ASTM C1402-98
(Guide)Standard Guide for High-Resolution Gamma-Ray Spectrometry of Soil Samples
Standard Guide for High-Resolution Gamma-Ray Spectrometry of Soil Samples
SCOPE
1.1 This guide covers the identification and quantitative determination of gamma-ray emitting radionuclides in soil samples by means of gamma-ray spectrometry. It is applicable to nuclides emitting gamma rays with energies greater than 20 keV. For typical gamma-ray spectrometry systems and sample types, activity levels of about 5 Bq are measured easily for most nuclides, and activity levels as low as 0.1 Bq can be measured for many nuclides. It is not applicable to radionuclides that emit no gamma rays such as the pure beta-emitting radionuclides hydrogen-3, carbon-14, strontium-90, and becquerel quantities of most transuranics. This guide does not address the in situ measurement techniques, where soil is analyzed in place without sampling. Guidance for in situ techniques can be found in Ref (1) and (2). This guide also does not discuss methods for determining lower limits of detection. Such discussions can be found in Refs (3), (4), (5), and (6).
1.2 This guide can be used for either quantitative or relative determinations. For quantitative assay, the results are expressed in terms of absolute activities or activity concentrations of the radionuclides found to be present. This guide may also be used for qualitative identification of the gamma-ray emitting radionuclides in soil without attempting to quantify their activities. It can also be used to only determine their level of activities relative to each other but not in an absolute sense. General information on radioactivity and its measurement may be found in Refs (7), (8), (9), (10), and (11) and General Methods E-181. Information on specific applications of gamma-ray spectrometry is also available in Refs (12) or (13). Practice D 3649 is a valuable source of information.
1.3 This standard may involve hazardous material, operations, and equipment. 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: C 1402 – 98
Standard Guide for
High-Resolution Gamma-Ray Spectrometry of Soil Samples
This standard is issued under the fixed designation C 1402; 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 2. Referenced Documents
1.1 This guide covers the identification and quantitative 2.1 ASTM Standards:
determination of gamma-ray emitting radionuclides in soil C 998 Practice for Sampling Surface Soil for Radionu-
samples by means of gamma-ray spectrometry. It is applicable clides
to nuclides emitting gamma rays with energies greater than 20 C 999 Practice for Soil Sample Preparation for the Deter-
keV. For typical gamma-ray spectrometry systems and sample mination of Radionuclides
types, activity levels of about 5 Bq are measured easily for C 1009 Guide for Establishing a Quality Assurance Pro-
most nuclides, and activity levels as low as 0.1 Bq can be gram for Analytical Chemistry Laboratories Within the
measured for many nuclides. It is not applicable to radionu- Nuclear Industry
clides that emit no gamma rays such as the pure beta-emitting D 3649 Practice for High-Resolution Gamma-Ray Spec-
radionuclides hydrogen-3, carbon-14, strontium-90, and bec- trometry of Water
querel quantities of most transuranics. This guide does not E 181 General Methods for Detector Calibration and Analy-
address the in situ measurement techniques, where soil is sis of Radionuclides
analyzed in place without sampling. Guidance for in situ E 380 Practice for Use of the International System of Units
2 6
techniques can be found in Ref (1) and (2). This guide also (SI) the Modernized Metric System
does not discuss methods for determining lower limits of 2.2 ANSI Standards:
detection. Such discussions can be found in Refs (3), (4), (5), N13.30 Performance Criteria for Radiobioassay
and (6). N42.14 Calibration and Use of Germanium Spectrometers
1.2 This guide can be used for either quantitative or relative for the Measurement of Gamma-Ray Emission Rates of
determinations. For quantitative assay, the results are expressed Radionuclides
in terms of absolute activities or activity concentrations of the N42.23 Measurement Quality Assurance for Radioassay
radionuclides found to be present. This guide may also be used Laboratories
for qualitative identification of the gamma-ray emitting radio- ANSI/IEEE-645 Test Procedures for High Purity Germa-
nuclides in soil without attempting to quantify their activities. nium Detectors for Ionizing Radiation
It can also be used to only determine their level of activities
3. Summary of Guide
relative to each other but not in an absolute sense. General
information on radioactivity and its measurement may be 3.1 High-resolution germanium detectors and multichannel
analyzers are used to ensure the identification of the gamma-
found in Refs (7), (8), (9), (10), and (11) and General Methods
E-181. Information on specific applications of gamma-ray ray emitting radionuclides that are present and to provide the
best possible accuracy for quantitative activity determinations.
spectrometry is also available in Refs (12) or (13). Practice
D 3649 is a valuable source of information. 3.2 For qualitative radionuclide identifications, the system
must be energy calibrated. For quantitative determinations, the
1.3 This standard may involve hazardous material, opera-
tions, and equipment. This standard does not purport to system must also be shape and efficiency calibrated. The
standard sample/detector geometries must be established as
address all of the safety concerns, if any, associated with its
use. It is the responsibility of the user of this standard to part of the efficiency calibration procedure.
