ASTM D3649-06(2014)

Standard Practice for High-Resolution Gamma-Ray Spectrometry of Water (Withdrawn 2023)

SIGNIFICANCE AND USE
5.1 Gamma-ray spectrometry is of use in identifying radionuclides and in making quantitative measurements. Use of a semiconductor detector is necessary for high-resolution measurements.  
5.2 Variation of the physical geometry of the sample and its relationship with the detector will produce both qualitative and quantitative variations in the gamma-ray spectrum. To adequately account for these geometry effects, calibrations are designed to duplicate all conditions including source-to-detector distance, sample shape and size, and sample matrix encountered when samples are measured.  
5.3 Since some spectrometry systems are calibrated at many discrete distances from the detector, a wide range of activity levels can be measured on the same detector. For high-level samples, extremely low-efficiency geometries may be used. Quantitative measurements can be made accurately and precisely when high activity level samples are placed at distances of 10 cm or more from the detector.  
5.4 Electronic problems, such as erroneous deadtime correction, loss of resolution, and random summing, may be avoided by keeping the gross count rate below 2000 counts per second (s–1) and also keeping the deadtime of the analyzer below 5 %. Total counting time is governed by the radioactivity of the sample, the detector to source distance and the acceptable Poisson counting uncertainty.
SCOPE
1.1 This practice covers the measurement of gamma-ray emitting radionuclides in water by means of gamma-ray spectrometry. It is applicable to nuclides emitting gamma-rays with energies greater than 45 keV. For typical counting systems and sample types, activity levels of about 40 Bq are easily measured and sensitivities as low as 0.4 Bq are found for many nuclides. Count rates in excess of 2000 counts per second should be avoided because of electronic limitations. High count rate samples can be accommodated by dilution, by increasing the sample to detector distance, or by using digital signal processors.  
1.2 This practice can be used for either quantitative or relative determinations. In relative counting work, the results may be expressed by comparison with an initial concentration of a given nuclide which is taken as 100 %. For quantitative measurements, the results may be expressed in terms of known nuclidic standards for the radionuclides known to be present. This practice can also be used just for the identification of gamma-ray emitting radionuclides in a sample without quantifying them. General information on radioactivity and the measurement of radiation has been published (1,2).2 Information on specific application of gamma spectrometry is also available in the literature (3-5). See also the referenced ASTM Standards in 2.1 and the related material section at the end of this standard.  
1.3 This standard does not purport to address the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitation prior to use.

General Information

• Status: Historical
• Publication Date: 31-May-2014
• ICS: 71.040.50 - Physicochemical methods of analysis
• Technical Committee: D19 - Water
• Drafting Committee: D19.04 - Methods of Radiochemical Analysis

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D3649 − 06 (Reapproved 2014)
Standard Practice for
High-Resolution Gamma-Ray Spectrometry of Water
This standard is issued under the fixed designation D3649; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This practice covers the measurement of gamma-ray 2.1 ASTM Standards:
emitting radionuclides in water by means of gamma-ray D1066Practice for Sampling Steam
spectrometry. It is applicable to nuclides emitting gamma-rays D1129Terminology Relating to Water
withenergiesgreaterthan45keV.Fortypicalcountingsystems D2777Practice for Determination of Precision and Bias of
and sample types, activity levels of about 40 Bq are easily Applicable Test Methods of Committee D19 on Water
measuredandsensitivitiesaslowas0.4Bqarefoundformany D3370Practices for Sampling Water from Closed Conduits
nuclides. Count rates in excess of 2000 counts per second D3648Practices for the Measurement of Radioactivity
should be avoided because of electronic limitations. High D4448GuideforSamplingGround-WaterMonitoringWells
count rate samples can be accommodated by dilution, by E181Test Methods for Detector Calibration andAnalysis of
increasing the sample to detector distance, or by using digital Radionuclides
signal processors.
3. Terminology
1.2 This practice can be used for either quantitative or
3.1 Definitions—For definitions of terms used in this
relative determinations. In relative counting work, the results
practice, refer toTerminology D1129. For terms not defined in
may be expressed by comparison with an initial concentration
thispracticeorinTerminologyD1129,referencemaybemade
of a given nuclide which is taken as 100%. For quantitative
to other published glossaries.
measurements,theresultsmaybeexpressedintermsofknown
nuclidic standards for the radionuclides known to be present.
