ASTM D4962-02(2009)
(Practice)Standard Practice for NaI(Tl) Gamma-Ray Spectrometry of Water
Standard Practice for NaI(Tl) Gamma-Ray Spectrometry of Water
SIGNIFICANCE AND USE
Gamma-ray spectrometry is used to identify radionuclides and to make quantitative measurements. Use of a computer and a library of standard spectra will be required for quantitative analysis of complex mixtures of nuclides.
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. This means that a complete set of library standards may be required for each geometry and sample to detector distance combination that will be used.
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 1 m or more from the detector.
Electronic problems, such as erroneous deadtime correction, loss of resolution, and random summing, may be avoided by keeping the gross count rate below 2 000 counts per second and also keeping the deadtime of the analyzer below 5 %. Total counting time is governed by the activity of the sample, the detector source distance, and the acceptable Poisson counting uncertainty.
SCOPE
1.1 This practice covers the measurement of radionuclides in water by means of gamma-ray spectrometry. It is applicable to nuclides emitting gamma-rays with energies greater than 50 keV. For typical counting systems and sample types, activity levels of about 40 Bq (1080 pCi) are easily measured and sensitivities of about 0.4 Bq (11 pCi) are found for many nuclides (1-10). 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 or by increasing the sample to detector distance.
1.2 This practice can be used for either quantitative or relative determinations. In tracer work, the results may be expressed by comparison with an initial concentration of a given nuclide which is taken as 100 %. For radioassay, the results may be expressed in terms of known nuclidic standards for the radionuclides known to be present. In addition to the quantitative measurement of gamma-ray activity, gamma-ray spectrometry can be used for the identification of specific gamma-ray emitters in a mixture of radionuclides. General information on radioactivity and the measurement of radiation has been published (11 and 12). Information on specific application of gamma-ray spectrometry is also available in the literature (13-16).
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard does not purport to address all of 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 limitations prior to use.
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Designation: D4962 − 02 (Reapproved 2009)
Standard Practice for
NaI(Tl) Gamma-Ray Spectrometry of Water
This standard is issued under the fixed designation D4962; 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 2. Referenced Documents
2.1 ASTM Standards:
1.1 This practice covers the measurement of radionuclides
D3648 Practices for the Measurement of Radioactivity
in water by means of gamma-ray spectrometry. It is applicable
D4962 Practice for NaI(Tl) Gamma-Ray Spectrometry of
to nuclides emitting gamma-rays with energies greater than 50
Water
keV. For typical counting systems and sample types, activity
E181 Test Methods for Detector Calibration andAnalysis of
levels of about 40 Bq (1080 pCi) are easily measured and
Radionuclides
sensitivities of about 0.4 Bq (11 pCi) are found for many
nuclides (1-10). Count rates in excess of 2000 counts per
3. Summary of Practice
second should be avoided because of electronic limitations.
3.1 Gamma-ray spectra are commonly measured with
High count rate samples can be accommodated by dilution or
modular equipment consisting of a detector, amplifier, multi-
by increasing the sample to detector distance.
channel analyzer device, and a computer(17 and 18).
1.2 This practice can be used for either quantitative or
3.2 Thallium-activated sodium-iodide crystals, NaI(Tl),
relative determinations. In tracer work, the results may be
which can be operated at ambient temperatures, are often used
expressed by comparison with an initial concentration of a
as gamma-ray detectors in spectrometer systems. However,
given nuclide which is taken as 100 %. For radioassay, the
their energy resolution limits their use to the analysis of single
results may be expressed in terms of known nuclidic standards
nuclides or simple mixtures of a few nuclides. Resolution of
for the radionuclides known to be present. In addition to the
about 7 % (45 keV full width at one half the Cs peak
quantitative measurement of gamma-ray activity, gamma-ray height) at 662 keV can be expected for a NaI(Tl) detector in a
spectrometry can be used for the identification of specific 76 mm by 76 mm-configuration.
