the
foil irradiated 20 minutes.
5. Determine the half-life of C u ^ by the four methods outlined in
the theory section of this experiment. How does each value compare with
the actual half-life of Cu^^?
6. From the decay curve of the foil irradiated for 24 hours, find
the half-life of Cu^^ and Cu^^. Compare these with actual values.647. Determine the relative saturation counting rates due to Cu
and C u ^ activity,
8. Calculate the relative activity of C u ^ and C u ^ from the acti
vation cross sections and irradiation time. Does this confirm the re
sults of Problem 7? Explain any differences.
Questions and Problems
1. From Equation E9-10, derive Equation E9-11.
2. Draw the decay schemes of Cu®^ and Cu^® and describe the
nuclear reactions involved in this experiment.
3. In the two-point method, show that the relation between the
instantaneous activity and the average activity is not dependent on
time. Hint: in Figure E9-3, compare the slope of the tangent of the
curve at t to the slope of chord connecting two points equidistant
either side of t.
Selected References
Evans, R. D., The Atomic Nucleus, McGraw-Hill Book Company, Inc.,New York, 1955, pp. 812-818.
EXPERIMENT 10REMOVAL CROSS SECTIONS
The purpose of this experiment is to study the shielding of
fast neutrons by water and several water-metal combinations.
Theory
The shielding of neutrons is actually a study of the manner in
which fast neutrons behave in matter. Normally, the shielding of neu
trons occurs by the slowing down of fast neutrons due to elastic and
inelastic collisions and the subsequent capture of the slowed neutrons
as a result of the higher absorption cross sections at the lower ener
gies. Because it is necessary to slow the fast neutrons, hydrogenous
materials constitute an important part of neutron shields. This experi
ment will show that hydrogenous materials in combination with certain
metals form effective neutron shields.
One measure of the effectiveness of neutron shields is the ef
fective removal cross section. The effective removal cross section of a
material is a cross section which describes the removal of neutrons by
any process from a beam of neutrons incident on the material. The equa
tion that describes this removal is
I - I0e“Ertd (E10-1)
Purpose
117
118
where
I * Intensity at td
I0 == intensity incident on the shield
Er = effective removal cross section of the shield
tj = thickness of the material.
Due to the fact that the BFg counter used in this experiment
efficiently detects only slow neutrons, a water moderator will be used
in combination with layers of iron and aluminum. To measure the removal
cross section of the shield, it will be necessary to eliminate the ef
fect of the water moderator. Therefore, the removal cross section of
the shield is determined by subtracting the effect of the water from the
overall effect of the water-metal combination. A more useful form of
Equation E10-1 is
The relaxation length (Xr) is the thickness of material which
causes a drop in intensity by a factor of e.
E lp^vat ~ lnIwat-met (E10-2)r td
(E10-3)
This experiment is designed to study the attenuation and ab
sorption of neutrons in water and certain water-metal combinations by
the concept of removal cross sections. Removal cross sections and re
laxation lengths for certain metals will be determined.
119
Apparatus
1. BF3 counter and associated equipment as shown in Figure E3-1,
Experiment 3.
2. Aluminum tub .
3. Mounting bracket for BF3 probe as shown in Figure E10-1.
4. Pu-Be source♦
5. Iron plates.
6 . Aluminum plates.
7. Lead bricks.
Procedure
1. Assemble the equipment as shown in Figure E3-1, Experiment 3,
and Figure E10-1, this experiment.
2. Fill the aluminum tub with water to one inch from the top.
3. Place the lead bricks in a square arrangement in the center of
the tub.
4. Place the Pu-Be source in the lead bricks.
5. Measure the activity (counts/minute) with the probe one inch
from the source.
6. Repeat this process every one inch until the count rate becomes
negligible.
7. Repeat Steps 5 and 6 with 2, 4, and 6 inches of iron over the
source.
8 . Repeat Steps 5 and 6 with 2, 4, and 6 inches of aluminum over
the source.