3.3 The soil samples typically need to be pretreated (for
establish appropriate safety and health practices and deter-
mine the applicability of regulatory limitations prior to use. example, dried), weighed, and placed in a standard container.
Annual Book of ASTM Standards, Vol 12.01.
1 4
This guide is under the jurisdiction of ASTM Committee C–26 on Nuclear Fuel Annual Book of ASTM Standards, Vol 11.02.
Cycle and is the direct responsibility of Subcommittee C26.05 on Methods of Test. Annual Book of ASTM Standards, Vol 12.02.
Current edition approved July 10, 1998. Published December 1998. Annual Book of ASTM Standards, Vol 14.02.
2 7
The boldface numbers in parentheses refer to the list of references at the end Available from American National Standards Institute, West 42nd St. 13th
of this standard. Floor, New York, NY 10036.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C 1402
For quantitative measurements, the dimensions of the container energy are predominant, the interpretation of minor, less
holding the sample and its placement in front of the detector energetic gamma-ray photopeaks becomes difficult due to the
must match one of the efficiency-calibrated geometries. If high Compton continuum and backscatter.
multiple geometries can be selected, the geometry chosen 5.3 True coincidence summing (also called cascade sum-
should reflect the detection limit and count rate limitations of ming) occurs regardless of the overall count rate for any
the system. Qualitative measurements may be performed in radionuclide that emits two or more gamma rays in coinci-
non-calibrated geometries. dence. Cobalt-60 is an example where both a 1173-keV and a
3.4 The identification of the radionuclides present is based 1332-keV gamma ray are emitted from a single decay. If the
on matching the energies of the observed gamma rays in the same is placed close to the detector, there is a finite probability
spectrum to computer-based libraries of literature references that both gamma rays from each decay interact with the
(see Refs (14), (15), (16), (17),or (18). The quantitative detector resulting in a loss of counts from both full energy
determinations are based on comparisons of observed count peaks. Coincidence summing and the resulting losses to the
rates to previously obtained counting efficiency versus energy photopeak areas can be considerable (>10 %) before a sum
calibration data, and published branching ratios for the radio- peak at an energy equal to the sum of the coincident gamma-
nuclides identified. ray energies becomes visible. Coincidence summing can be
reduced to the point of being negligible by increasing the
4. Significance and Use
source to detector distance. Coincidence summing can be a
4.1 Gamma-ray spectrometry of soil samples is used to severe problem if a well-type detector is used. See General
identify and quantify certain gamma-ray emitting radionu-
Methods E 181 and (7) for more information.
clides. Use of a germanium semiconductor detector is neces- 5.4 Random summing is a function of count rate (not dead
sary for high-resolution gamma-ray measurements.
time) and occurs in all measurements. The random summing
4.2 Much of the data acquisition and analysis can be rate is proportional to the total count squared and to the
automated with the use of commercially available systems that
resolving time of the detector and electronics. For most
include both hardware and software. For a general description
systems, uncorrected random summing losses can be held to
of the typical hardware in more detail than discussed in Section
less than 1 % by limiting the total counting rate to less than
6, see Ref (19).
1000 counts. However, high-precision analyses can be per-
4.3 Both qualitative and quantitative analyses may be per-
formed at high count rates by the use of pileup rejection
formed using the same measurement data.
circuitry and dead-time correction techniques. Refer to General
4.4 The procedures described in this guide may be used for
Methods E 181 for more information.
a wide variety of activity levels, from natural background
6. Apparatus
levels and fallout-type problems, to determining the effective-
6.1 Germanium Detector Assembly—The detector should
ness of cleanup efforts after a spill or an industrial accident, to
have an active volume of greater than 50 cm , with a full width
tracing contamination at older production sites, where wastes
at one half the peak maximum (FWHM) less than 2.0 keV for
were purposely disposed of in soil. In some cases, the
the cobalt-60 gamma ray at 1332 keV, certified by the
combination of radionuclide identities and concentration ratios
manufacturer. A charge-sensitive preamplifier using low-noise
can be used to determine the source of the radioactive
field-effect transistors should be an integral part of the detector
materials.
assembly.