gamma-ray emitters in a mixture of radionuclides. General
3.3 Interaction of a gamma-ray with the atoms in a NaI(Tl)
information on radioactivity and the measurement of radiation
detector results in light photons that can be detected by a
has been published (11 and 12). Information on specific
multiplier phototube.The output from the multiplier phototube
application of gamma-ray spectrometry is also available in the
and its preamplifier is directly proportional to the energy
literature (13-16).
deposited by the incident gamma-ray. These current pulses are
fed into an amplifier of sufficient gain to produce voltage
1.3 The values stated in SI units are to be regarded as
output pulses in the amplitude range from 0 to 10 V.
standard. No other units of measurement are included in this
3.4 A multichannel pulse-height analyzer is used to deter-
standard.
mine the amplitude of each pulse originating in the detector,
1.4 This standard does not purport to address all of the
and accumulates in a memory the number of pulses in each
safety concerns, if any, associated with its use. It is the
amplitude band (or channel) in a given counting time(17 and
responsibility of the user of this standard to establish appro-
18).Fora0to2MeV spectrum two hundred data points are
priate safety and health practices and determine the applica-
adequate.
bility of regulatory limitations prior to use.
3.5 The distribution of the amplitudes (pulse heights) of the
pulse energies, represented by the pulse height, can be sepa-
rated into two principal components. One of these components
This practice is under the jurisdiction ofASTM Committee D19 on Water and
has a nearly Gaussian distribution and is the result of total
is the direct responsibility of Subcommittee D19.04 on Methods of Radiochemical
Analysis.
Current edition approved Feb. 1, 2009. Published March 2009. Originally
approved in 1989. Last previous edition approved in 2002 as D4962 – 02. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/D4962-02R09. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
The boldface numbers in parentheses refer to the references at the end of this Standards volume information, refer to the standard’s Document Summary page on
practice. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4962 − 02 (2009)
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
the photopeak. This continuous curve is referred to as the
Compton continuum and results from interactions wherein the
gamma photons lose only part of their energy to the detector.
Other peaks components, such as escape peaks, backscattered
gamma-rays,orx-raysfromshields,areoftensuperimposedon
the Compton continuum. These portions of the curve are
shown in Fig. 1 and Fig. 2. Escape peaks will be present when
gamma-rays with energies greater than 1.02 MeV are emitted
from the sample (19-24). The positron formed in pair produc-
tionisusuallyannihilatedinthedetectorandoneorbothofthe
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
FIG. 2 Single and Double Escape Peaks
continuum serves no useful quantitative purpose in photopeak
analysis and must be subtracted from the photopeak to obtain
the correct number of counts before peaks are analyzed.
nated by prerequisites incorporated in the program. Analysis
may also be terminated when a preselected time or total counts
3.6 If the analysis is being directed and monitored by an
in a region of interest or in a specified channel is reached.
online computer program, the analysis period may be termi-
Visual inspection of the computer monitor can also be used as
a criterion for manually terminating the analysis.
3.7 Upon completion of the analysis, the spectral data are
interpreted and reduced to nuclide activity of becquerels
(disintegrations per second) or related units suited to the
particular application. At this time, the spectral data may be
inspected on the monitor 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 means
ofanequationrelatingchannelnumberandgamma-rayenergy.
If the system is calibrated for 10 keVper channel with channel
zero representing 0 keV, the energy can be immediately
calculated. In some systems the channel number or gamma-ray
energyinkeVcanbedisplayedonthemonitorforanyselected
channel. Identification of nuclides may be aided by libraries of
gamma-ray spectra and other nuclear data tabulations (25-30).
3.7.1 Data reduction of spectra involving mixtures of
nuclides is usually accomplished using a library of standard
spectra of the individual nuclides acquired under conditions
identical to that of the unknown sample (25-30).
4. Significance and Use
4.1 Gamma-ray spectrometry is used to identify radionu-
clides and to make quantitative measurements. Use of a
computer and a library of standard spectra will be required for
quantitative analysis of complex mixtures of nuclides.