121
R e s u l t s a n d P r e s e n t a t i o n o f D a t a
1. Plot on the same graph the normalized count rate versus dis
tance from the source for water and for each water-aluminum combination.
(Use semi-log graph paper.)
2. Plot on the same graph the normalized count rate versus dis
tance from the source for water and for each water-iron combination.
3. Calculate the removal cross sections of water, iron, and alu
minum by calculating the values at several different distances from the
source and averaging the results.
4. C a l c u l a t e t h e r e l a x a t i o n o f l e n g t h s o f w a t e r , i r o n , a n d a l u
m i n u m .
Questions and Problems
1. Why should the removal cross section not be calculated from the
data taken near the source?
2. Discuss in detail the reasons why the curves of the water-metal
combinations have values greater than those of just the water alone.
3. If a stronger source is used, what modifications would have to
be made to the experimental setup? Explain.
S e l e c t e d R e f e r e n c e s
Goldstein, Herbert, Fundamental Aspects of Reactor Shielding. Addison- Wesley Publishing Company, Inc., Reading, Massachusetts, Chapter 6 .
Valenti, Frank A., A Manual of Experiments in Reactor Physics, The Macmillan Company, New York, pp. 177-188.
EXPERIMENT 11CHEMICAL SEPARATION
The purpose of this experiment Is to separate a metallic Ion by
solvent extraction and determine distribution coefficients by measuring
the activities of the resulting aqueous solutions and alcohol extrac
tions .
Theory
The processes of radiochemical separation have important appli
cations in the field of nuclear engineering. Basically, the methods
used to separate radioactive substances are the same standard methods
used to separate non-radioactive substances. These methods include, but
are not limited to, precipitation, ion exchange, filter paper chromatog
raphy, and solvent extraction. This experiment will be concerned with
solvent extraction.
Solvent extraction is a method by which one or more extractable
solutes are separated by their preferential solubility in an immiscible
extracting solvent. The extent to which a given solute partitions can
be expressed in terms of a distribution coefficient (extraction coeffi
cient), k d .
When the two phases are at equilibrium (rates of transfer in
both directions are equal), the distribution coefficient is given by
Purpose
122
123
Kjj - C2/CL (Ell-1)
where and Cg represent the concentration of the solute In the two
solvents. If the solute Is a radioactive substance, the concentration
in each phase is directly proportional to the activity of each phase.
Therefore, the distribution coefficient can be expressed as the ratio of
the activities,
K d - A 2/A^ (Ell-2)
This ratio gives an easy method of determining Kp. Therefore, the
amount of solute that has been extracted can be readily measured.
In this experiment we will extract ions of copper and iron,
measure their activity, and determine distribution coefficients. The
ions will initially be dissolved in sulfuric acid and the extracting so
lution will be iso-butyl alcohol.
Apparatus
1. Scintillation detector and associated equipment assembled as
shown in Figure E7-1, Experiment 7.
2. Following chemicals:
Sulfuric acid, I^SO^, 1 N
Potassium thiocynate, KSCN, 0.5 N
Copper sulfate, CuSO^
Ferrous ammonium sulfate, Fe(NH^)g(SO^)g
Iso-butyl alcohol
Hydrogen peroxide, H 2O2
124
3. Bottles, 8 ounces (2)
4. Graduated cylinders, 10 ml (28)
5. Volumetric pipettes, 1 ml (2)
6 . Volumetric pipettes, 2 ml (2)
7. Droppers (20)
8 . Plastic vials (16)
Procedure
1. Irradiate 1 gram each of GtiSO^ and Fe(NH^)2(SO^)2 «
2 . Mix each irradiated salt with 100 ml of 1 H
3. Pour 10 ml of each mixed solution into separate graduated cyl
inders .
4. Into each of four graduated cylinders, marked 1 - 4 , transfer
one ml of Fe(NH^)g^SO^)2 solution. Use one ml volumetric pipette for
the transfer.
