Total absorption scintillation spectrometer

Item Type text; Thesis-Reproduction (electronic)

Authors Kielkopf, Edward C., 1933-

Publisher The University of Arizona.

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TOTAL ABSORPTION SCINTILLATION SPECTROMETER

by

Edward C. Kielkopf» Jr.

A Thesis Submitted to the Faculty of the

DEPARTMENT OF NUCLEAR ENGINEERING

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 6 7

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 Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission is 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 author.

SIGNED

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the data shown below:

ROY G. POST Professor of Nuclear Engineering

Date

ACKNOWLEDGMENTS

The author wishes to express his appreciation to Dr. Roy G. Post

and Dr. Morton E. Wacks for their guidance and assistance in accomplish­

ing this work. The help of Dr. Monte V. Davis and Dr. Robert L. Seale

is gratefully acknowledged.

He would like to thank Mr. Donald M« Fiehl for his. work in pro­

ducing the machined components of the system.

A special note of thanks is due the author*s wife and children

for their encouragement and patience while this work was being performed.

Finally, he extends his appreciation to the United States Army

for the opportunity to complete this work.

iii

TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS ........................................ . v

ABSTRACT ....... vi

INTRODUCTION ............. 1

THEORY......................... 3

Gamma Radiation...... 3Background Radiation ............ 5Scintillation Spectrometry........ 7

THE TOTAL ABSORPTION PROBLEM ............... 9

Large Crystals .............................. 11Anticoincidence Shielding ................................... 12Sum Coincidence ..... 16

SYSTEM DESIGN AND CONSTRUCTION ................................. 19

OPERATION ....... 30

CONCLUSION ........................ 33

APPENDIX .................................................. 35

LIST OF REFERENCES ........... 41

iv

LIST OF ILLUSTRATIONS

Figure ^age

1 Absorption Cross Sections for Gamma Radiationof Nal (Tl) ........................................ 6

2 Cobalt 60 Spectra Using a 4 3/4 x 5 1/2 inchNal (Tl) Well Crystal . ............................ 10

3 Block Diagram of Anticoincidence ScintillationSpectrometer ....... 13

4 An Anticoincidence Scintillation Spectrometer ............. 15

5 Cobalt 60 Spectrum Using Two 3 x 3 inch Nal (Tl)Crystals in Sum Coincidence ............. 17

6 Sll Response of RCA-6342A Photomultiplier Tube ...... 22

7 The Sphere with End Flanges in Place ............ 23

8 Carrier for 13/4 inch Crystal Assembly ................... 25

9 Attachments to Crystal Carrier ............. 26

10 Photomultiplier Tube Carrier ....... 28

11 Total Absorption Scintillation Spectrometer .............. 29

12 Cobalt 60 Spectrum from the Crystal ....... 31

13 Cobalt 60 Spectrum from the Anticoincidence Shield ........ 32

14 The Compton Effect ..... 36

v

ABSTRACT

Gamma ray spectrometry is the measurement of the energy dis­

tribution of the gamma emission of radioisotopes * Losses of energy

from Compton scattering precludes complete definition of the spectra,

particularly in the lower energy part. An anticoincidence scintilla­

tion spectrometer was constructed to reduce the effect of Compton

scattering on spectra. The system consists of a crystal detector

surrounded by a liquid scintillator. The only pulses recorded are

those from the crystal detector which are not in coincidence with

pulses from the liquid detector. Since most scattered photons will

escape from the crystal and be absorbed by the liquid, only those

events resulting in complete energy absorption in the crystal will be

detected thus reducing the Compton portion of the gamma ray spectrum.

This arrangement also helps minimize the natural background contribu­

tions to spectra.

INTRODUCTION

The analysis of complex mixtures of radionuclides by gamma ray

spectrometry has become a very common technique„ Definition of the

spectrum is facilitated by obtaining total absorption of the gamma

radiation in the detector. Total absorption results in a spectrum con­

sisting wholly of energy peaks from radionuclides present in the mixture

whose intensities are proportional to those of the incident gamma rays.

Total absorption may be precluded by the escape of secondary photons

from the detector. Background also tends to complicate a spectrum but

its effect can be minimized by subtraction.