4.2 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 ad-
equately account for these geometry effects, calibrations are
designed to duplicate all conditions including source-to-
FIG. 1 Compton Continuum detector distance, sample shape and size, and sample matrix
D4962 − 02 (2009)
encountered when samples are measured. This means that a
complete set of library standards may be required for each
geometryandsampletodetectordistancecombinationthatwill
be used.
4.3 Sincesomespectrometrysystemsarecalibratedatmany
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 pre-
cisely when high activity level samples are placed at distances
of1mor more from the detector.
4.4 Electronic problems, such as erroneous deadtime
correction, loss of resolution, and random summing, may be
avoidedbykeepingthegrosscountratebelow2 000countsper
second and also keeping the deadtime of the analyzer below
5 %. Total counting time is governed by the activity of the
sample, the detector source distance, and the acceptable
Poisson counting uncertainty.
5. Interferences
5.1 In complex mixtures of gamma-ray emitters, the degree
of interference of one nuclide in the determination of another
isgovernedbyseveralfactors.Ifthegamma-rayemissionrates
from different radionuclides are similar, interference will occur
when the photopeaks are not completely resolved and overlap.
FIG. 3 Gamma Spectrometry System
Amethod of predicting the gamma-ray resolution of a detector
is given in the literature (31). If the nuclides are present in the
mixture in unequal portions radiometrically, and nuclides of
an inner sample well, 51 to 102 mm in diameter, 44 to 102 mm
higher gamma-ray energies are predominant, there are serious
high, and hermetically sealed in an opaque container with a
interferences with the interpretation of minor, less energetic
transparentwindow.Thecrystalshouldcontainlessthan5µg/g
gamma-ray photopeaks. The complexity of the analysis
of potassium, and should be free of other radioactive materials.
methodisduetotheresolutionoftheseinterferencesand,thus,
In order to establish freedom from other radioactive materials,
one of the main reasons for computerized systems.
themanufacturershouldsupplythegamma-rayspectrumofthe
background of the crystal between 80 and 3000 keV. The
5.2 Cascade summing may occur when nuclides that decay
crystal should be attached and optically coupled to a photo-
by a gamma-ray cascade are analyzed. Cobalt-60 is an ex-
multiplier. (The photomultiplier requires a preamplifier or a
ample; 1172 and 1333 keV gamma-rays from the same decay
cathode follower compatible with the amplifier). The resolu-
may enter the detector to produce a sum peak at 2505 keV and
tion(FWHM)oftheassemblyforthephotopeakofcesium-137
cause the loss of counts from the other two peaks. Cascade
should be less than 9 %.
summing may be reduced by increasing the source to detector
6.1.2 Shield—The detector assembly shall be surrounded by
distance.Summingismoresignificantifawell-typedetectoris
an external radiation shield made of massive metal, equivalent
used.
to 102 mm of lead in gamma-ray attenuation capability. It is
5.3 Random summing occurs in all measurements but is a
desirable that the inner walls of the shield be at least 127 mm
function of count rate. The total random summing rate is
distant from the detector surfaces to reduce backscatter. If the
proportional to the square of the total number of counts. For
shield is made of lead or a lead liner, the shield must have a
most systems, random summing losses can be held to less than
gradedinnershieldof1.6mmofcadmiumortinlinedwith0.4
1 % by limiting the total counting rate to 2 000 counts per
mmofcopper,toattenuateleadx-raysat88keV,onthesurface
second (see Methods E181).
near the detector. The shield must have a door or port for
5.4 The density of the sample is another factor that can
inserting and removing samples.
affect quantitative results. This source of error can be avoided 6.1.3 High Voltage Power/Bias Supply—High-voltage
by preparing the standards for calibration in matrices of the
power supply of range (usually from 500 to 3000 V and up to
same density of the sample under analysis. 10 mA) sufficient to operate a NaI(Tl) detector,
photomultiplier, and its preamplifier assembly. The power
6. Apparatus
supply shall be regulated to 0.1 % with a ripple of not more
6.1 Gamma Ray Spectrometer, consisting of the following than 0.01 %. Line noise caused by other equipment shall be
components, as shown in Fig. 3: removed with radiofrequency filters and additional regulators.
