A GAMMA-RAY DETECTOR FOR IN-SITU MEASUREMENT OF CS-137 RADIOACTIVITY IN SNOWFIELDS AND GLACIERS

The rate of snow deposition at various cold regions on the earth is an important quantity for glaciological and climatological studies. Radioactive debris from above-ground tests of nuclear weapons (mainly 1954-1970) and from the Chernobyl accident (1986) have been deposited on glaciers and snowfields, where they can be used as time and depth markers to determine the subsequent accumulation of snow. We discuss a technique to locate these markers that has been used just recently - in-situ measurement of gamma-rays from Cs-137. These gamma-rays, which are associated with radioactive fallout, have a distinctive depth profile and serve as markers of the historical nuclear events. The gamma-ray measurement involves lowering a scintillation detector down a borehole in the snow or ice and recording the response to the Cs-137 gamma-rays as a function of depth. The in-situ measurement can be done relatively quickly and can replace sample retrieval, or it can be used to decide which ice or snow samples should be transported for later analysis in the laboratory. The feasibility of in-situ gamma-ray measurement has been demonstrated at sites in the French Alps and Greenland. We report on a portable detector system that is being developed for use in Antarctica. It is based, as much as possible, on inexpensive, commercially available detectors and electronics. The advantages and disadvantages of this approach are discussed. The problems involved with making these measurements in a harsh environment and the steps taken to deal with them are also presented.