5. Into cylinders 2 and 4, put 5 drops of H 202 .
6 . Into cylinders 3 and 4, add 0.5 ml of 0.5 H KSCN.
7. Fill graduated cylinders 1 - 4 to two ml using 1 N H 2S0^.
8 . Using a two ml volumetric pipette, add two ml of iso-butyl al
cohol to cylinders 1 - 4 .
9. Place polyethylene stoppers on cylinders and agitate suffi
ciently to cause solvent extraction.
10. Remove one ml each of the aqueous and extracted solution, using
droppers to decant and graduated cylinders to measure. Mark the , : ■
graduated cylinders containing aqueous solution A-D and the graduated
cylinders containing extracted solutions E-H.
11. Transfer the contents of A-H into plastic vials marked A-H.
12. Count each vial for two minutes.
13. Wash all glassware used in Steps 4 - 1 0 .
14. Repeat Step 4 using CuSO^ solution in place of Fe(NH^)gCSO/p2
solution.
15. Repeat Steps 5 - 1 3 .
Results and Presentation of Data
1. Calculate the distribution coefficient for each extraction
system. Be sure to include with these coefficients, a description of#|-|- , | I, t
the ions which were extracted in each case (i.e., Fe , Fe , etc.)
Questions and Problems
1. Describe the nuclear reactions involved in this experiment.
Sketch the decay schemes of the radioisotopes.
2. Show that it is unnecessary to correct for the differences in
the times between irradiation and counting. Is a correction necessary
to account for the short half-life of C u ?
3. Write the chemical equations describing the processes taking
place in each of the eight cylinders before the iso-butyl alcohol is
125
added
126
Selected References
Flagg, John F ., Chemical Processing of Reactor Fuels, Academic Press, New .York and London, 1961, Chapter 4.
Overman, Ralph T. and H. M, Clark, Radioisotope Techniques, McGraw-Hill Book Company, Inc., New York, 1960, Chapter 9.
EXPERIMENT 12ABSORPTION OF BETA PARTICLES
The purpose of this experiment is to study the absorption of
beta particles in matter.
Theory
One of the identifying characteristics of beta radiation is its
range. The range of a beta particle is generally defined as the minimum
thickness of material necessary to absorb the most energetic beta par
ticle of the particular spectrum under consideration. Since each beta
emitting isotope has a characteristic beta spectrum, it also has a char
acteristic beta range.
The results of many experiments have shown that there is a def
inite range-energy relationship. Katz and Penfold (see selected refer
ences) have proposed the following empirical relations:
R0(mg/cm2) - 412 En , n * 1.265 - 0.0954 In E (E12-1)
0.01 Mev < E < ~ 3 Mev
R0(mg/cm2) = 530 E - 106, Mev < E < ~ 2 0 Mev (E12-2)
In the above equations, R@ is the range and E is the maximum energy of
the beta spectrum under consideration.
Purpose
127
128
One of the more widely used experimental methods to determine
the range of beta particles was developed by Feather (see selected ref
erences) . Feather*s method compares the absorption curves of the parti
cles whose range is to be determined with the absorption curve of a
well-known standard.
To determine an unknown range by Feather*s method, absorption
measurements (counts versus absorber thicknesses) are made for the known
and unknown isotope (i.e., the isotope with the unknown range). From
these measurements the instrument and ganana backgrounds are subtracted.
The resulting pure beta absorption curves are then plotted after they
have been normalized to the same initial point (see Figure E12-1). The
known range of the standard is then divided into N equal parts, T%,
Tg,... T^, and the fractional beta transmissions are found for each T^.
The thicknesses of the absorber giving the same fractional transmissions
are then found for the unknown isotope by horizontal lines shown in
Figure E12-1. These thicknesses are labeled T^, T^,* * * T^. Each of
Absorber Thickness (mg/cnr)
Figure E12-1. Pure Beta Absorption Curves
129
these thicknesses are multiplied by N/i, and the resulting products are
then plotted against i. This plot is known as a Feather plot. To de
termine the unknown range, the Feather plot is extrapolated to i * N,
and the unknown range is taken from the extrapolated point. (See Figure
E12-2.)
