Low-background Ge(Li) detector systems for radioenvironmental studies

Abstract

Two Ge(Li) detector systems designed to measure low-level radioenvironmental samples are described. The materials used in the construction of these systems and the effect of these materials on the background radioactivities are discussed. One system, designed to count mid-range specific activities, contains a closed, coaxial, 10% Ge(Li) detector that is housed in electrolytic magnesium and operated within a standard lead cave. Continuum background levels in this system vary from 0.1 c/min keV at 200 keV to 0.01 c/min keV at 1 MeV. The second system, designed for maximum sensitivity, contains a 15%-relative-efficiency Ge(Li) detector housed in electrolytic magnesium and operated in anticoincidence with a large plastic detector. This entire system is placed inside a lead-and steel armor plate shield located in a counting room below ground level. For single gamma-ray emitters from point sources, the maximum continuum suppression factor is greater than 8. Under suppression conditions the background continuum, as well as the usual contaminant peaks, are the lowest yet reported for Ge(Li) environmental counting systems. Continuum values decrease from about 6 × 10 −3 c/min keV at 200 keV to less than 3 × 10 −4 c/min keV at 1 MeV. A 1000 min count of 250 g of soil shows that this system can measure a 137Cs specific activity of 20 fCi/g with a precision of 15%. The lowest specific activity per gram detectable in a one-week count under threshold conditions (± 50%) for a single gamma-ray emitter with E γ >500 keV is several hundred aCi/g.