Capabilities and limitations of high-resolution gamma spectrometry in environmental research

摘要

The availability of large intrinsic germanium detectors has opened new perspectives for fast, accurate and reliable environmental measurements. Many important isotopes in the natural and the technologically enhanced radiation environment have some suitable gamma lines, allowing quantitative determination of the radionuclides by high-resolution gamma spectrometry. In this paper, the capabilities and the limitations of a typical gamma spectrometry set-up that can be used for in-situ applications and laboratory measurements on soil samples are discussed and emphasis is laid on the sensitivity of laboratory measurements as well as on the applications for systematic surveys. The results of a survey for post-Chernobyl ~(137)Cs and natural radiation (~(40)K, U- and Th-series) in Belgium, obtained using a portable Ge-detector, clearly illustrate the advantages of high-resolution spectroscopy for in-situ applications. Such techniques can also be employed for measuring depleted uranium (DU) in Kosovo. The question of determining the Minimum Detectable Activity (MDA) of the set-up is always very important. A usual approach defines MDA as the maximum amount of activity for a particular nuclide that could be present and remain undetected in a real spectrum, with a given fractional error (e.g. 20%). The MDA for DU in soil samples (100-150 grams) was extensively investigated for the given set-up. The calculated 20% MDA value for DU is as low as 15 Bq for a 18 h long measuring time. Illustrating the capabilities of the set-up, the results of a 50 sampling point survey in Kosovo are presented. A total of 150 soil samples were measured for natural radionuclides, fallout isotopes and DU content. In none of the 150 measured samples was a significant trace of the 1001 keV line found. The mean values for the natural isotopes in Kosovo soil are quite similar to those found in the south of Belgium. The mean value of ~(137)Cs concentration for the top Kosovo soil layers investigated is 53 Bq kg~(-1).