environmental laboratory accreditation course for radiochemistry: day two presented by minnesota...

Environmental Laboratory Accreditation Course for Radiochemistry: DAY TWO

Presented byMinnesota Department of HealthPennsylvania Department of Environmental ProtectionU.S. Environmental Protection AgencyWisconsin State Laboratory of Hygiene

Instrumentation & Methods: Alpha Scintillation CounterRa226, Ra228

Lynn West

Wisconsin State Lab of Hygiene

Method Review

Radium 226 (EPA 903.1) Radium 228 (EPA 904.0) Alpha-Emitting Radium Isotopes

(EPA 903.0)

Radium Chemistry

Chemically similar to Ca & Ba +2 oxidation state in solution Insoluble salts include: CO3, SO4, &

CrO4

Forms a complex with EDTA Property used extensively in analytical

procedures

Radiochemical Characteristics

Isotope T1/2 Decay Mode

Series

223Ra 11.1 D Alpha Actinium (235U)

224Ra 3.6 D Alpha Thorium (232Th)

226Ra 1622 A Alpha Uranium (238U)

228Ra 5.8 A Beta Thorium (232Th)

Radium 226 (EPA 903.1)

Prescribed Procedures for Measurement of Radioactivity in Drinking Water

EPA 600 4- 80-032 August 1980

Interferences

No radioactive interferences The original method does not use a

yield correction

238U decay series

903.1 Method Summary

1 L acidified sample Ra co-precipitated with stable Ba as

SO4

Precipitate is separated from sample matrix & supernate is discarded

Method summary cont.

(Ba-Ra)SO4 is dissolved in EDTA Solution Transferred to a “bubbler” After a period of ingrowth, 222Rn is

purged for sample & collected in scintillation cell

.

A typical radon de-emanation system

Helium gas in

Vacuum applied

Stopcock 1

Stopcock 2

Stopcock 4

Stopcock 3

Sintered disc

Vacuum gaugeScintillation cell

Stopcock 5

Solutionlevel

Bubbler

Stopcock

O-ring joint

Support

Components

Bubbler Scintillation Cell Vacuum System

& gauge Avoid using Hg

manometer if possible

Scintillation Cell

222Rn from sample is collected in the cell

Progeny establish secular equilibrium in about 4 hrs

The alpha counts from 222Rn & its progeny are collected

Zn(Tl)S

Quartz Window

Alpha Scintillation Cell Counter

Sample counted 4 hrs after de-emanantion

Alpha particles interact with Zn(Ag)S coating & emit light

Light flashes are counted on a scaler

Radon Cell Counters

Instrument Calibration

Each instrument system & scintillation cell needs to be calibrated

Calibration samples should be prepared in the same manner as the samples.

The entire de-emanation system effects the calibration measurement

Use NIST traceable standards

Perform yearly or after repairs

Calculations

)exp(1)exp(

1

)exp(1

1

22.23

3

21t

t

ttYVE

BGD

)exp(1)exp(

1

)exp(1

1

22.2

/)(96.1

3

3

21

3

t

t

ttYVE

tBGUNC

)exp(1)exp(

1

)exp(1

1

22.2

/66.4

3

3

21

3

t

t

ttYVE

tBDL

Calculations cont.

Computer programs should be hand verified

Decay constants and time intervals must be in the same units of time

Minimum background count time should be equal to the minimum sample count time

Method Quality Control

Per each batch of 20 samples, analyze the following: Method blank Laboratory control sample Precision sample Matrix spike sample

Established action limits for each

Method Quality Control, cont.

Instrument operating procedure should describe Daily control charts and acceptance

limits Required action Preventative maintenance

Method SOP main sections

SCOPE AND APPLICATION SUMMARY OF METHOD REGULATORY DEVIATIONS METHOD PERFORMANCE SAFETY SAMPLE HANDLING &

PRESERVATION INTERFERENCES DEFINITIONS EQUIPMENT REAGENTS

METHOD: DETERMINATION OF 226RA

CALIBRATION OF SCINTILLATION CELLS

CALCULATIONS QUALITY CONTROL WASTE DISPOSAL POLLUTION PREVENTION REFERENCES FIGURES

Radium 228 (EPA 904.0)


EPA 600 4- 80-032 August 1980

Interferences

The presence of 90Sr in the water samples gives a positive bias to the measured 228Ra activity.