\\\ extrapolated
1 . 2 3 4 .i
N
Figure E12-2. Feather Plot
In this experiment the ranges and energies of several beta
emitting isotopes will be found by Feather's method. These ranges will210be compared with generally accepted values. Bi , with a range of
o508 mg/cm , will be used as the standard.
Apparatus
1. GM counter assembled as shown in Figure El-2.
2. Aluminum absorber set. Atomic Accessories.
3. B i ^ ® , P a ^ ^ , and Co**® beta sources.
130Procedure
1. Place the Bi source in the chamber and count for one minute.
2. Place a small aluminum absorber between the source and the de
tector and count for one minute.
3. Repeat Step 2 with increasing absorber thicknesses until the
count rate is reduced essentially to background.
4. Repeat steps 1 through 3 for the other beta sources.
5. Estimate and record the thicknesses of the GH chamber window
and the air between the window and the source.
Results and Presentation of Data
1. Plot counts versus absorber thickness for each source. Note:
the thicknesses of the GH window and the air between the source and the
window must be included in the thickness of the absorber.
2. Graphically subtract gamma and instrument backgrounds from the
absorption curves. (This gives the pure beta absorption curves.)
3. Normalize each of the pure absorption curves to the initial
point of the B i ^ ® curve and plot the P a ^ 4 an<j co60 each with the B i ^ ®
curve.
4. Determine the ranges of the P a ^ ^ and the Co**® sources using
Feather's method and compare these ranges with accepted values. (Use
N = 10.)
5. Determine the beta energies of the isotopes and compare these
with accepted values.
210
131Q u e s t i o n s a n d P r o b l e m s
1« Would the accuracy of Feather's method be Increased by making
N > 10? Explain.
2. L i s t s e v e r a l i s o t o p e s w h i c h w o u l d m a k e g o o d s t a n d a r d s . G i v e
t h e i r e n e r g i e s a n d r a n g e s . W h a t m a k e s t h e m g o o d s t a n d a r d s ?
3. H o w d o e s b a c k s c a t t e r i n g a f f e c t t h e r e s u l t s o f t h i s e x p e r i m e n t ?
4. Using equations E12-1 and E12-2, plot R0 versus beta energy
(0.01 Mev to 10 Mev). Use log-log graph paper.
S e l e c t e d R e f e r e n c e s
Evans, Robley D . , The Atomic Nucleus. McGraw-Hill!.Book Company, .New York, 1955, pp. 621-629.
F e a t h e r , N . , P r o c e e d i n g s o f t h e C a m b r i d g e P h i l o s o p h i c a l S o c i e t y ,Vol. 34, p. 599, (1938).
Katz, L. and A. S. Penfold, Reviews of Modern Physics, Vol. 24, p. 28, (1952).
EXPERIMENT 13DECONTAMINATION OF SURFACES
The purpose of this experiment is to study the procedures used
to decontaminate surfaces.
Theory
Radioactive contamination is the undesired presence of radioac
tive materials in amounts that may be harmful to personnel and material.
In addition, this contamination may cause inaccurate experimental re
sults. Contamination originates from a loss of material that would
often be inconsequential except for its radioactivity. Such things as
loss of a gas, evaporation of a liquid, liquid transfer, manipulation of
a solid, and absorption on surfaces all may lead to contamination. Sur
face contamination is the deposition and attachment of radioactive mate
rials to the surface, and it is the decontamination of a surface with
which this experiment is concerned.
The decontamination of a surface is a measure of the extent to
which radioisotopes can be removed from a contaminated surface. This
decontaminability is normally expressed as a decontamination factor
(DF).
DF - aCtlVlty (E13-1)final activity
Purpose
132
133Although there are many different means of decontaminating sur
faces, certain established procedures will yield the best results. A
list of decontaminants for various surfaces is given in Table 4-4 of
Overman and Clark (see selected references).