The escape of gamma photons may be limited by employing a large

detector or by compensation for the effect of their escape. Sodium

iodide crystals with diameters of fourteen inches are available and the

spectra obtained by using them can be expected to reflect nearly total

absorption. Large liquid scintillators have been used extensively for

whole body counting but for routine gamma ray spectrometry their poor

response to low energy radiation limits their usefulness»

Anticoincidence shielding has been found to be very useful in

reducing the intensity of the Compton continuum. The detector is sur­

rounded by a scintillator which is connected in anticoincidence with it

so that only pulses representing total absorption in the detector are

counted. Davis, et al., (1956) used a liquid scintillator shield around

a sodium iodide crystal while Perkins, et_ al. 9 (1960) surrounded one

sodium iodide crystal with a sodium iodide crystal and another with a

plastic scintillator. All systems provided significant reductions in

the intensity of the Compton continuum.

An additional solution to the total absorption problem has been

the sum coincidence method which records only coincident events in two

detectors where total absorption occurs. The two events are summed to­

gether giving a peak at the summed energy which is satisfactory if it

is known where this peak will appear, JL.jB. , the components of the sample

have been identified.

The construction of a total absorption scintillation spectro­

meter will simplify investigative work in neutron activation analysis

and serve as an educational tool. It should provide spectra which are

relatively free of the effects of Compton scattering and background

radiation. The energy peaks should be well resolved. The system must

be versatile enough to present accurately spectra of radioisotopes emit­

ting gamma rays at any energy.

THEORY

Gamma Radiation

Gamma radiation is detected by its interaction with matter.

There are three main interactions which occur: photoelectric, Compton

scattering, and pair production. The photoelectric effect occurs when

photons transfer all of their energy to a bound electron of an atom,

usually in the K or L shell, which is in turn ejected with a kinetic

energy equal to the energy of the incident photon less the ionization

potential of the electron. The range of this electron is very short in

most elements and the probability of escape from a scintillator is quite

small. The cross section per atom, t , for photoelectric absorption in

the K shell is given by Kaplan (1962) as:

a x - v W 4 / 2 ( ^ ) 7/2 (1)

where

2<t>o = -y ( - ^ y ) 2 = 6.65 x 10“25cm2 (2)

m c o

2Z is the atomic number of the absorbing material, m e is the rest massoof the electron, hv is the energy of the incident photon and e is the

charge on the electron. (f>o is the cross section for Thomson scattering.

4The Compton effect occurs in an interaction between an atomic

electron and a photon. The electron is separated from the atom and

ultimately absorbed while the photon is scattered from its original

course with a reduced amount of energy. The electron’s energy is the

difference between that of the incident and scattered photon. Since

the scattering is random, recoil electrons over the whole energy range

are produced and, upon their absorption in the scintillator, the Compton

continuum appears. The Compton edge denotes the maximum energy of the

recoil electrons. A detailed treatment of this effect may be found in

the Appendix. The cross section for Compton scattering, , is the sum

of the cross sections for energy transfer to the absorbing material and

the energy in scattered radiation. From Kaplan (1962) :

hole in the negative energy state. This hole constitutes a positron

while the electron in the positive energy state remains an electron. The

interaction occurs in the coulombic field of the nucleus. Disregarding

the small amount of recoil energy of the nucleus, all of the energy of

the incident photon is transferred to the pair of particles in the form

ln(l+2a)]+ ln(l+2a)

where <j> is defined in Equation (2) ando

hv (4)a = 2m c o

Pair production occurs when an electron is raised to a state of9positive energy by a photon of at least 2 m c*- (= 1.02 MeV) leaving a

of their kinetic energies and rest masses. At the end of its range the

positron interacts with an electron producing two annihilation radiation

quanta having energies of 0.511 MeV each. One or both of these photons

may escape or be absorbed in the material through Compton or photoelec­

tric processes. The probability for pair production interaction, K, is

normally expressed in units of <t> where

2 2J = JJJ = Z2 x 5.796 x 10~28cm2 (5)

m c o

Z is the atomic number of the absorbing material, e is the charge on the 2electron and m^c is the rest mass of the electron (Kaplan 1962).

Figure 1 shows the regions of dominance for each of the three

effects in sodium iodide and is typical of all matter. The broad range

of the Compton effect indicates that in any spectrum there will be a

continuum.

Background Radiation

Ionizing radiation, produced directly or indirectly by the inter­

action of cosmic radiation with the detector, is the fundamental source

of background in counters (Watt and Ramsden 1964). Other sources are

radioisotopes and electronic devices in the vicinity of, or connected to,

the detection system.

Cosmic radiation consists of protons, electrons, neutrons, other

particles and photons. The effects of this radiation upon a scintilla­

tion system can best be suppressed by surrounding the detector with a

shield. The most effective shield is one which maximizes photon

Cross

Section

(cm

100

10 -

i H

Total

0.1 Compton

Photoelectric

f Pair Production

0.0110 .01 0.1 10

Energy (MeV)

Figure 1. Absorption Cross Sections for Gamma Radiation of Nal (Tl) .(Data from NBS Circular No. 583, 1957. Reprinted from Harshaw Scintillation Phosphors)

attenuation but limits the resultant production of bremsstrahlung. Such

a shield will readily attenuate particulate radiation.