Due to the short half-life of 228Ac, a emitter of similar energy is substituted during instrument calibration. A high or low bias may result depending on which isotope is selected.

Natural Ba may result in falsely high chemical yield.

232Th- decay series


228Ra in a drinking water sample is co-precipitated with Ba & Pb as SO4

The (Ba-Ra)SO4 precipitate is dissolved in basic EDTA. The progeny, 228Ac, is chemically separated from its parent by repeatedly forming the (Ba-Ra)SO4

Allow at least 36 hrs for the ingrowth of 228Ac & secular equilibrium

904.0 Method Summary, cont.

228Ac is then separated from 228Ra by precipitation as a OH-. (Save supernate)

This is the end of ingrowth & the beginning of 228Ac decay

228Ac is co-precipitated with Y as (Ac-Y2(C2O4)3)

904.0 Method Summary, cont.

Transferred to a planchet & counted on a low-background / proportional counter

The Ba carrier yield is found by precipitating the Ba from the supernatant as BaSO4

Instrumentation

Low background gas flow proportional counter P-10 counting gas (10% CH4 & 90% Ar)

Due to short half-life of 228Ac, a multi-detector system is desirable 6.13 hr Processing time from start of decay to

count is about 250 m

Gas flow proportional counterwindow assembly

Instrument Calibration

Each instrument system needs to be calibrated

Calibration samples should be prepared in the same manner as the samples.

Use isotope with beta energy approximately equal to 0.404 keV



Calculations

)exp(

1

)exp(1

1

)exp(122.2132

2

ttt

t

RVE

BGD

)exp(1

)exp(11

)exp(122.2

/)(96.1

132

22

tttt

RVE

tBGUNC

)exp(1

)exp(11

)exp(122.2

/66.4

132

22

tttt

RVE

tBDL

Method Quality Control


Established action limits for each

Method Quality Control, cont.



Method SOP main sectionsMethod SOP main sections




CALIBRATION OF INSTRUMENT


Alpha-Emitting Radium Isotopes (EPA 903.0)


EPA 600 4- 80-032 August 1980

Interferences (EPA 903.0)

Natural Ba may result in falsely high chemical yield

Ingrowth of progeny must be corrected for Method only corrects for 226Ra progeny

Does not accurately measure 226Ra if other alpha emitting isotopes are present

Calibration based only on 226Ra

Ac-2286.13 hours

Th-2321.4×1010 y

Ra-2285.75 y

Th-2281.90 y

Ra-2243.64 days

Rn-22054.5 s

Tl-2083.1 m

Po-212300 ns

Po-216158 ms

Pb-208stable

Pb-21210.6 hours

Bi-21260.6 m

Massnumber(N)

Atomicnumber(Z)

67%

33%

beta decay

alpha decay

232Th- decay series

Pa-2341.18 m

238U decay series

beta decay

Atomicnumber(Z)

Massnumber(N)

alpha decay

Po-210138.4 d

U-2384.4×109 y

Th-23424.1 d

U-2342.48×105 y

Th-2308.0×104 y

Ra-2261622 y

Rn-2223.825 d

Pb-206stable

Po-2141.6×10-4 s

Po-2183.05 m

Bi-2105.0 d

Pb-21022 a

Pb-21426.8 m

Bi-21419.7 m

Pa-2313.48×104 y

235U decay series

beta decay

Atomicnumber(Z)

Massnumber(N)

alpha decay

U-2357.3×108 y

Th-23125.6 h

Th-22718.17 d

Ra-22311.7 d

Rn-2193.92 s

Po-2110.52 s

Po-2151.83×10-3 s

Bi-2105.0 d

Pb-207stable

Pb-21136.1 m

Bi-2112.15 m

Ac-22722.0 y

Fr-22322 m

At-2190.9 m

Bi-2158 m

At-21510-4 s

Tl-2074.79 m


1 L acidified sample Ra co-precipitated with stable Ba &

Pb as SO4 223Ra 224Ra 226Ra

Precipitate is separated from sample matrix & supernate is discarded

903.0 Method Summary, Cont.

Progeny ingrowth starts with the final (Ba-Ra)SO4 precipitation. Since a correction factor is applied to

correct for ingrowth, care needs to be taken to avoid disturbing the radon progeny ingrowth after this step

Transfer to tared planchet & dry under infra-red heat lamp

Instrumentation (EPA 903.0)

Low background gas flow proportional counter P-10 counting gas (10% CH4 & 90% Ar)