6.1.1 Detector Assembly—Sodium iodide crystal, activated 6.1.4 Amplifier—An amplifier compatible with the pream-
with about 0.1 % thallium iodide, cylindrical, with or without plifier or emitter follower and with the pulse-height analyzer.
D4962 − 02 (2009)
6.1.5 Data Acquisition and Storage Equipment—A multi- ume of an NIST traceable radionuclide standard solution
channel pulse-height analyzer (MCA) or stand-alone analog- containing 100 to 10 000 Bq in a container and placing the
to-digital-converter(ADC)undersoftwarecontrolofaseparate container on the detector or in the detector well.
computer, performs many functions required for gamma-ray
8.1.2 Preparation of Apparatus:
spectrometry.An MCAor computer collects the data, provides
8.1.2.1 Follow the manufacturer’s instructions, limitations,
a visual display, and outputs final results or raw data for later
andcautionsforthesetupofandthepreliminarytestingforall
analysis.The four major components of an MCAare theADC,
of the spectrometry equipment to be used in the analysis. This
the memory, the control, and the input/output circuitry and
equipment could include detectors, power supplies,
devices.TheADCdigitizestheanalogpulsesfromthedetector
preamplifiers, amplifiers, multichannel analyzers, and comput-
amplifier. These pulses represent energy. The digital result
ing systems.
selects a memory location (channel number) which is used to
8.1.2.2 Place an appropriate volume of an NIST traceable
store the number of events which have occurred with that
standard or an NIST traceable mixed standard of radionuclides
energy. Simple data analysis and control of the MCA is
in a sealed container and place the container at a desirable and
accomplished with microprocessors, which control factors
reprod
...
This document is not anASTM standard and is intended only to provide the user of anASTM 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.
´1
Designation:D 4962–95 Designation:D4962–02 (Reapproved 2009)
Standard Practice for
NaI(Tl) Gamma-Ray Spectrometry of Water
This standard is issued under the fixed designation D 4962; 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. ´ NOTE—Editorial changes were made in
October 1995.
1. Scope
1.1 This practice covers the measurement of gamma-ray emitting radionuclides in water by means of gamma-ray spectrometry.
Itisapplicabletonuclidesemittinggamma-rayswithenergiesgreaterthan50keV.Fortypicalcountingsystemsandsampletypes,
activitylevelsofabout40Bq(1080pCi)areeasilymeasuredandsensitivitiesofabout0.4Bq(11pCi)arefoundformanynuclides
(11-10). 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 or by increasing the sample to detector distance.
1.2 This practice can be used for either quantitative or relative determinations. In tracer work, the results may be expressed by
comparison with an initial concentration of a given nuclide which is taken as 100 %. For radioassay, the results may be expressed
in terms of known nuclidic standards for the radionuclides known to be present. In addition to the quantitative measurement of
gamma radioactivity,gamma-ray activity, gamma-ray spectrometry can be used for the identification of specific gamma-ray
emitters in a mixture of radionuclides. General information on radioactivity and the measurement of radiation has been published
(11 and 12). Information on specific application of gamma-ray spectrometry is also available in the literature (3(13-16).
1.3
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard does not purport to address all of 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
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D 3648 Practices for Measurement of Radioactivity Practices for the Measurement of Radioactivity
D 4962 Practice for NaI(Tl) Gamma-Ray Spectrometry of Water
E 181 Test Methods for Detector Calibration and Analysis of RadionuclidesRadionuclides
3. Summary of Practice
3.1 Gamma-ray spectra are commonly measured with modular equipment consisting of a detector, an analyzer, a memory
amplifier, multi-channel analyzer device, and a permanent storage device. computer (17 and 18).