4. Summary of Practice
This practice can also be used just for the identification of
4.1 Gamma ray spectra are measured with modular equip-
gamma-ray emitting radionuclides in a sample without quan-
ment consisting of a detector, high-voltage power supply,
tifying them. General information on radioactivity and the
preamplifier,amplifierandanalog-to-digitalconverter(ordigi-
measurement of radiation has been published (1,2). Informa-
tal signal processor), multichannel analyzer, as well as a
tion on specific application of gamma spectrometry is also
computer with display.
available in the literature (3-5). See also the referencedASTM
Standards in 2.1 and the related material section at the end of
4.2 High-purity germanium (HPGe) detectors, p-type or
this standard.
n-type,areusedfortheanalysisofcomplexgamma-rayspectra
because of their excellent energy resolution.These germanium
1.3 This standard does not purport to address the safety
systems, however, are characterized by high cost and require
concerns, if any, associated with its use. It is the responsibility
cooling.Liquidnitrogenorelectromechanicalcooling,orboth,
of the user of this standard to establish appropriate safety and
can be used.
health practices and determine the applicability of regulatory
limitation prior to use.
4.3 In a germanium semiconductor detector, gamma-ray
photons produce electron-hole pairs. The charged pair is then
1 collected by an applied electric field. A very stable low noise
This practice is under the jurisdiction ofASTM Committee D19 on Water and
is the direct responsibility of Subcommittee D19.04 on Methods of Radiochemical
preamplifier is needed to amplify the pulses of electric charge
Analysis.
CurrenteditionapprovedJune1,2014.PublishedJuly2014.Originallyapproved
in 1978. Last previous edition approved in 2006 as D3649–06. DOI: 10.1520/ For referenced ASTM standards, visit the ASTM website, www.astm.org, or
D3649-06R14. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
The boldface numbers in parentheses refer to a list of references at the end of Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3649 − 06 (2014)
resultingfromgammaphotoninteractions.Theoutputfromthe double escape peaks at energies of 0.511 or 1.022 MeV less
preamplifier is directly proportional to the energy deposited by than the photopeak energy. In the plot of pulse height versus
the incident gamma-ray. These current pulses are fed into an count rate, the size and location of the photopeak on the pulse
amplifier of sufficient gain to produce voltage output pulses in height axis is proportional to the number and energy of the
the amplitude range from 0 to 10 V. incident photons, and is the basis for the quantitative and
qualitative application of the spectrometer. The Compton
4.4 A multichannel pulse-height analyzer is used to deter-
continuum serves no useful purpose in photopeak analysis and
mine the amplitude of each pulse originating in the detector,
must be subtracted when peaks are analyzed.
and accumulates in a memory the number of pulses in each
amplitude band (or channel) in a given counting time. Com- 4.6 If the analysis is being directed and monitored by an
puterizedsystemswithstoredprogramsandinterfacehardware online computer program, the analysis period may be termi-
can accomplish the same functions as hardwired multichannel nated by prerequisites incorporated in the program. If the
analyzers.Theprimaryadvantagesofthecomputerizedsystem analysis is being performed with a modern multichannel
include the capability of programming the multi-channel ana- analyzer, analysis may be terminated when a preselected time
or total counts in a region of interest or in a specified channel
lyzer functions and the ability to immediately perform data
reduction calculations using the spectral data stored in the is reached. Visual inspection of a display of accumulated data
computer memory or mass storage device. Fora0to 2-MeV can also be used as a criterion for manually terminating the
spectrum, 4000 or more channels are typically needed in order analysis on either type of data acquisition systems.
to fully utilize a germanium detector’s excellent energy reso-
4.7 Upon completion of the analysis, the spectral data are
lution.
interpreted and reduced to include activity of Bq (disintegra-
4.5 The distribution of the amplitudes (pulse heights) of the tion per second) or related units suited to the particular
pulses can be separated into two principal components. One of application.At this time the spectral data may be inspected to
these components has a nearly Gaussian distribution and is the identify the gamma-ray emitters present. This is accomplished
result of total absorption of the gamma-ray energy in the by reading the channel number from the x-axis and converting
detector. This peak is normally referred to as the full-energy to gamma-ray energy by multiplying by the appropriate keV/
peak or photopeak. The other component is a continuous one channel(systemgain).Insomesystemsthechannelnumberor
lower in energy than that of the photopeak. This continuous gamma-ray energy in keV can be displayed for any selected
curve is referred to as the Compton continuum and is due to channel. Identification of nuclides may be aided by catalogs of
interactions wherein the gamma photons deposit only part of gamma-ray spectra and other nuclear data tabulations (3,6-8).
