PERFORMANCE AND PROMISE OF THE COMPTON SUPPRESSION WELL COUNTER

Determination of gamma-ray activities in small environmental samples poses a challenge to measurement technologies: to achieve the lowest possible detection limits, an instrument must combine high efficiency with superior peak-to-background characteristics. These requirements are simultaneously met by a Compton-suppression well counter (CSWC) of intrinsic germanium. The Brookhaven CSWC has a 5-cm deep, 1-cm diameter well and active volume of similar to 125 cm(3). It is positioned in the 3.3-in through-hole of 10 X 12 in NaI(Tl) crystal. This CSWC was first assembled in the mid 1980s, and used in several research studies: double beta(-) decay of Ca-48, the radioactivity of Ta-180m, and high-sensitivity neutron activation analysis of iridium in rocks and minerals. More recently it has been employed in environmental radioactivity research on the transport of the members of the uranium and thorium series in natural soils. The counter typically measures samples of similar to 3 g of soil, has a peak-to-Compton ratio of similar to 300:1, and a sensitivity for U-235 of similar to 50 ng. It can detect gamma rays of less than 30 keV, hence can be used to measure environmentally significant nuclides like Pb-210, Th-230 and Am-241 in small soil samples or soil fractions. Because of the simple geometry self-absorption corrections are straightforward. The paper will discuss the performance of this device, and the projected performance of an advanced model which will incorporate a number of changes materially increasing the sensitivity.