3.2 Thallium-activated sodium-iodide crystals, NaI(Tl), which can be operated at ambient temperatures, are often used as
gamma-ray detectors in spectrometer systems. However, their energy resolution limits their use to the analysis of single nuclides
or simple mixtures of a few nuclides. Resolution of about 7 % (45 keV full width at one half the Cs-137 Cs peak height) at
662 keV can be expected for a NaI(Tl) detector in a 7576 mm by 7576 mm-configuration.
3.3 Interaction of a gamma-ray with the atoms in a NaI(Tl) detector results in light photons that can be detected by a multiplier
phototube. The output from the multiplier phototube and its 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.
3.4 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
systemswithstoredprogramsandinterfacehardwarecanaccomplishthesamefunctionsashardwiredmultichannelanalyzers.The
This practice is under the jurisdiction of Committee D-19 on Water and is the direct responsibility of Subcommittee D19.04 on Methods of Radiochemical Analysis.
Current edition approved July 15, 1995. Published September 1995. Originally published as D4962–89. Last previous edition D4962–89.
ThispracticeisunderthejurisdictionofASTMCommitteeD19onWaterandisthedirectresponsibilityofSubcommitteeD19.04onMethodsofRadiochemicalAnalysis.
Current edition approved Feb. 1, 2009. Published March 2009. Originally approved in 1989. Last previous edition approved in 2002 as D 4962 – 02.
The boldface numbers in parentheses refer to the references at the end of this practice.
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book ofASTM Standards
, Vol 11.02.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.
D4962–02 (2009)
primary advantages of the computerized system include the capability of programming the multichannel analyzer functions and
the ability to immediately perform data reduction calculations using the spectral data stored in the computer memory or mass
storage device time (417 and 18). ).Fora0to2MeV spectrum two hundred data points are adequate.
3.5 The distribution of the amplitudes (pulse heights) of the pulse energies, represented by the pulse height, 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; thisphotopeak. This continuous curve is referred to
as the Compton continuum and results from interactions wherein the gamma photons lose only part of their energy to the detector.
Other peaks components, such as escape peaks, backscattered gamma-rays, or x-rays from shields, are often superimposed on the
Compton continuum. These portions of the curve are shown in Fig. 1 and Fig. 2. Escape peaks will be present when gamma-rays
with energies greater than 1.02 MeV are emitted from the sample (5(19-24). 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 quantitative purpose in photopeak analysis and must be subtracted when from the photopeak
to obtain the correct number of counts before peaks are analyzed.
3.6 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
Analysis may also 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 cathode-ray tube (CRT) display of accumulated data the computer monitor can also be used as a criterion
for manually terminating the analysis on either type of data acquisition systems. analysis.
3.7 Upon completion of the analysis, the spectral data are interpreted and reduced to nuclide activity of becquerels
(disintegrations per second) or related units suited to the particular application. At this time, the spectral data may be inspected
on the CRTmonitor to identify the gamma-ray emitters present. This is accomplished by reading the channel number from the
x-axisandconvertingtogamma-rayenergybymeansofanequationrelatingchannelnumberandgamma-rayenergy.Ifthesystem
is calibrated for 10 keV per channel with channel zero representing 0 keV, the energy can be immediately calculated. In some
FIG. 1 Compton Continuum
D4962–02 (2009)
FIG. 2 Single and Double Escape Peaks
systems the channel number or gamma-ray energy in keV can be displayed on the CRTmonitor for any selected channel.
Identification of nuclides may be aided by catalogslibraries of gamma-ray spectra and other nuclear data tabulations (625-30).
