PERFORMANCE AND PROMISE OF THE COMPTON SUPPRESSION WELL COUNTER

Determination of gamma-ray activities in small environmental samples p oses a challenge to measurement technologies: to achieve the lowest po ssible detection limits, an instrument must combine high efficiency wi th superior peak-to-background characteristics. These requirements are simultaneously met by a Compton-suppression well counter (CSWC) of in trinsic 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 th e 3.3-in through-hole of 10 X 12 in NaI(Tl) crystal. This CSWC was fir st assembled in the mid 1980s, and used in several research studies: d ouble beta(-) decay of Ca-48, the radioactivity of Ta-180m, and high-s ensitivity neutron activation analysis of iridium in rocks and mineral s. More recently it has been employed in environmental radioactivity r esearch on the transport of the members of the uranium and thorium ser ies in natural soils. The counter typically measures samples of simila r 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 sig nificant nuclides like Pb-210, Th-230 and Am-241 in small soil samples or soil fractions. Because of the simple geometry self-absorption cor rections are straightforward. The paper will discuss the performance o f this device, and the projected performance of an advanced model whic h will incorporate a number of changes materially increasing the sensi tivity.