Alpha scintillation counter

Instrument Calibration (EPA 903.0)

Each instrument system needs to be calibrated

Calibration samples should be prepared using 226Ra



Calculations (EPA 903.0)

RIVEBG

D

22.2

RIVE

tBGUNC

22.2

/)(96.1

RIVEtB

DL

22.2

/66.4

Method Quality Control (EPA 903.0)


Established action limits for each Demonstration of capability

Method Quality Control, Cont. (903.0)



Method SOP main sectionsMethod SOP main sections (903.0)




CALIBRATION OF INSTRUMENT


Instrumentation & Methods: Gamma Spectroscopy

Lynn West

Wisconsin State Lab of Hygiene

Instrumentation – Gamma Spectroscopy/Alpha Spectroscopy

Quick review of Radioactive Decay (as it relates to σ & spectroscopy)

Interaction of Gamma Rays with matter Basic electronics Configurations Semi-conductors Resolution Spectroscopy Calibration/Efficiency Coincidence summing Sample Preparation Daily instrument checks

Review of Radioactive Modes of Decay

Properties of Alpha Decay Progeny loses of 4 AMU. Progeny loses 2 nuclear charges Often followed by emission of gamma

226

88Ra 22286

Rn + 42He + energy

Review of Radioactive Modes of Decay, Cont.

Properties of Alpha Decay Alpha particle and

progeny (recoil nucleus) have well-defined energies

spectroscopy based on alpha-particle energies is possible

Energy (MeV)

Cou

nts

4.5 5.5

Alpha spectrum at the theoretical limit of energy resolution


Properties of beta (negatron) decay No change in mass number of progeny. Progeny gains 1 nuclear charge Beta particle, antineutrino, and recoil

nucleus have a continuous range of energies

no spectroscopy of elements is possible Often followed by emission of gamma

Review of Radioactive Modes of Decay, cont.

Cou

nts

Ar-36

Cl-36

Energy (MeV)

Beta Emission from Cl-36.

From G. F. Knoll,Radiation Detection and Measurement, 3rd Ed., (2000).


Properties of Positron decay No change in mass number of progeny Progeny loses 1 nuclear charge Positron, neutrino, and recoil nucleus

have a continuous range of energies no spectroscopy of elements is possible

Positron is an anti-particle of an electron


Properties of Positron decay When the positron comes in contact

with an electron, the particles are annihilated

Two photons are created each with an energy of 511 keV (the rest mass of an electron)

The annihilation peak is a typical feature of a spectrum


Other modes of decay Electron Capture

Neutron deficient isotopes Electron is captured by the nucleus from

an outer electron shell Vacancy left from captured electron is

filled in by electrons from higher energy shells

X-rays are released in the process


Other modes of decay Auger electrons

Excitation of the atom resulting in the ejection of an outer electron

Internal conversion electrons Excitation of the nucleus resulting in the

ejection of an outer electron Bremsstrahlung

“Braking” radiation Photon emitted by a charged particle as it

slows down Adds to the continuum


Gamma Emission No change in mass, protons, or

neutrons Excess excitation energy is given off as

electromagnetic radiation, usually following alpha or beta decay

Gamma emissions are high-energy, short-wave-length

Source:

http://lasp.colorado.edu


Gamma Emission Decay Schemes

Pb S

hie

ldin

g

Pb S

hie

ldin

g

e-

e

511 γ

511 γ

γ

γ

Pb X Ray

e-

PE

e-

e-

CS

γ γ

Source

γ

γ

γe 511 γ

511 γ

e-

e-

e-PP

CS

CS

KEYPE Photoelectric absorptionCS Compton scatteringPP Pair productionγ gamma-raye- ElectronePositron

Gamma Spectrum Features

Source: Practical Gamma-Ray Spectrometry, Gilmore & Hemingway

Resolution

Basic Electronic Schematic – Gamma Spectroscopy

Detector Bias Supply

Detector PreamplifierMultichannel Analyzer (MCA)Amplifier

Low Voltage Supply

Configurations of Ge Detectors

Electrical contact

True coaxial Closed-end coaxial

Holes

Electrons

+

Holes

Electrons

p-type coaxial, ∏-type

n-type coaxial, v-type

p+ contact

n+ contact

Nature of Semi-conductors

Good conductors are atoms with less than four valence electrons

atoms with only 1 valance electron are the best conductors

examples copper silver gold

Nature of Semi-conductors, Cont.