Scintillation Spectrometry

A scintillator, or phosphor, converts the energy in the course

of the three processes (photoelectric, Compton and pair production) into

visible light. The light excites the photocathode of the photomultiplier

tube, which is optically connected to the scintillator, causing the emis­

sion of electrons from it. These electrons are accelerated by the

potential between the cathode and the first dynode of the tube. Upon

striking the first dynode the excess energy of each electron causes the

ejection of more electrons which are accelerated to the second dynode by

the potential between dynodes one and two. This process is repeated for

each of several dynodes until the electrons reach the anode of the tube

and an electrical signal, proportional to the incident photon, leaves the

tube. The output signal is amplified and stored according to pulse

height by a multichannel analyzer.

The scintillation process varies with the materials used. In

the case of activated sodium iodide crystals, the chain of events has

been postulated as the production of electron-hole pairs by interaction

of a gamma photon with the crystal material and the subsequent absorption

of the resultant photoelectron (Birks 1964, pp. 68-93). These pairs

recombine to produce an exciton, which diffuses through the lattice ulti­

mately being captured by a luminescence center. This excitation energy

is quickly brought to thermal equilibrium in the lowest excited state 3Pq , the excess energy being dissipated as phonons to the lattice. The

energy from this excited state is then expended in thermal activation,3by capture of phonons, to the level, from which luminescence emis­

sion occurs. Luminescence is the emission of light (visible or ultra­

violet) with a characteristic spectrum.

The conjugated and aromatic organic molecules have luminescence

associated with them as an inherent molecular property in contrast to

the alkali halides in which luminescence is a property of the crystal

lattice. Luminescence in organics is a result of the electronic struc­

ture of the molecules. This property is exhibited in the vapor state,

in liquid and solid solutions, in the crystalline state and to some

extent in the liquid, plastic and glassy states. The structure of

organic molecules is largely determined by the electronic structure of

the carbon atom and the molecules in turn are loosely bound together

and retain their individual identity, electronic structure and lumines­

cence. The organic compounds generally used as scintillators include

anthracene, toluene, xylene and trans-s tilbene.

THE TOTAL ABSORPTION PROBLEM

The deviation of gamma ray spectra from the ideal is primarily

a result of Compton scattering. If a scintillation spectrometer can be

constructed to minimize this effect, the spectrum obtained from a par­

ticular sample should consist of the energy peaks corresponding to the

energies of the emitted photons. Figure 2 presents spectra for Cobalt

60 obtained with and without the use of an anticoincidence shield in

the detection system.

The primary absorption of gamma rays is an exponential function

of distance which is dependent on energy and the material through which

it passes.

The intensity. I, of monoenergetic gamma radiation at any dis­

tance, x, from a point isotropic source is given by:

I = Bl^exp (-yx) (6)

where

I is the initial radiation intensity o

B is the buildup factor

y is the mass attenuation coefficient.

The mass attenuation coefficient is a function of the material

through which the radiation passes and the energy of the radiation.

That portion which is due to the Compton effect accounts for the partial

Coun

ts/s

ec

10

1000

Without Anticoincidence

1.17 MeV1.33 MeV

1002.5 MeV

ComptonContinuum

10 - Backscatter peak

With Anticoincidence

1000800600400200Pulse Height

3 1Figure 2. Cobalt 60 Spectra Using a 4 v x 5 y inch Nal (Tl) WellCrystal. (From R. C. Davis, P. R. Bell, G. G. Kelley and N. H. Lazar, IRE Trans. on Nucl. Sci., NS-3, Nov. 1956)

11transfer of energy to the electron. As the absorber thickness increases,

the number of secondary photons produced builds up and, although many

are absorbed, the number of photons at a particular point in the absorber

is larger than that predicted by use of the mass attenuation coefficient

alone. The buildup factor accounts for this fact.

Large Crystals

The alkali halide crystals are probably the most widely used

scintillators. Their absorption properties are superior to the organic

scintillators because of their higher densities and the higher atomic

numbers of their constituent elements. For example, sodium iodide has3 3a density of 3.76 gm/cm as compared to 0.86 gm/cm for toluene. The

significance of higher atomic numbers lies in the correspondingly higher

photoelectric absorption coefficients. (Sodium 11 and iodine 53 vs

hydrogen 1 and carbon 6.)

