ASTM D4962-95e1
(Practice)Standard Practice for NaI(Tl) Gamma-Ray Spectrometry of Water
Standard Practice for NaI(Tl) Gamma-Ray Spectrometry of Water
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 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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e1
Designation: D 4962 – 95
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 (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Editorial changes were made in October 1995.
1. Scope 3. Summary of Practice
1.1 This practice covers the measurement of gamma-ray 3.1 Gamma-ray spectra are measured with modular equip-
emitting radionuclides in water by means of gamma-ray ment consisting of a detector, an analyzer, a memory device,
spectrometry. It is applicable to nuclides emitting gamma-rays and a permanent storage device.
with energies greater than 50 keV. For typical counting systems 3.2 Thallium-activated sodium-iodide crystals, NaI(Tl),
and sample types, activity levels of about 40 Bq are easily which can be operated at ambient temperatures, are often used
measured and sensitivities of about 0.4 Bq are found for many as gamma-ray detectors in spectrometer systems. However,
nuclides (1). Count rates in excess of 2000 counts per second their energy resolution limits their use to the analysis of single
should be avoided because of electronic limitations. High nuclides or simple mixtures of a few nuclides. Resolution of
count rate samples can be accommodated by dilution or by about 7 % (45 keV full width at one half the Cs-137 peak
increasing the sample to detector distance. height) at 662 keV can be expected for a NaI(Tl) detector in a
1.2 This practice can be used for either quantitative or 75 mm by 75 mm-configuration.
relative determinations. In tracer work, the results may be 3.3 Interaction of a gamma-ray with the atoms in a NaI(Tl)
expressed by comparison with an initial concentration of a detector results in light photons that can be detected by a
given nuclide which is taken as 100 %. For radioassay, the multiplier phototube. The output from the multiplier phototube
results may be expressed in terms of known nuclidic standards and its preamplifier is directly proportional to the energy
for the radionuclides known to be present. In addition to the deposited by the incident gamma-ray. These current pulses are
quantitative measurement of gamma radioactivity, gamma-ray fed into an amplifier of sufficient gain to produce voltage
spectrometry can be used for the identification of specific output pulses in the amplitude range from 0 to 10 V.
gamma-ray emitters in a mixture of radionuclides. General 3.4 A multichannel pulse-height analyzer is used to deter-
information on radioactivity and the measurement of radiation mine the amplitude of each pulse originating in the detector,
has been published (2). Information on specific application of and accumulates in a memory the number of pulses in each
gamma-ray spectrometry is also available in the literature (3). amplitude band (or channel) in a given counting time. Com-
1.3 This standard does not purport to address all of the puterized systems with stored programs and interface hardware
safety concerns, if any, associated with its use. It is the can accomplish the same functions as hardwired multichannel
responsibility of the user of this standard to establish appro- analyzers. The primary advantages of the computerized system
priate safety and health practices and determine the applica- include the capability of programming the multichannel ana-
bility of regulatory limitations prior to use. lyzer functions and the ability to immediately perform data
reduction calculations using the spectral data stored in the
2. Referenced Documents
computer memory or mass storage device (4).Fora0to2MeV
2.1 ASTM Standards: spectrum two hundred data points are adequate.
D 3648 Practices for Measurement of Radioactivity
3.5 The distribution of the amplitudes (pulse heights) of the
E 181 Test Methods for Detector Calibration and Analysis pulses can be separated into two principal components. One of
of Radionuclides
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
This practice is under the jurisdiction of Committee D-19 on Water and is the
peak or photopeak. The other component is a continuous one
direct responsibility of Subcommittee D19.04 on Methods of Radiochemical
lower in energy than that of the photopeak; this continuous
Analysis.
Current edition approved July 15, 1995. Published September 1995. Originally
curve is referred to as the Compton continuum and results from
published as D 4962 – 89. Last previous edition D 4962 – 89.
interactions wherein the gamma photons lose only part of their
The boldface numbers in parentheses refer to the references at the end of this
energy to the detector. Other peaks, such as escape peaks,
practice.
Annual Book of ASTM Standards, Vol 11.02. backscattered gamma-rays, or x-rays from shields, are often
Annual Book of ASTM Standards, Vol 12.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 4962
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). 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.
3.6 If the analysis is being directed and monitored by an
online computer program, the analysis period may be termi-
nated 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
FIG. 2 Single and Double Escape Peaks
is reached. Visual inspection of a cathode-ray tube (CRT)
display of accumulated data can also be used as a criterion for
(disintegrations per second) or related units suited to the
manually terminating the analysis on either type of data
particular application. At this time, the spectral data may be
acquisition systems.
inspected on the CRT to identify the gamma-ray emitters
3.7 Upon completion of the analysis, the spectral data are
present. This is accomplished by reading the channel number
interpreted and reduced to nuclide activity of becquerels
from the x-axis and converting to gamma-ray energy by means
of an equation relating channel number and gamma-ray energy.