Good insulators are atoms with more than four valence electrons

atoms with 8 valance electron are the best insulators

examples noble gases


Semiconductors are made of atoms with four valence electrons

they are neither good conductors nor good insulators

examples germanium silicon


Energy Band Diagram

VALENCE BANDVALENCE BAND

FORBIDDEN BAND

VALENCE BAND

FORBIDDEN BAND

CONDUCTION BAND CONDUCTION

BANDCONDUCTION

BAND

Insulator Semiconductor Conductor


Covalent bonds are formed in semiconductors the atoms are arranged in definite

crystalline structure the arrangement is repeated

throughout the material each atom is covalently bonded to 4

other atoms

Nature of Semi-conductors, cont. Pure Semi-conductor

Each atom has 8 shared electrons there are no free electrons

or no electrons in the conduction band however, thermal energy can cause

some valence electrons to gain enough energy to move in to the conduction band this leads to the formation of a “hole”


Both holes (+) & free electrons (-) are current carriers

a pure semi conductor has few carriers of either type

more carriers lead to more current doping is the process used to

increase the number of carriers in a semiconductor


Impurities can be added during the production of the semiconductor, this is called doping

The impurities are either trivalent or pentavalent

trivalent examples indium, gallium, boron

pentavalent examples arsenic, phosphorus, antimony

n-type Semiconductor

An impurity with 5 valence electrons (group V) will form 4 covalent bonds with the atoms of the semiconductor

One electron is left over & loosely held by the atom

This type of impurity is known as donor impurities.

There are more negative carriers

n-type Semiconductor

VALENCE BAND

CONDUCTION BAND

Valence electron forbidden band

Donor electron forbidden band

Donor electron Energy level

p-type semiconductors

An impurity with 3 valence electrons (group III) will form 3 covalent bonds with the atoms of the semiconductor

The absence of the fourth electron leaves a hole

This type of impurity is known as acceptor impurities.

There are more positive carriers

p-type Semiconductor, cont.

VALENCE BAND

CONDUCTION BAND

Valence electron forbidden band

Acceptor hole forbidden band

Acceptor hole Energy level

Depletion Zone

In the depletion zone the charge carriers have canceled each other out

voltage is developed across the depletion zone due to the charge separation

+-

p-type n-type

Vc

Depletion zone

V

+

++

+ ++

+

+

++

+ ---

-

- --- -

++++

+++

++

++

+

------

---

-

Calibration/Efficiency

Ideally, calibration sources would be prepared such that a point calibration is performed for each nuclide reported this is totally impractical for analyzing

routine unknown samples Sources should be prepared to have

identical shape and density as the sample


Differences in density are less important than differences in geometry Newer software packages allow the

user to create different efficiencies mathematically

Source strength should not be so great as to cause pile-up


The calibration energies should cover the entire range of interest

For close to the detector geometries, choose a multi-lined source made from a combination of nuclides which do not suffer from True Coincidence Summing (TCS). See Table 7.2 pg 153 Gilmore, G. and Hemingway, J. 1995. Practical Gamma-Ray Spectrometry. John Wiley & Sons, New York

Coincidence Summing

True Coincidence Summing (TCS) The summing of gamma rays emitted

almost simultaneously from the nucleus resulting in a negative bias from the true value

Larger detectors suffer more from TCS than do smaller detectors

TCS can be expected whenever samples contain nuclides with complicated decay schemes

Coincidence Summing

True Coincidence Summing (TCS) TCS can be expected whenever

samples contain nuclides with complicated decay schemes

The degree of TCS is not dependent on count rate

TCS is geometry dependent and is worse for close to the detector geometries

Coincidence Summing

True Coincidence Summing (TCS) TCS is geometry dependent and is

worse for close to the detector geometries

Summed pulses will not be rejected by the pile-up rejection circuitry because the pulses will not be misshapen

For detectors with thin windows X-rays that would normally be absorbed in the end cap may contribute to TCS

Well detectors suffer the worst from TCS

Coincidence Summing

True Coincidence Summing (TCS) Newer software packages have systems

for reduces this problem

Coincidence Summing

Random Coincidence Summing Also known as pile-up Two or more gamma rays being

detected at nearly the same time Counts are lost from the full-energy

peaks in the spectrum Affected by count rate Pile-up rejection circuitry reduces

problem

Sample Preparation

Acidify water samples Note: Iodine is volatile in acidic solutions

Active material should be distributed evenly throughout the geometry Samples should be homogenous