The size of a sodium iodide crystal required to obtain 90%

absorption of 3 MeV gamma radiation from a point isotropic source can

be calculated from Equation (6)

Y" = 0.1 = 2.72 exp (0.036)(3.76)(x) = 24 cm or 9.45 in. (7) o

The buildup factor in this case is that for tin whose absorption coeffi­

cient closely approximate those of sodium iodide and whose atomic

number is close to that of iodine (Goldstein 1959).

Four pi geometry should be used, jL.je. , the sample should be

completely surrounded by the detector. This is attained by utilizing a

12

cylindrical crystal with a well drilled in it for the source. In the

example, the crystal would have to be about 19 inches in diameter by 19

inches long. The resultant spectra using such a scintillator should

lose a minimum of secondary photons from Compton scattering.

The most frequently used background shield is lead. Four pi

geometry is desirable for the shield as well as detector. Care must be

taken to utilize low noise electronic equipment with the detector because

the shield only protects from external radiation and noise.

Anticoincidence Shielding

The simplicity of a scintillation system such as a properly

shielded large sodium iodide crystal would seem to be ideal for total

absorption work. Use of a smaller crystal with an anticoincidence shield

can provide comparable results. Although the percentage of radiation

absorbed is considerably lower, the distortion caused by Compton scat­

tering can be reduced.

The use of an anticoincidence shield has found wide acceptance

in total absorption scintillation spectrometry because it is economically

attainable where a large crystal may not be. The advantage exists of

eliminating unwanted pulses from the spectrum of the sample under inves­

tigation. Where lead passively absorbs external radiation, the anticoin­

cidence shield detects this radiation and eliminates it from consideration,

in the anticoincidence circuit of the counting equipment. Figure 3 is

a block diagram of such a system. If a photon, no matter what its origin,

enters the shield and interacts with it in any manner so that it is

detected by one of the photomultiplier tubes, a pulse goes to the

13

Photomultiplier Tubes

Photomultiplier Tubes

CrystalDetector

AnticoincidenceCircuit

MultichannelAnalvzer

Scintillator

Figure 3. Block Diagram of Anticoincidence Scintillation Spectrometer.(From W. H. Ellett, "Proc. of Total Absorption Gamma-Ray Spectrometry Symposium,M p. 60, TID 7594 1960)

14

anticoincidence circuit. If there is a time coincident pulse coming

from the principal detector, it is not counted. Only those pulses from

the principal detector which are not in coincidence with pulses coming

from the shield are passed and counted.

The effect of the shield on a spectrum can be evaluated by deter­

mining the reduction in the Compton continuum. Equation (6) without the

buildup factor, can be used to calculate the fraction interacting in the

shield.

1 - ~ = exp (-yx) = 1 - e (8)o

where y is the mass attenuation coefficient of the shield and x is its

thickness. The buildup factor is not included because any photon inter­

action is effective and total absorption is not necessary. For an

organic shield ten inches thick, 55.8% of 3 MeV gamma radiation which

escapes the crystal will interact in the shield and the Compton continuum

will be reduced that much. The effect will be the same if the organic is

liquid or solid.

Four pi geometry in the shield system provides strong assurance

that secondary photons will interact with the shield resulting in a coin­

cident event. A spherical liquid shield appears to be the best all

around solution. Perkins et̂ al., (1960) and Ellett (1960) employed solid

shields in cylindrical configurations and achieved satisfactory results

for their purposes. Davis et al., (1956) used the liquid shield arrange­

ment shown in Figure 4. The solution phosphor is xylene, p-terphenyl

and a-NPO. The tank interior surface is a-alumina reflector. The tank

Sheet Iron Tank

Figure 4

PMPMSolutionPhosphor

Source0.005 Aluminum

Solution Phosphor

PM PM

CrystalandPM

PM

An Anticoincidence Scintillation Spectrometer. (From R. C. Davis, P. R. Bell, G. G. Kelley and N. H. Lazar, IRE Trans, on Nucl. Sci., NS-3, Nov. 1956)

16

is 28 inches in diameter and the sodium iodide well crystal is 4 3/4 x

5 1/2 inches» The spectra in Figure 2 were obtained using this scin­

tillation spectrometero

Sum Coincidence

The sum coincidence method of total absorption provides another

means of minimizing the Compton portion of the spectra. It is especially

good for the measurement of gamma radiation spectra emitted in cascade

disintegrations, jl.jb. , decay occurs by positron or negatron emission22followed by gamma radiation coincident with annihilation quanta. Na

and Na^ are isotopes which decay in this manner. The gamma rays are

detected in coincidence by two crystals butted end to end and counted

separately. If both counters have the same energy response a sum pulse

which is proportional to the energies absorbed in the two crystals is

sent to a differential discriminator. This component must be adjusted

so that an output pulse appears only when the input signal indicates that

total energy absorption has occurred in the crystals. The only way that

a proper adjustment may be made is if the constituent elements of the

sample being measured are known to be present in it. Figure 5 presents

the results of one such arrangement»

Perkins (1965) describes a coincidence-anticoincidence scintilla­

tion spectrometer which seems to optimize the features of both systems.