If the system is calibrated for 10 keV per channel with channel
zero representing 0 keV, the energy can be immediately
calculated. In some systems the channel number or gamma-ray
energy in keV can be displayed on the CRT for any selected
channel. Identification of nuclides may be aided by catalogs of
gamma-ray spectra and other nuclear data tabulations (6).
3.7.1 Computer programs for data reduction have been used
extensively although calculations for some applications can be
performed effectively with the aid of a desktop or pocket
calculator (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 (6).
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-
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
FIG. 1 Compton Continuum discrete distances from the detector, a wide range of activity
D 4962
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 cor-
rection, loss of resolution, and random summing, may be
avoided by keeping the gross count rate below 100 000 counts
per minute 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 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.
A method of predicting the gamma-ray resolution of a detector
is given in the literature (7). 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. The complexity of the analysis
method is due to the resolution of these interferences and, thus,
FIG. 3 Gamma Spectrometry System
one of the main reasons for computerized systems.
5.2 Cascade summing may occur when nuclides that decay
the manufacturer should supply the gamma-ray spectrum of the
by a gamma-ray cascade are analyzed. Cobalt-60 is an ex-
background of the crystal between 80 and 3000 keV. The
ample; 1172 and 1333 keV gamma-rays from the same decay
crystal should be attached and optically coupled to a multiplier
may enter the detector to produce a sum peak at 2505 keV and
phototube. (The multiplier phototube requires a preamplifier or
cause the loss of counts from the other two peaks. Cascade
a cathode follower compatible with the amplifier). The resolu-
summing may be reduced by increasing the source to detector
tion (FWHM) of the assembly for the photopeak of cesium-137
distance. Summing is more significant if a well-type detector is
should be less than 9 %.
used.
6.1.2 Shield—The detector assembly shall be surrounded by
5.3 Random summing occurs in all measurements but is a
an external radiation shield made of massive metal, equivalent
function of count rate. The total random summing rate is
to 102 mm of lead in gamma-ray attenuation capability. It is
proportional to the square of the total number of counts. For
desirable that the inner walls of the shield be at least 127 mm
most systems, random summing losses can be held to less than
distant from the detector surfaces to reduce backscatter. If the
1 % by limiting the total counting rate to 1000 counts per
shield is made of lead or a lead liner, the shield must have a
second (see General Methods E 181).
graded inner shield of 1.6 mm of cadmium or tin lined with 0.4
5.4 The density of the sample is another factor that can
mm of copper, to attenuate lead x-rays at 88 keV, on the surface
affect quantitative results. This source of error can be avoided
near the detector. The shield must have a door or port for
by preparing the standards for calibration in matrices of the
inserting and removing samples.
same density of the sample under analysis.
6.1.3 High Voltage Power/Bias Supply—High-voltage
6. Apparatus
power supply of range (usually from 500 to 3000 V and up to
10 mA) sufficient to operate a NaI(Tl) detector, multiplier
6.1 Gamma Ray Spectrometer, consisting of the following
phototube, and its preamplifier assembly. The power supply
components, as shown in Fig. 3:
shall be regulated to 0.1 % with a ripple of not more than
6.1.1 Detector Assembly—Sodium iodide crystal, activated
0.01 %. Line noise caused by other equipment shall be re-
with about 0.1 % thallium iodide, cylindrical, with or without
moved with radiofrequency filters and additional regulators.
an inner sample well, 51 to 102 mm in diameter, 44 to 102 mm
6.1.4 Amplifier—An amplifier compatible with the pream-
high, and hermetically sealed in an opaque container with a
plifier or emitter follower and with the pulse-height analyzer.
transparent window. The crystal should contain less than 5 μg/g
6.1.5 Data Acquisition and Storage Equipment—A multi-
of potassium, and should be free of other radioactive materials.
channel pulse-height analyzer (MCA) or stand-alone analog-
In order to establish freedom from other radioactive materials,
to-digital-converter (ADC) under software control of a separate
computer, performs many functions required for gamma-ray
Refer to Annual Book of ASTM Standards, Vols 12.01 and 12.02. spectrometry. An MCA or computer collects the data, provides
D 4962
a visual display, and outputs final results or raw data for later and cautions for the set up of and the preliminary testing for all
analysis. The four major components of an MCA are the ADC, 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, preampli-
devices. The ADC digitizes the analog pulses from the detector fiers, amplifiers, multichannel analyzers, and computing sys-
amplifier. These pulses represent energy. The digital result tems.
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
en
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