theirenergyinthedetector.Thesetwoportionsofthecurveare 4.7.1 Computerprogramsfordatareductionhavebeenused
shown in Fig. 1. Other peaks, such as escape peaks, backscat- extensively although calculations for some applications can be
tered gamma rays or X rays from shields, are often superim- performed effectively with the aid of a scientific calculator.
posed on the Compton continuum. Escape peaks will be Data reduction of spectra taken with germanium spectrometry
presentwhengamma-rayswithenergiesgreaterthan1.02MeV systems is usually accomplished by integration of the photo-
are emitted from the sample. The positron formed in pair peaks above a definable background (or baseline) and subse-
production is usually annihilated in the detector and one or quent activity calculations using a library which includes data
both of the 511–keV annihilation quanta may escape from the such as nuclide name, half-life, gamma-ray energies, and
detectorwithoutinteraction.Thisconditionwillcausesingleor absolute gamma intensity.
FIG. 1 Cesium-137 Spectrum
D3649 − 06 (2014)
5. Significance and Use avoided by preparing the standards for calibration in solutions
or other matrices with a density comparable to the sample
5.1 Gamma-ray spectrometry is of use in identifying radio-
being analyzed.
nuclides and in making quantitative measurements. Use of a
semiconductor detector is necessary for high-resolution mea-
7. Apparatus
surements.
5.2 Variation of the physical geometry of the sample and its 7.1 Gamma Ray Spectrometer, consisting of the following
relationshipwiththedetectorwillproducebothqualitativeand
components:
quantitative variations in the gamma-ray spectrum. To ad-
7.1.1 Detector Assembly:
equately account for these geometry effects, calibrations are
7.1.1.1 Germanium Detector—The detector may have a
designed to duplicate all conditions including source-to-
volumeofabout50to150cm ,withafullwidthatone-halfthe
detector distance, sample shape and size, and sample matrix
peak maximum (FWHM) less than 2.2 keV at 1332 keV,
encountered when samples are measured.
certified by the manufacturer. A charge-sensitive preamplifier
5.3 Sincesomespectrometrysystemsarecalibratedatmany
using low noise field effect transistors should be an integral
discrete distances from the detector, a wide range of activity
part of the detector assembly.Aconvenient support should be
levels can be measured on the same detector. For high-level
provided for samples of the desired form.
samples, extremely low-efficiency geometries may be used.
7.1.1.2 Shield—The detector assembly may be surrounded
Quantitative measurements can be made accurately and pre-
by an external radiation shield made of a dense metal,
cisely when high activity level samples are placed at distances
equivalent to 102 mm of lead in gamma-ray attenuation
of 10 cm or more from the detector.
capability. It is desirable that the inner walls of the shield be at
5.4 Electronic problems, such as erroneous deadtime
least 127 mm distant from the detector surfaces to reduce
correction, loss of resolution, and random summing, may be
backscatter. If the shield is made of lead or a lead liner, the
avoidedbykeepingthegrosscountratebelow2000countsper
shield may have a graded inner shield of 1.6 mm of cadmium
–1
second (s ) and also keeping the deadtime of the analyzer
or tin lined with 0.4 mm of copper, to attenuate the 88-keVPb
below 5%. Total counting time is governed by the radioactiv-
X-rays.The shield should have a door or port for inserting and
ity of the sample, the detector to source distance and the
removing samples.
acceptable Poisson counting uncertainty.
7.1.1.3 High Voltage Power/Bias Supply—The bias supply
6. Interferences requiredforgermaniumdetectorsusuallyprovidesavoltageup
to 5000 V and up to 100 µA. The power supply should be
6.1 In complex mixtures of gamma-ray emitters, the degree
regulated to 0.1% with a ripple of not more than 0.01%. Line
of interference of one nuclide in the determination of another
noise caused by other equipment should be removed with rf
isgovernedbyseveralfactors.Ifthegamma-rayemissionrates
filters and additional regulators.
fromdifferentradionuclidesaresimilar,interferencewilloccur
7.1.1.4 Amplifier—Anamplifiercompatiblewiththepream-
when the photopeaks are not completely resolved and overlap.