The shield enables the investigator to obtain a true spectrum of the

sample under examination and the coincidence system permits attainment

of specific total absorption peaks,. For examination of samples with

unknown constituent elements the sum coincidence method does not offer

Coun

ts/m

in/K

eV17

15

10

5

00 1 2

Energy (MeV)

Figure 5. Cobalt 60 Spectrum Using Two 3 x 3 inch Nal (Tl) Crystals inSum Coincidence. (From W. H. Ellett, Proc. of Total Absorption Gamma-Ray Spectrometry Symposium, p. 60, TID 7594 I960)

18

much assistance but for particular work when these elements are known it

is an excellent adjunct to an anticoincidence system.

The best solution to the total absorption problem appears to lie

in using a large size alkali halide well crystal surrounded by an alkali

halide crystal sphere in anticoincidence. This provides best absorption

in the principal detector at any energy and the highest probability for

interaction in the shield. The expense involved in such an undertaking,

however, is prohibitive. The alternative of a large crystal without a

shield is attractive hut again expensive. A smaller crystal in a four

pi organic shield seems to offer something which is attainable. Further,

use of a liquid shield offers the capability of using different sized

crystals with the same shield. This versatility permits the spectro­

meter to be used in experiments involving radiations over a wide range

of energies.

SYSTEM DESIGN AND CONSTRUCTION

The primary consideration in the design and construction of any

scintillation spectrometer is to obtain a system with the best possible

resolution. Resolution is a function of the performance of the compo­

nents of the system. The variables which contribute most to it are the

emission of light photons by the scintillator, the collection of these

photons by the photocathode, the emission of photoelectrons and their

collection at the first dynode, and the electron multiplication process.

The optical coupling must be complete between scintillator and photo­

multiplier tube and a light tight seal must enclose both components to

minimize the loss of visible light photons. The scintillator should be

covered with an efficient diffuse reflector at all points not in con­

tact with a photomultiplier tube.

The particular aim in the design of this scintillation spectrom­

eter is to minimize the effects of Compton scattering on spectra. In

addition, it would be advantageous to have a portable system for maximum

versatility.

It has been explained that the effectiveness of an anticoinci­

dence shield is dependent upon its constituent elements, thickness and

geometry and the energy of the impinging radiation. The presence of

background and Compton counts in spectra from this system will vary

inversely with the probability of interaction in the shield, hence the

material to be used must have the properties necessary to obtain maximum

19

20interaction. The use of any organic liquid for this task immediately

suggests a container large enough to assure a high probability of inter­

action because of the low mass attenuation coefficient.

The liquid scintillator chosen for use as the anticoincidence

shield was toluene with p-terphenyl and POPOP (p-Bis(2-5-phenyloxazolyl)

-Benzene)» Toluene is an aromatic solvent which scintillates quite well.

It has good efficiency when used in large volumes because it is highly

transparent. Its disadvantages are its low flash point and high toxicity

which require that extreme caution must be exercised when using it. A

disadvantage which it has in common with other aromatic solvents such as

xylene and trimethylbenzene is that they dissolve conventional polyethy­

lene and polypropylene over long periods of time. The latter two also

require caution in their use and their scintillation properties are not

as good as toluene.

Although toluene by itself is an adequate scintillator3 its

efficiency is increased by the addition of p-terphenyl and POPOP. Birks

(1964, p. 295) reports that use of 4 grams of p-terphenyl and 0.1 gram

of POPOP per liter of toluene has been found to be a satisfactory compro­

mise between efficiency and material economy. The quick transfer of the

excitation energy in the solvent to the p-terphenyl serves to reduce the

non-radiative dissipation of this energy by quenching in the solvent.

POPOP is added to increase the wavelength of the light so that it more

nearly matches the spectral response of the photomultiplier tube viewing

the liquid scintillator. For tubes with an Sll response a concentration

of 0.1 gram of POPOP per liter of toluene with 5 grams of p-terphenyl

gives a near optimum result.

