American National Standard for Calibration of Germanium Detectors for In-Situ Gamma-Ray Measurements
|Publication Date:||1 January 2002|
In-situ gamma-ray spectrometry using portable high-purity germanium (HPGe) detectors, collimated or uncollimated, is a technique for measuring the gamma-ray fluence rate from a gamma-emitting source for the purpose of obtaining or estimating a specific activity or activity concentration. The most common and well-established application of in-situ spectrometry has been the measurements of gamma-emitting radionuclides in surface soils. Recently, the technique is being applied to large extended objects such as walls, pipes, and containers of radioactive material. The impracticality of constructing standards for these geometries precludes the use of traditional approaches to detector calibration. Therefore, those wishing to use the technique have developed alternative approaches that use a combination of measurements and calculations. The growing need for quality assurance for in-situ measurements of radioactivity, along with the variety of calibration techniques, requires that performance tests be developed for assessing the calibration of HPGe detectors for the sources currently being measured.
The purpose of this standard is to specify performance tests for the calibrations of in-situ gamma-ray measurement systems for a range of test configurations. This standard applies to service providers and their customers performing or requiring radioassay of sources using in-situ gamma-ray spectrometry with HPGe detectors (hereafter referred to as "detectors"). It is generally assumed, but not required, that the sources are large (with respect to the detector). It is also assumed that the calibration is performed without the benefit of certified reference materials that are identical, in terms of the radionuclide mix and geometry, to the objects being measured. This standard applies to the calibration of detectors, uncollimated or collimated, for the measurement of the following objects:
- Uniformly contaminated surface soils
- Surficial contamination of flat objects
- Volumetric contamination of cylindrical and rectangular objects
- Containerized homogeneous sources (cylindrical and rectangular containers)
- Attenuated sources
The energy of the gamma-rays addressed in this standard is 60 keV to 3000 keV.
This standard does not treat the specialized cases of aerial gamma-ray spectrometry, in-situ spectrometry in boreholes. Moreover, this standard does not address in-situ gamma-ray spectrometry measurements for detectors operated in a gross counting mode. Discussions of procedures as they pertain to peak-finding algorithms, peak-position, peak-area measurements, and energy calibration, as well as discussions of performance tests of spectrometry systems and analysis software are beyond the scope of this standard; refer to ANSI N42.14-1999 and IEC 61275-1997-09 for such methods.
CAUTION-While guidance on the application of in-situ gamma-ray spectrometry and the use of in-situ measurement data is beyond the scope of this standard, some general remarks concerning the technique are warranted. As with any measurement technique, one must be aware of assumptions that give rise to limitations on the range of potential applications. In-situ measurements carry specific assumption regarding many measurement parameters. The most notable parameter is the distribution of activity in the source. A detector measures only the gamma-ray fluence from which one can infer an activity concentration. The extent to which the measurement assumptions reflect the actual or true measurement conditions is the extent to which the measurements results will satisfy their intended purpose. For example, a detector that is calibrated to measure sources with uniformly distributed activity will produce inconsistent and misleading results if the source is highly heterogeneous. However, there could be an acceptable degree of heterogeneity that would still allow for useful data. Service providers and their customers should consider using data quality objectives as a guide or a means to evaluate a measurement program that uses or intends to use this technique.