Decontamination methods, regardless of the type of surface to
which they are applied, should concentrate on a thorough cleaning of the
surface without damaging the surface itself. If, however, noncorrosive
methods do not result in proper decontamination, then harsher treatments
should be used. For example, if it is necessary to decontaminate a piece of wood, and if normal washing does not produce the desired results, it may be necessary to sand away the contaminated surface. The
basic rule is to employ a treatment that is as mild as possible but will
still decontaminate successfully. Mild reagents and extensive scrubbing
are generally successful in reducing the contamination to a desired
level.
No list of decontamination methods will be given in this theory
section. However, in order to illustrate the proper techniques, decon
tamination of painted wood will be discussed. After a surface of
painted wood is contaminated, the first step in decontamination is to
scrub the surface thoroughly with a household detergent. If the level
of the radiation is not reduced to background, this process should be
repeated. If the radiation level is still too high after a second
scrubbing with detergent, a solution of trisodium phosphate is then used
to scrub the surface. If the radiation level is still too high after
134
this has been accomplished. It may be necessary to sand away the painted
surface.
In this experiment salts of sodium chloride, ferrous ammonium
sulfate, copper sulfate, and ammonium phosphate will be irradiated in
solution. The irradiated solutions will then be used to contaminate
surfaces of glass, glazed brick, unglazed brick, painted wood, unpainted
wood, asphalt tile, sheet iron, stainless steel, linoleum, and plastic.
Proper decontamination procedures will then be used to reduce the level
of radiation on the contaminated surfaces to background.
Apparatus
1. Portable GM counter, Johnson, Model GSM-5.
2. Cutie Pie survey meter. Nuclear-Chicago, Model 2586.
3. The following chemicals:
Sulfuric acid, HgSO^, 1 N
Trisodium phosphate, Na^PO^
Sodium chloride, NaCl
Ferrous ammonium sulfate, Fe(NH^)gCSO^)g
Copper sulfate, CuSO^
Ammonium phosphate, (NH^)gPO^
4. Graduated cylinders, 10 ml, (4).
5. Plastic vials (4).
6. Droppers (20).
7. Household detergent.
. Beakers, 600 ml, (5).8
1359. Bench top laboratory oven, Planchet.
Procedure
1. Mix 0.1 gram of NaCl, Fe(NH^)2(504)2 * CuSO^, and (NH4)3PO4 to
5 ml of 1 N H 2SO4 . Use the 10 ml graduated cylinders.
2. Mark the plastic vials 1 - 4 and half fill these vials by
placing solutions of NaCl into vial 1, FeCNH^)2(SO4)2 into vial 2, CuSO^
into vial 3, and (NH4)3PO4 into vial 4.
3. Irradiate these solutions to obtain the desired activity.
Note: after the solutions have been irradiated they will be stored in a
radioactive hood.
4. Transfer 2 drops of the irradiated NaCl solution onto the sur
faces that are to be contaminated.
5. Dry the irradiated surfaces in the laboratory oven.
6 . Monitor the activity with the proper survey meter.
7. Use the necessary decontamination procedures to reduce the con
taminated surface to background. After each step of decontamination,
monitor the activity with the proper survey meter. Note: be certain to
record all intensities before and after decontamination.
8 . Repeat steps 4 through 7 using solutions of F e C N H ^ 2(804)2*
CUSO4 , and (NH4)gP04.
9. Be certain that all materials used in this experiment are thor
oughly decontaminated before the laboratory period is concluded.
Results and Presentation of Data
1. Determine the decontamination factor after each step in the
decontamination procedure. Do this for all surfaces used In this ex
periment.
2. Plot radioisotope type as a function of radiation intensity for
each surface after each step in decontamination.
Questions and Problems
1. Sketch the decay schemes of the radioisotopes used in this ex
periment .