21The detection of scintillations in the liquid anticoincidence

shield must trigger quick reactions in the photomultiplier tubes so that

the anticoincidence circuit may correctly compare coincident events. If

the liquid is sufficiently transparent, as toluene is, the photomulti­

plier tube must be responsive to the incident light to provide a correct

output pulse. The RCA-6342A tube has these characteristics. The antic­

ipated light wavelength in the.solution is 4440 angstroms and this tube

has its maximum response at 4400 + 500 angstroms. Figure 6 presents the

response curve for this tube. The transit time from photocathode to

anode is four nanoseconds which is the best for its size.

The efficiency of a liquid scintillator can be reduced by quench­

ing due to oxygen dissolved in the solvent. Experiments have shown that

quenching is very significant in toluene. Various methods have been

employed to eliminate the oxygen but the one best suited for this appar­

atus consists of bubbling an inert gas through the liquid for about

fifteen minutes. Argon will produce as good results as nitrogen and it

need not be purified before use (Ott jBt _al. , 1955) . Increases in pulse

heights of 20 to 30% have been achieved through this process.

Two aluminum hemispheres, shown in Figure 7, were obtained for

the liquid container. The dimensions are such that with the carrier for

a five inch crystal assembly in place, at least ten inches of liquid

remain for an anticoincidence shield. There is a 74% probability of

interaction in this thickness of organic solvent for 1.17 MeV photons.

The modifications that were made to each hemisphere to accommo­

date the crystal carriers are also shown in Figure 7. In both cases

the flange heights are such that their horizontal planes are tangent to

Relative

Sens

itiv

ity

22

Maximum ResponseIOC-

10

40003000 60005000Wave1en g th-Angst roms

Figure 6. Sll Response of RCA-6342A Photomultiplier Tube. (Radio Corporation of America)

ml co

23

■jr Drain PipeGas Pipe

Figure 7. The Sphere with End Flanges in Place.

m|oc

24the sphere at the polar axis thus preserving the spherical configuration

of the tank. The openings are large enough to pass the carriers for

crystal-photomultiplier tube combinations up to five inches in nominal

diameter.

The length of the crystals with their photomultiplier tubes and

preamplifiers caused their carriers to be so long that they became tunnels

passing completely through the sphere. Figure 8 depicts the carrier for

a 1 3/4 inch crystal detector. The assembly is inserted from the bottom

of the tank. For larger sized crystals a separate container with its own

photomultiplier tube could be placed in the vacant space in the carrier

to complete the four pi geometry. In the event that a two crystal coin­

cidence arrangement were used, the second assembly would be inserted in

the open end of the carrier. The wall thickness of the carrier is minimal

to reduce radiation absorption. Aluminum has a high mass attenuation

coefficient and any absorption which occurs will result in more counts

in the Compton portion of the spectrum. It is desired to have maximum

absorption in the crystal, but of the photons escaping, the intent is

that there be maximum interaction in the liquid, that is, minimum absorp­

tion in the region between the two detectors. There will be a certain

amount of backscatter because of this wall but its thinness should mini­

mize the effect. The assembly is fixed to the lower flange by eight

screws at intervals of 45°, Figure 9 presents the fixtures which attach

to the crystal container. The plate, a, is connected to the flange of

the upper hemisphere by eight screws. The container passes through the

center of the plate which serves as a cover for the tank. The tank is

filled with liquid by removal of the plate. A spacer, b, fits in the

25

Figure 8.

m I ooCM

i"ifsZZZZzT

U l x J

x / X

■3 inch Aluminum Tube 0.035 inch Wall Thickness

? 7 7 7 3<

Carrier for 1— inch Crystal Assembly.

<cr

Figure 9. Attachments to Crystal Carrier.

27bottom of the container between the base of the detector assembly and

the retainer cap, c„ Both spacer and cap are hollow so that the power

and signal cables may pass between the preamplifier and the counter.

The cap is fixed to the crystal container by eight screws, The spacer

positions the detector assembly at the center of the sphere.

Figure 10 is a drawing of the photomultiplier tube container.

The small lip is the same thickness as the sphere so that heliarc weld­

ing is facilitated. Four flanges have been placed on each hemisphere

at 90° intervals. Initially, only four photomultiplier tubes are being

used, two up and two down. The additional four, when installed, should

provide a significant improvement in the spectra because of the increase

in collection of visible light photons. The face of each photomultiplier

tube will extend into the liquid 1/8 inch to take full advantage of the

field of vision of the photocathode. It is estimated that this field

covers 165° and that the dead space around each tube will amount to 33,25 in . With eight tubes in operation coverage will overlap and the

dead spaces will be eliminated. With only four tubes in place it is

expected that the dead spaces will not be covered even if the lower set

of tubes were to be oriented 90° away from the upper set.

