ASTM D4962-02
(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 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.
General Information
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Standards Content (Sample)
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Designation:D4962–02
Standard Practice for
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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.
1. Scope 3. Summary of Practice
1.1 This practice covers the measurement of radionuclides 3.1 Gamma-ray spectra are commonly measured with
in water by means of gamma-ray spectrometry. It is applicable modular equipment consisting of a detector, amplifier, multi-
to nuclides emitting gamma-rays with energies greater than 50 channel analyzer device, and a computer (17 and 18).
keV. For typical counting systems and sample types, activity 3.2 Thallium-activated sodium-iodide crystals, NaI(Tl),
levels of about 40 Bq (1080 pCi) are easily measured and which can be operated at ambient temperatures, are often used
sensitivities of about 0.4 Bq (11 pCi) are found for many as gamma-ray detectors in spectrometer systems. However,
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nuclides (1-10). Count rates in excess of 2000 counts per their energy resolution limits their use to the analysis of single
second should be avoided because of electronic limitations. nuclides or simple mixtures of a few nuclides. Resolution of
137
High count rate samples can be accommodated by dilution or about7 %(45keVfullwidthatonehalfthe Cspeakheight)
by increasing the sample to detector distance. at 662 keV can be expected for a NaI(Tl) detector in a 76 mm
1.2 This practice can be used for either quantitative or by 76 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-ray activity, 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 (11 and 12). Information on specific and accumulates in a memory the number of pulses in each
application of gamma-ray spectrometry is also available in the amplitude band (or channel) in a given counting time (17 and
literature (13-16). 18).Fora0to2MeV spectrum two hundred data points are
1.3 This standard does not purport to address all of the adequate.
safety concerns, if any, associated with its use. It is the 3.5 The distribution of the amplitudes (pulse heights) of the
responsibility of the user of this standard to establish appro- pulse energies, represented by the pulse height, can be sepa-
priate safety and health practices and determine the applica- rated into two principal components. One of these components
bility of regulatory limitations prior to use. has a nearly Gaussian distribution and is the result of total
absorption of the gamma-ray energy in the detector; this peak
2. Referenced Documents
is normally referred to as the full-energy peak or photopeak.
2.1 ASTM Standards:
The other component is a continuous one, lower in energy than
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D 3648 Practices for Measurement of Radioactivity the photopeak. This continuous curve is referred to as the
E 181 Test Methods for Detector Calibration and Analysis
Compton continuum and results from interactions wherein the
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of Radionuclides gamma photons lose only part of their energy to the detector.
Other peaks components, such as escape peaks, backscattered
gamma-rays,orx-raysfromshields,areoftensuperimposedon
1
This practice is under the jurisdiction ofASTM Committee D19 on Water and
the Compton continuum. These portions of the curve are
is the direct responsibility of Subcommittee D19.04 on Methods of Radiochemical
shown in Fig. 1 and Fig. 2. Escape peaks will be present when
Analysis.
Current edition approved Feb. 10, 2002. Published May 2002. Originally
gamma-rays with energies greater than 1.02 MeV are emitted
published as D 4962 – 89. Last previous edition D 4962 – 95.
from the sample (19-24). The positron formed in pair prod
...
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