Calibration materials should simulate samples (actual or mathematical)

Daily Instrument Checks

Short background count Linearity check Resolution check Additionally, a long background

count is needed for background subtraction

Instrumentation & Methods: Gamma Emitting Radionuclides USEPA 901.1

Jeff Brenner

Minnesota Department of Health

EPA Method 901.1Gamma Emitting Radionuclides

Gamma Emitting Radionuclides

EPA Method 901.1What we’ll cover Scope of the method Summary of the method Calibration

Determining energy calibration Determining efficiency calibration Determining system background

Quality control Interferences Application Calculations

Activity

EPA Method 901.1Scope

The method is applicable for analyzing water samples

Measurement of gamma photons emitted from radionuclides without separating them from the sample matrix.

Radionuclides emitting gamma photons with the following energy range of 60 to 2000 keV.

EPA Method 901.1 Gamma Emitting Radionuclides Summary

Water sample is preserved in the field or lab with nitric acid

Homogeneous aliquot of the preserved sample is measured in a calibrated geometry.


Sample aliquots are counted long enough to meet the required sensitivity.

EPA Method 901.1 Calibrations Gamma Emitting Radionuclides

Library of radionuclide gamma energy spectra is prepared

Use known radionuclide concentrations in standard sample geometries to establish energy calibration.

Single solution containing a mixture of fission products emitting Low energy Medium energy High energy Example (Sb-125, Eu154, and Eu-155)


86.54 Eu-155 105.31 Eu-155 123.07 Eu-154 176.33 Sb-125 247.93 Eu-154 427.89 Sb-125 463.38 Sb-125 591.76 Eu-154 600.56 Sb-125 635.90 Sb-125 692.42 Eu-154 723.30 Eu-154 756.86 Eu-154 873.20 Eu-154 996.30 Eu-1541004.76 Eu-1541274.51 Eu-1541596.45 Eu-154

EPA Method 901.1 Gamma Emitting Radionuclides

Counting efficiencies for the various gamma energies are determined from the activity counts of those known standard values.

A counting efficiency vs. gamma energy curve is determined for each container geometry and for each detector.


86.54 Eu-155

105.31 Eu-155

176.33 Sb-125

427.89 Sb-125

463.38 Sb-125

600.56 Sb-125

996.30 Eu-154

1004.76 Eu-154

1274.51 Eu-154

EPA Method 901.1 Calibrations Gamma Emitting Radionuclides

FWHM used to monitor peak shape Smaller tolerance for low energy Greater tolerance for high energy

Document a few FWHM to determine instrument drift


86.54 Eu-155 105.31 Eu-155 123.07 Eu-154 176.33 Sb-125 247.93 Eu-154 427.89 Sb-125 463.38 Sb-125 591.76 Eu-154 600.56 Sb-125 635.90 Sb-125 692.42 Eu-154 723.30 Eu-154 756.86 Eu-154 873.20 Eu-154 996.30 Eu-1541004.76 Eu-1541274.51 Eu-1541596.45 Eu-154

EPA Method 901.1 (Determine System Background)

Contribution of the background must be measured

Measure under the same conditions, counting mode, as the samples

Background determination is performed every time the liquid nitrogen is filled

EPA Method 901.1 (Batch Quality Control)

Instrument efficiency check Analyzed daily Control chart Establish action limits

Low background check Analyzed weekly Control chart Establish action limits

Analytical Batch Sample Duplicates at a 10% frequency Sample Spikes at a 5% frequency Control chart Establish action limits

EPA Method 901.1Interferences Significant interference occurs when

counting a sample with a NaI(Tl) detector. Sample radionuclides emit gamma

photons of nearly identical energies. Sample homogeneity is important to

gamma count reproducibility and counting efficiency. Add HNO3 to water sample container to

lessen the problem of radionuclides adsorbing to the container

EPA Method 901.1Application

The limits set forth in PL 93-523, 40 CFR 34324 recommend that in the case of man-made radionuclides, the limiting concentration is that which will produce an annual dose equivalent to 4 mrem/year.

If several radionuclides are present, the sum of their annual dose equivalent must not exceed 4 mrem/year.

environmental laboratory accreditation course for radiochemistry: day two presented by minnesota...

Documents

th slide

possible slide

scaler slide

analytical procedures

yield correction slide

ac secular equilibrium

method summary

zntls quartz window