If the nuclides are present in the mixture in unequal portions plifier and with the pulse-height analyzer shall be provided.
radiometrically, and if nuclides of higher gamma-ray energies
7.1.2 Data Acquisition and Storage Equipment:
are predominant, there are serious interferences with the
7.1.2.1 Data Acquisitions—A multichannel pulse-height
interpretation of minor, less energetic gamma-ray photopeaks.
analyzer (MCA) or stand-alone analog-to-digital-converter
The complexity of the analysis method is due to the resolution
(ADC) under software control of a separate computer, per-
of these interferences and, thus, one of the main reasons for
forms many functions required for gamma-ray spectrometry.
computerized systems.
An MCA or computer collects the data, provides a visual
6.2 Cascade summing may occur when nuclides that decay
display, and outputs final results or raw data for later analysis.
by a gamma-ray cascade are analyzed. Cobalt-60 is an ex-
The four major components of an MCA are the ADC, the
ample; 1172 and 1333-keV gamma rays from the same decay
memory, control, and input/output. More recently, digital
may enter the detector to produce a sum peak at 2505 keVand
signal processors (DSP) can directly amplify and digitize
cause the loss of counts from the other two peaks. Cascade
signals from the preamplifier, replacing individual amplifier
summing may be reduced by increasing the source to detector
and ADC components. The ADC digitizes the analog pulses
distance.Summingismoresignificantifawell-typedetectoris
from the amplifier. These pulses represent energy. The digital
used.
resultisusedbytheMCAtoselectamemorylocation(channel
6.3 Random summing is a function of counting rate and number) which is used to store the number of events which
occurs in all measurements. The random summing rate is have occurred with that energy. Simple data analysis and
proportional to the total count squared and the resolving time control of the MCA is accomplished with microprocessors.
of the detector. For most systems random summing losses can These processors control the input/output, channel summing
be held to less than 1% by limiting the total counting rate to
over set regions of interest, and system energy calibration to
–1
2000 counts per second (s ). Refer to Test Methods E181 for
name a few examples.
more information.
7.1.2.2 Data Storage—Because of the use of microproces-
6.4 The density of the sample is another factor that can sors modern MCAs provide a wide range of input and output
effect quantitative results. Errors from this source can be (I/O) capabilities.
D3649 − 06 (2014)
8. Sampling 10.2 Procedure:
10.2.1 Prep
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D3649 − 06 D3649 − 06 (Reapproved 2014) An American National Standard
Standard Practice for
High-Resolution Gamma-Ray Spectrometry of Water
This standard is issued under the fixed designation D3649; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This practice covers the measurement of gamma-ray emitting radionuclides in water by means of gamma-ray spectrometry.
It is applicable to nuclides emitting gamma-rays with energies greater than 45 keV. For typical counting systems and sample types,
activity levels of about 40 Bq are easily measured and sensitivities as low as 0.4 Bq are found for many nuclides. Count rates in
excess of 2000 counts per second should be avoided because of electronic limitations. High count rate samples can be
accommodated by dilution, by increasing the sample to detector distance, or by using digital signal processors.
1.2 This practice can be used for either quantitative or relative determinations. In relative counting work, the results may be
expressed by comparison with an initial concentration of a given nuclide which is taken as 100 %. For quantitative measurements,
the results may be expressed in terms of known nuclidic standards for the radionuclides known to be present. This practice can
also be used just for the identification of gamma-ray emitting radionuclides in a sample without quantifying them. General
information on radioactivity and the measurement of radiation has been published (1,2). Information on specific application of
gamma spectrometry is also available in the literature (3-5). See also the referenced ASTM Standards in 2.1 and the related material
section at the end of this standard.
1.3 This standard does not purport to address the safety concerns, if any, associated with its use. It is the responsibility of the
user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitation
prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D1066 Practice for Sampling Steam
D1129 Terminology Relating to Water
D2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
D3370 Practices for Sampling Water from Closed Conduits
D3648 Practices for the Measurement of Radioactivity
D4448 Guide for Sampling Ground-Water Monitoring Wells
E181 Test Methods for Detector Calibration and Analysis of Radionuclides
3. Terminology
3.1 Definitions—For definitions of terms used in this practice, refer to Terminology D1129. For terms not defined in this practice
or in Terminology D1129, reference may be made to other published glossaries.
4. Summary of Practice
4.1 Gamma ray spectra are measured with modular equipment consisting of a detector, high-voltage power supply, preamplifier,
amplifier and analog-to-digital converter (or digital signal processor), multichannel analyzer, as well as a computer with display.
4.2 High-purity germanium (HPGe) detectors, p-type or n-type, are used for the analysis of complex gamma-ray spectra because
of their excellent energy resolution. These germanium systems, however, are characterized by high cost and require cooling. Liquid
nitrogen or electromechanical cooling, or both, can be used.
