ASTM D7784-20

5.1?This practice was developed for the rapid determination of gamma-emitting radionuclides in environmental media. The results of the test may be used to determine if the activity of these radionuclides in the sample exceeds the action level for the relevant incident or emergency response. The detection limits will be dependent on sample size, counting configuration, and the detector system in use.

5.2?In most cases, a sample container which is large in diameter and short in height relative to the detector will provide the best gamma-ray detection efficiency. For samples of water or other low-Z materials (for example, vegetation), the re-entrant or Marinelli-style beaker may yield the best gamma-ray detection efficiency.

5.3?The density of the sample material and physical parameters of the sample container (for example, diameter, height, material) may have significant consequences for the accuracy of the sample analysis as compared to the calibration. For this reason, the ideal calibration material and container (often referred to as geometry) will be exactly the same as the samples to be analyzed. Differences in sample container or sample matrix may introduce significant errors in detector response, especially at low gamma-ray energies. Every effort should be made to account for these differences if the exact calibration geometry is not available.

5.4?This practice establishes an empirical gamma-ray spectrometer calibration using standards traceable to the SI via a national metrology institute (NMI) such as the National Institute of Standards and Technology (NIST) in the United States and the National Physical Laboratory (NPL) in the United Kingdom in a specific geometry selected to ensure that the container, density, and composition of the standard matches that of the samples as closely as possible. However, in some cases it may be beneficial to modify such initial calibrations using mathematical modeling or extrapolations to an alternate geometry. Use of such a model may be acceptable, depending on the measurement quality objectives of the analysis process, and provided that appropriate compensation to uncertainty estimates are included. The use of such calibration models is best supported by the successful analysis of a method validation reference material (MVRM).

5.5?This practice addresses the analysis of numerous gamma-emitting radionuclides in environmental media. This practice should be applicable to non-environmental media (for example, urine, debris, or rubble) that have similar physical properties. The key determination of similar physical properties is the ability to demonstrate that the gamma spectrometry system response to the sample configuration is suitably similar to that for which the system is calibrated.

5.6?For the analysis of radionuclides with low gamma-ray emission energies (<100 keV), self-absorption of the gamma-rays in the sample matrix can have a significant adverse effect on detection and quantification. The user should verify that instrument calibrations appropriately account for any self-absorption that may result from the sample matrix.

5.7?Commonly available energy and efficiency calibration standards cover the energy range of approximately 60 keV to 1836 keV. Results obtained using gamma-ray peaks outside the efficiency calibrated energy range will have greater uncertainty not accounted for in the uncertainty calculations of this practice. Great care should be taken to review the efficiency calibration values and the shape of the efficiency curve outside this range. For greater accuracy in the analysis of radionuclides whose gamma-ray energies are outside this range, a calibration standard which includes radionuclide(s) whose gamma-ray energies span the energy range of radionuclides of interest is advised.

1.1?This practice covers the quantification of radionuclides in environmental media (for example, water, soil, vegetation, food) by means of simple preparation and counting with a high-resolution gamma ray detector. Because the practice is designed for rapid analysis, extensive efforts to ensure homogeneity or ideal sample counting conditions are not taken.

1.2?The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.

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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.4?This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.