Figure 11 is a partial cutaway of the assembled apparatus, "A"

is the detector assembly while !tBn and "C" are the preamplifiers in the

anticoincidence shield. The lower two photomultiplier tubes and preampli­

fiers are not shown since they are 90° away from the upper set. The two

hemispheres as shown are welded together at the center plane by a bead

weld. ;

'4 “20 Tap

iDrill

ra

4Tr̂ loo

H

i>-

TL4r~ vDL Mi—1| 00

2.129 x _ i . 20 Tar

Figure 10. Photomultiplier Tube Carrier.

29

Teflon packing

Teflon packing

Gas PipeDrain Pipe

Teflon packing

Figure 11. Total Absorption Scintillation Spectrometer.

OPERATION

The initial checkout of the assembled scintillation spectrometer

was performed by operating the crystal detector and the anticoincidence

shield separately. This insured that each was operating properly with

respect to the other.

Figure 12 is the Cobalt 60 spectrum obtained by the crystal

detector alone which was used as a basis for comparison with spectra

obtained by the anticoincidence shield. Initially each detector assembly

in the shield was operated individually until common operating conditions

were established for all four. The detectors were then connected in

parallel so that the anticoincidence shield was performing as a single

detector assembly. Figure 13 is the spectrum obtained from the anti-

coincidence shield using the same sample in the same position as for the

spectrum obtained in Figure 12. The poor resolution obtained in the

anticoincidence shield may be attributed to the absorption in the crystal

and the size of the liquid.

It was not possible to employ the anticoincidence shield in its

designed mode of operation. The difficulty appears to be in the elec­

tronics of properly matching the output signals from the crystal detec­

tor with those from the anticoincidence shield to obtain the desired

effect.

30

31

CM

00

OO vO00 Oi—I i—I

(^0TX) a^nufui/s^unoo Figure 12. Cobalt 60 Spectrum from the Crystal.

Channel

Numb

er

32

o1—4

CN r—I

or—I

c00

00 VO oCM

(£0Tx) a^nujui/s^unoj

Figure 13. Cobalt 60 Spectrum from the Anticoincidence Shield.

Channel

Numb

er

CONCLUSION

This total absorption scintillation spectrometer should demon­

strate an excellent capability for unscrambling the spectra of complex

mixtures of radionuclides» Significant reductions in the Compton and

background portions of the gamma spectra simplify the task of identifying

the radionuclides producing the spectra. The system is versatile in that

it can handle varying sized crystals ̂ either singly or in pairs and it

is mobile in that it can be moved about for use in different projects.

With the addition of four more photomultiplier tubes to the anticoin­

cidence shield there should be a further improvement in the spectra

obtained. Additional experimentation may be desirable in order to

improve the spectra by using larger photomultiplier tubes or more of

the two inch size. Variations in the solute concentrations in the

anticoincidence shield may also tend to improve the spectra.

Replacement of the aluminum crystal carrier by one of methyl

methacrylate will permit the assembly to be used as a multidimensional

coincidence spectrometer for high energy work rather than just two

dimensional. This change will require a use of different liquid since

toluene attacks methyl methacrylate, Faissner et al. ? (1963) report

that a solvent, Shellsol A, which is not available in this country at

present, has scintillation properties which are nearly equal to those

of toluene. It is safer to handle and lacks the solvent power of toluene

and xylene.33

34The incorporation of a sample transfer system would enlarge

further the operation of the spectrometer. It could then be used for

analysis of elements with very short half lives.

\

APPENDIX

A. Ho Compton (1923) developed the mathematical description of

an effect which results from the scattering of electromagnetic radiation

by matter. He found that when monochromatic X-rays were scattered by a

light element, such as carbon, the scattering radiation fell into two

components, one of the original wavelength and one of a longer wave­

length. The difference in wavelength was found to vary directly with

the scattering angle. At an angle of 90° the difference in wavelength

was found to be .0236 x 10 cm. regardless of the wavelength of the

original beam and the scattering material.

Compton assumed that a collision had occurred between a photon

and a free electron in which there was conservation of energy and momen­

tum. By quantum theory electromagnetic radiation has energy hv and hvmomentum — c

where-27h is Planck's constant, 6.61 x 10 erg sec

v is the frequency of radiation, and

c is the velocity of light, 2.998 x 10*^ cm/sec.