This practice is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.04 on Methods of Radiochemical Analysis.
Current edition approved Dec. 15, 2006June 1, 2014. Published January 2007July 2014. Originally approved in 1978. Last previous edition approved in 19982006 as
D3649 – 98a.D3649 – 06. DOI: 10.1520/D3649-06.10.1520/D3649-06R14.
The boldface numbers in parentheses refer to thea list of references at the end of this test method.standard.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3649 − 06 (2014)
4.3 In a germanium semiconductor detector, gamma-ray photons produce electron-hole pairs. The charged pair is then collected
by an applied electric field. A very stable low noise preamplifier is needed to amplify the pulses of electric charge resulting from
gamma photon interactions. The output from the preamplifier is directly proportional to the energy deposited by the incident
gamma-ray. These current pulses are fed into an amplifier of sufficient gain to produce voltage output pulses in the amplitude range
from 0 to 10 V.
4.4 A multichannel pulse-height analyzer is used to determine the amplitude of each pulse originating in the detector, and
accumulates in a memory the number of pulses in each amplitude band (or channel) in a given counting time. Computerized
systems with stored programs and interface hardware can accomplish the same functions as hardwired multichannel analyzers. The
primary advantages of the computerized system include the capability of programming the multi-channel analyzer functions and
the ability to immediately perform data reduction calculations using the spectral data stored in the computer memory or mass
storage device. For a 0 to 2-MeV spectrum, 4000 or more channels are typically needed in order to fully utilize a germanium
detector’s excellent energy resolution.
4.5 The distribution of the amplitudes (pulse heights) of the pulses can be separated into two principal components. One of these
components has a nearly Gaussian distribution and is the result of total absorption of the gamma-ray energy in the detector. This
peak is normally referred to as the full-energy peak or photopeak. The other component is a continuous one lower in energy than
that of the photopeak. This continuous curve is referred to as the Compton continuum and is due to interactions wherein the gamma
photons deposit only part of their energy in the detector. These two portions of the curve are shown in Fig. 1. Other peaks, such
as escape peaks, backscattered gamma rays or X rays from shields, are often superimposed on the Compton continuum. Escape
peaks will be present when gamma-rays with energies greater than 1.02 MeV are emitted from the sample. The positron formed
in pair production is usually annihilated in the detector and one or both of the 511–keV annihilation quanta may escape from the
detector without interaction. This condition will cause single or double escape peaks at energies of 0.511 or 1.022 MeV less than
the photopeak energy. In the plot of pulse height versus count rate, the size and location of the photopeak on the pulse height axis
is proportional to the number and energy of the incident photons, and is the basis for the quantitative and qualitative application
of the spectrometer. The Compton continuum serves no useful purpose in photopeak analysis and must be subtracted when peaks
are analyzed.
4.6 If the analysis is being directed and monitored by an online computer program, the analysis period may be terminated by
prerequisites incorporated in the program. If the analysis is being performed with a modern multichannel analyzer, analysis may
be terminated when a preselected time or total counts in a region of interest or in a specified channel is reached. Visual inspection
of a display of accumulated data can also be used as a criterion for manually terminating the analysis on either type of data
acquisition systems.
4.7 Upon completion of the analysis, the spectral data are interpreted and reduced to include activity of Bq (disintegration per
second) or related units suited to the particular application. At this time the spectral data may be inspected to identify the
gamma-ray emitters present. This is accomplished by reading the channel number from the x-axis and converting to gamma-ray
energy by multiplying by the appropriate keV/channel (system gain). In some systems the channel number or gamma-ray energy
in keV can be displayed for any selected channel. Identification of nuclides may be aided by catalogs of gamma-ray spectra and
other nuclear data tabulations (3,6-8).
FIG. 1 Cesium-137 Spectrum
D3649 − 06 (2014)
4.7.1 Computer programs for data reduction have been used extensively although calculations for some applications can be
performed effectively with the aid of a scientific calculator. Data reduction of spectra taken with germanium spectrometry systems
is usually accomplished by integration of the photopeaks above a definable background (or baseline) and subsequent activity
calculations using a library which includes data such as nuclide name, half-life, gamma-ray energies, and absolute gamma intensity.
5. Significance and Use
5.1 Gamma-ray spectrometry is of use in identifying radionuclides and in making quantitative measurements. Use of a
semiconductor detector is necessary for high-resolution measurements.