3.7.1Computer programs for data reduction have been used extensively although calculations for some applications can be
performed effectively with the aid of a desktop or pocket calculator (1
3.7.1 Data reduction of spectra involving mixtures of nuclides is usually accomplished using a library of standard spectra of
the individual nuclides acquired under conditions identical to that of the unknown sample (25-30). Data reduction of spectra
involvingmixturesofnuclidesisusuallyaccomplishedusingalibraryofstandardspectraoftheindividualnuclidesacquiredunder
conditions identical to that of the unknown sample (6).
4. Significance and Use
4.1 Gamma-ray spectrometry is used to identify radionuclides and to make quantitative measurements. Use of a computer and
a library of standard spectra will be required for quantitative analysis of complex mixtures of nuclides.
4.2 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. This means that a complete set of library standards may be required for each geometry and sample to
detector distance combination that will be used.
4.3 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 of1mor more from
the detector.
4.4 Electronic problems, such as erroneous deadtime correction, loss of resolution, and random summing, may be avoided by
keeping the gross count rate below 1000002 000 counts per minutesecond and also keeping the deadtime of the analyzer below
5 %. Total counting time is governed by the activity of the sample, the detector source distance, and the acceptable Poisson
counting uncertainty.
5. Interferences
5.1 In complex mixtures of gamma-ray emitters, the degree of interference of one nuclide in the determination of another is
governedbyseveralfactors.Ifthegamma-rayemissionratesfromdifferentradionuclidesaresimilar,interferencewilloccurwhen
the photopeaks are not completely resolved and overlap. A method of predicting the gamma-ray resolution of a detector is given
in the literature (7(31). If the nuclides are present in the mixture in unequal portions radiometrically, and nuclides of higher
gamma-ray energies are predominant, there are serious interferences with the interpretation of minor, less energetic gamma-ray
photopeaks.Thecomplexityoftheanalysismethodisduetotheresolutionoftheseinterferencesand,thus,oneofthemainreasons
for computerized systems.
5.2 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
D4962–02 (2009)
lossofcountsfromtheothertwopeaks.Cascadesummingmaybereducedbyincreasingthesourcetodetectordistance.Summing
is more significant if a well-type detector is used.used.
5.3 Randomsummingoccursinallmeasurementsbutisafunctionofcountrate.Thetotalrandomsummingrateisproportional
to the square of the total number of counts. For most systems, random summing losses can be held to less than 1 % by limiting
the total counting rate to 10002 000 counts per second (see General Methods E 181).
5.4 The density of the sample is another factor that can affect quantitative results. This source of error can be avoided by
preparing the standards for calibration in matrices of the same density of the sample under analysis.
6. Apparatus
6.1 Gamma Ray Spectrometer, consisting of the following components, as shown in Fig. 3:
6.1.1 Detector Assembly—Sodium iodide crystal, activated with about 0.1 % thallium iodide, cylindrical, with or without an
innersamplewell,51to102mmindiameter,44to102mmhigh,andhermeticallysealedinanopaquecontainerwithatransparent
window. The crystal should contain less than 5 µg/g of potassium, and should be free of other radioactive materials. In order to
establish freedom from other radioactive materials, the manufacturer should supply the gamma-ray spectrum of the background
of the crystal between 80 and 3000 keV. The crystal should be attached and optically coupled to a multiplier phototube.
photomultiplier. (The multiplier phototubephotomultiplier requires a preamplifier or a cathode follower compatible with the
amplifier). The resolution (FWHM) of the assembly for the photopeak of cesium-137 should be less than 9 %.
6.1.2 Shield—The detector assembly shall be surrounded by an external radiation shield made of massive 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 must have a graded inner
shieldof1.6mmofcadmiumortinlinedwith0.4mmofcopper,toattenuateleadx-raysat88keV,onthesurfacenearthedetector.
The shield must have a door or port for inserting and removing samples.
6.1.3 High Voltage Power/Bias Supply —High-voltage power supply of range (usually from 500 to 3000 V and up to 10 mA)
sufficienttooperateaNaI(Tl)detector,multiplierphototube,photomultiplier,anditspreamplifierassembly.Thepowersupplyshall
be regulated to 0.
...
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