Gamma-ray bore-hole logging for determining radioactive fallout layers in snow

Following large atmospheric thermonuclear test series conducted at specified dates (1954 and 1962), fission products (137Cs-90Sr) produced reference levels in snow layers. A gamma-ray spectrometry of 137Cs was made in bore-holes which were also sampled in order to measure in the laboratory the global low-level beta and gamma activities. The in-situ gamma-ray spectrometer is a scintillation detector coupled to a multichannel analyser. A motorized winch with an electronic control allows automatic exploration of the bore-holes. For calibration purposes, a low-level gamma-ray spectrometer with 1 count/s background and 20% efficiency at 662 keV and a low-level beta counter with a sample changer is used. It shows 5 counts/h background, with an efficiency of 20% (90Sr), for snow samples filtered on ion-exchange resin-papers. These measurements concern ice bore-holes from east Antarctica, covering the time from 1950 up to 1980, and from the French Alps (1960 up to 1980). The successful determination of the reference levels corresponding to the thermonuclear tests of 1954 and 1962 was carried out in Antarctica and the French Alps, respectively. In the laboratory, the low-level analyses performed on the bore-hole samples confirmed these results. The most important aspect of the in-situ gamma-ray spectrometer is its ability to determine rapidly the mean annual accumulation of snow in the field for several locations during one traverse. From these results, information can be obtained on certain meteorological parameters and the absolute value of the amount of deposited 137Cs. (author)