5.2 Variation of the physical geometry of the sample and its relationship with the detector will produce both qualitative and
quantitative variations in the gamma-ray spectrum. To adequately account for these geometry effects, calibrations are designed to
duplicate all conditions including source-to-detector distance, sample shape and size, and sample matrix encountered when
samples are measured.
5.3 Since some spectrometry systems are calibrated at many discrete distances from the detector, a wide range of activity levels
can be measured on the same detector. For high-level samples, extremely low-efficiency geometries may be used. Quantitative
measurements can be made accurately and precisely when high activity level samples are placed at distances of 10 cm or more
from the detector.
5.4 Electronic problems, such as erroneous deadtime correction, loss of resolution, and random summing, may be avoided by
–1
keeping the gross count rate below 2000 counts per second (s ) and also keeping the deadtime of the analyzer below 5 %. Total
counting time is governed by the radioactivity of the sample, the detector to source distance and the acceptable Poisson counting
uncertainty.
6. Interferences
6.1 In complex mixtures of gamma-ray emitters, the degree of interference of one nuclide in the determination of another is
governed by several factors. If the gamma-ray emission rates from different radionuclides are similar, interference will occur when
the photopeaks are not completely resolved and overlap. If the nuclides are present in the mixture in unequal portions
radiometrically, and if nuclides of higher gamma-ray energies are predominant, there are serious interferences with the
interpretation of minor, less energetic gamma-ray photopeaks. The complexity of the analysis method is due to the resolution of
these interferences and, thus, one of the main reasons for computerized systems.
6.2 Cascade summing may occur when nuclides that decay by a gamma-ray cascade are analyzed. Cobalt-60 is an example;
1172 and 1333-keV gamma rays from the same decay may enter the detector to produce a sum peak at 2505 keV and cause the
loss of counts from the other two peaks. Cascade summing may be reduced by increasing the source to detector distance. Summing
is more significant if a well-type detector is used.
6.3 Random summing is a function of counting rate and occurs in all measurements. The random summing rate is proportional
to the total count squared and the resolving time of the detector. For most systems random summing losses can be held to less than
–1
1 % by limiting the total counting rate to 2000 counts per second (s ). Refer to Test Methods E181 for more information.
6.4 The density of the sample is another factor that can effect quantitative results. Errors from this source can be avoided by
preparing the standards for calibration in solutions or other matrices with a density comparable to the sample being analyzed.
7. Apparatus
7.1 Gamma Ray Spectrometer, consisting of the following components:
7.1.1 Detector Assembly:
7.1.1.1 Germanium Detector—The detector may have a volume of about 50 to 150 cm , with a full width at one-half the peak
maximum (FWHM) less than 2.2 keV at 1332 keV, certified by the manufacturer. A charge-sensitive preamplifier using low noise
field effect transistors should be an integral part of the detector assembly. A convenient support should be provided for samples
of the desired form.
7.1.1.2 Shield—The detector assembly may be surrounded by an external radiation shield made of a dense metal, equivalent to
102 mm of lead in gamma-ray attenuation capability. It is desirable that the inner walls of the shield be at least 127 mm distant
from the detector surfaces to reduce backscatter. If the shield is made of lead or a lead liner, the shield may have a graded inner
shield of 1.6 mm of cadmium or tin lined with 0.4 mm of copper, to attenuate the 88-keV Pb X-rays. The shield should have a
door or port for inserting and removing samples.
7.1.1.3 High Voltage Power/Bias Supply—The bias supply required for germanium detectors usually provides a voltage up to
5000 V and up to 100 μA. The power supply should be regulated to 0.1 % with a ripple of not more than 0.01 %. Line noise caused
by other equipment should be removed with rf filters and additional regulators.
7.1.1.4 Amplifier—An amplifier compatible with the preamplifier and with the pulse-height analyzer shall be provided.
7.1.2 Data Acquisition and Storage Equipment:
7.1.2.1 Data Acquisitions—A multichannel pulse-height analyzer (MCA) or stand-alone analog-to-digital-converter (ADC)
under software control of a separate computer, performs many functions required for gamma-ray spectrometry. An MCA or
D3649 − 06 (2014)
computer collects the data, provides a visual display, and outputs final results or raw data for later analysis. The four major
components of an MCA are the ADC, the memory, control, and input/output. More recently, digital signal processors (DSP) can
directly amplify and digitize signals from the preamplifier, replacing individual amplifier and ADC components. The ADC digitizes
the analog pulses from the
...