4.5 Collecting samples and bringing them to a data acqui-
6.2 Sample Holder Assembly—As reproducibility of results
sition system for analysis may be used as the primary method
depends directly on reproducibility of geometry, the system
to detect deposition of radionuclides in soil. For obtaining a
should be equipped with a sample holder that will permit using
representative set of samples that cover a particular area, see
reproducible sample/detector geometries for all sample con-
Practice C 998. Soil can also be measured by taking the data
acquisition system to the field and measuring the soil in place tainer types that are expected to be used at several different
sample-to-detector distances.
(in situ). In situ measurement techniques are not discussed in
this guide. 6.3 Shield—The detector assembly should be surrounded by
a radiation shield made of material of high atomic number
5. Interferences
providing the equivalent attenuation of 100 mm (or more in the
5.1 In complex mixtures of gamma-ray emitters, the degree case of high background radiation) of low-activity lead. It is
of interference of one nuclide in the determination of another desirable that the inner walls of the shield be at least 125 mm
is governed by several factors. Interference will occur when the distant from the detector surfaces to reduce backscatter and
photopeaks from two separate nuclides overlap within the annihilation radiation. If the shield is made of lead or has a lead
resolution of the gamma-ray spectrometer. Most modern analy- liner, the shield should have a graded inner shield of appropri-
sis software can deconvolute multiplets where the separation of ate materials, for example, 1.6 mm of cadmium or tin-lined
any two adjacent peaks is more than 0.5 FWHM (see Refs (20) with 0.4 mm of copper, to attenuate the induced 88-keV lead
and (21)). For peak separations that are smaller than 0.5 fluorescent Xrays. The shield should have a door or port for
FWHM, most interference situations can be resolved with the inserting and removing samples. The materials used to con-
use of automatic interference correction algorithms (22). struct the shield should be prescreened to ensure that they are
5.2 If the nuclides are present in the mixture in very unequal not contaminated with unacceptable levels of natural or man-
radioactive portions and if nuclides of higher gamma-ray made radionuclides. The lower the desired detection capability,
C 1402
the more important it is to reduce the background. For very low that permits some kind of batch processing and automated
activity samples, the detector assembly itself, including the operation is recommended.
preamplifer, should be made of carefully selected low back-
ground materials. 7. Container for a Test Sample
6.4 High-Voltage Power/Bias Supply—The bias supply re-
7.1 Sample holders and containers must have a reproducible
quired for germanium detectors usually provides a voltage up
geometry. Considerations include commercial availability, ease
to 65000 V and 1 to 100 μA. The power supply should be
of use and disposal, and the containment of radioactivity for
regulated to 0.1 % with a ripple of not more than 0.01 %. Noise
protection of the working environment, personnel, and the
caused by other equipment should be removed with r-f filters
gamma-ray spectrometer from contamination. For small soil
and power line regulators.
samples (up to a few grams), plastic bottles are convenient
6.5 Amplifier—A spectroscopy amplifier which is compat-
containers, while large samples (up to several kilograms),
ible with the preamplifier. If used at high count rates, a model
which require greater sensitivity, are frequently packaged in
with pile-up rejection should be used. The amplifier should be
Marinelli beakers. For analyzing low-energy gamma rays at
pole-zeroed properly prior to use.
close geometries, the consistency of the wall thickness of the
6.6 Data Acquisition Equipment—A multichannel pulse- sample container facing the detector becomes an important
factor in the variability of the analysis results.
height analyzer (MCA) with a built-in or stand-alone analog-
to-digital converter (ADC) compatible with the amplifier 7.2 Measurements may require precautions to prevent the
output and pileup rejection scheme. The MCA (hardwired or a loss of volatile radionuclides. For example, the determination
computer-software-based) collects the data, provides a visual of radium-226 in soil by the measurement of the 609-keV
display, and stores and processes the gamma-ray spectral data. gamma ray of bismuth-214 assumes secular equilibrium be-
The four major components of an MCA are: ADC, memory, tween radium-226 and its progency and that the radon-222
control, and input/output. The ADC digitizes the analog pulses
daughter was not lost from the sample.
from the amplifier. The height of these pulses represents energy
7.3 A beta absorber consisting of about 6 mm of aluminum,
deposited in the detector. The digital result is used by the MCA
beryllium, or plastic should be placed between the detector and
to select a memory location (channel number) which is used to sample for samples that have significant quantities of high-
store the number of events which have occurred at the energy.
energy beta emitters.
The MCA must also be able to extend the data collection time
for the amount of time that the system is dead while processing
8. Calibration and Standardization
pulses (live time correction).
8.1 Overview:
6.7 Data Output Equipment—Modern MCAs provide a
8.1.1 Commission and operate the instrumentation and de-
wide range of input and output (I/O) capabilities. Typically,
tector in accordance with the manufacturer’s instructions.
these include the ability to transfer any section of data to one
Initial set-up includes all electronic adjustments to provide
or more of th
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