2. A bottle containing one curie of Co^® is dropped and broken on
a stairwell. The solution runs down a painted wooden staircase, over a
linoleum floor, out to a concrete porch, down unpainted wooden stairs,
and finally to the lawn. Describe the necessary decontamination proce
dures.
3. What is the difference between a ’major' and a 'minor* spill?
Selected References
Blatz, Hanson, Introduction to Radiological Health. McGraw-Hill Book Company, New York, 1964, Chapter 7.
Blatz, Hanson, ed.. Radiation Hygiene Handbook. McGraw-Hill Book Company, New York, 1959, Section 18.
Overman, Ralph T . , and H. M. Clark, Radioisotope Techniques. McGraw-Hill Book Company, New York, 1960, Chapter 4.
136
EXPERIMENT 14
AUTORADIOGRAPHY
The purpose of this experiment is to study autoradiography as a
means of radiation detection.
Theory
In this manual, ionization chambers, semiconductor detectors,
and scintillation counters have been studied as methods by which ion
izing radiation may be detected. The use of photographic film in film
badges to measure radiation doses has also been briefly discussed in
Chapter 2. This experiment will consider another use of photographic
emulsions in radiation detection. This method is autoradiography.
Autoradiography is the determination of the distribution of ra
dioactivity in a specimen by the use of photographic emulsions.
The photographic process that allows for the use of emulsions
as radiation detectors is basically an interaction between the ionizing
radiation and the photosensitive substance of the emulsion. The most
widely used photosensitive substances are silver halide crystals. The
silver halide crystal absorbs energy from the incident photons or
charged particles. Under the action of a chemical reducing agent, the
energized crystals convert more readily from the halide to metallic
silver than do the non-energized crystals. This physical condition in
Purpose
137
138the crystal that makes it 'developable* is called a 'latent image *. Al
though it has been used for many years, the process of latent image for
mation is not yet completely understood.
The components of any photographic emulsion are the photosensi
tive material (silver halide), a dispersal medium for the photosensitive
material, and the emulsion base (cellulose acetate for most films).
In determining which emulsion should be used for a particular
experiment, the following factors should be considered: sensitivity to
the type of radiation of interest, crystal size, crystal concentration,
thickness of the emulsion, and the effect of background on the emulsion.
In this experiment indium, in combination with aluminum, will
be irradiated. Photographic film will then be used to determine the
distribution of the indium within the aluminum.
Apparatus
1. Polaroid Land 4 X 5 film packet. Type 57, one per student.
2. One aluminum-indium packet per student.
3. Polaroid Land film holder. Number 500.
Procedure
1. Each student will be issued a packet consisting of indium foils
distributed within aluminum holders. The foil arrangement will be dif-
erent for each student.
2. Irradiate the packet to an intensity of approximately 5 mr/hr.
3. Place the Polaroid film over the irradiated packet for approxi
mately one hour. (Keep in a dark place such as a drawer.)
4. Use the Polaroid Land film holder to develop the film.
5. Note the distribution of the indium within the aluminum.
Results and Presentation of Data
1. Sketch the location of the indium foils within the aluminum
packet.
Questions and Problems
1. Discuss the effects of crystal size, crystal concentration,
thickness of emulsion, and the effect of background on the latent image.
2. How could autoradiography be used in leak detection?
3. What type of radiation in this experiment produces the latent
image on the emulsion? Why?
4. Discuss how photographic emulsions are used in neutron detec
tion.
139
Selected References
Barkac, Walter H., Nuclear Research Emulsions, Academic Press, New York, 1963.
Norris, William P., and L. A. Woodruff, 'The Fundamentals ofRadioautography', Annual Review of Nuclear Science, Vol. 5, pp. 227-326, 1955.
Overman, Ralph T., and H. M. Clark, Radioisotope Techniques, McGraw-Hill Book Company, Inc., New York, 1960.
Yagoda, Herman, Radioactive Measurements with Nuclear Emulsions, - John Wiley and Sons, Inc., New York, 1949.