Figure 14 is a representation of the scattering process. For conservation

of momentum the electron involved in the collision recoils with a momentum

equal to the vector difference between that of the incident and scattered

photon. The reduced momentum of the scattered photon means a lower fre­

quency or a longer wavelength than that of the incident photon.

35

36

Scattered PhotonhvMomentum

Incident Photonhv

Momentum =

Recoiling Electron m v

Momentum = — -V T 7

Figure 14. The Compton Effect.

For conservation of energy

hv = hv + m c' o o - 1 (1)

where:

is the frequency of the incident radiation

v is that of the scattered radiation

v is the recoil velocity of the electron,-27mo is mass of the electron, 0.9107 x 10 gram.

and.

c is the velocity of light.

For conservation of momentum there are two equations for the two compon­

ents :

hv , m vo hv . . o x-component: --- = — cos® + ,, • -• -i— - cosic c V I W

(2)

hv moVy-component: 0 = — sind) - sinB (3)

where <p is the scattering angle of the photon and 0 is the recoil angle

of the electron.

Let 0 = “c

then these three equations may now be solved for v, v and 0 for any

particular scattering angle, The respective wavelengths are

38

so Equations (2) and (3) become

, , m 8cT T cos* = --7T- COS6 (4)xo x

m 8c— slncf) = 7- -rr sin0 (5)x V i - s 2

Squaring these and adding

9 9 9 2*2 2 _ 2 2h h 2h cos* mo 8 C o c 2 2 f n-4- — — - :— :— - ---- = — - m c (o)X 2 X2 XoX 1-62 1-B2 °

Also Equation (1) becomes

, , m e— - — + m c = - °— (7)X X o ¥

Vl-B2

and squaring it

75 + 71_ rr + 2™0ch(r" - b + mo2c2 “ TTI (8)A A O O 1—P

Y Y (cos<f>-l) + 2m^ch(^- - ^) = 0 o o

or

AX = X -X = --- (1-cos4>)o m e o(9)

39

It can be seen that the shift in wavelength is dependent only upon the

scattering angle of the radiation since " • is constant.m c o

h = 0.0242 x 10 8 cmm c oso

AX = 0.0242 (l-cos4>) angstroms (10)

The kinetic energv of the recoil electron is

T = hv - hv o

hvh^(l-cos$)^§Y (ID

or, if

a

hvH — ‘C 1-cos 4))

m c o

hvo2m c o

a(l-cos^)hv l+a( 1-cos <{))T = -r— TT --- (12)

This will vary from <t> = 0°, 9 - 90°

where T = 0 to a maximum

when (J> = 180°, 6 = 0°

40This energy denotes the Compton edge. The corresponding energy of the

scattered photon is

hvhv = i^a-cos*) (14)

Some photons are scattered back by the housing of the scintil­

lator and photomultiplier tube. The peak of the distribution of the

resultant pulses in the spectrum is the back scatter peak and corres­

ponds to the energy of photons scattered at an angle of 180°,

hvEbs - iTit <15>

by Equation (14).

LIST OF REFERENCES

Birks, J» B.5 The Theory and Practice of Scintillation Counting, NewYork: The Macmillan Company, (1964).

Compton, Arthur H. 5 Phys» Rev. 9 21.) 483 (1923) .

Davis, R. C., P. R. Bell, G. G» Kelley and N, H. Lazar, IRE Trans., NS-3, (November 1956).

Ellett, W. H., Proc- of the Total Absorption Gamma-Ray Spectrometry Symposium, TIP 7594, (1960), 60-70.

Faissner, H., F. Ferrero, A. Ghani and M. Reinharz, Nucleonics, 21,(February 1963), 50.

Goldstein, Herbert, Fundamental Aspects of Reactor Shielding, Reading Addison-Wesley Publishing Company, Inc., (1959), 221.

■Kaplan, Irving, Nuclear Physics, 2d ed., Reading: Addison-WesleyPublishing Company, Inc., (1962), Chap. 15.

Ott, Donald.G., F. Newton Hayes, Jay E. Hammel and John F. Kephart, Nucleonics, 13, (May 1955), 62.

Perkins, R. W., J. M. Nielson and R. N. Diebel, Proc. of the TotalAbsorption Gamma-Ray Spectrometry Symposium, TIP 7594, (1960) 48-59.

Perkins, R. W. , Nucl. Instr. & Meth. , 3_3? 71, (1965)

Watt, D. E« and D. Ramsden. High Sensitivity Counting Techniques,New York: The Macmillan Company, (1964).