Construction of an anticompton Ge(Li) gamma-ray spectrometer with compton shield NaI(Tl) scintillators and its applications to low-level counting

A Compton suppressed Ge(Li) gamma-ray spectrometer using anticompton NaI(Tl) scintillation detectors has been constructed to measure low-level radioactive samples and also to measure highly active samples by detecting the collimated gamma-rays. The detector system consists of a coaxial Ge(Li) detector having 2keV FWHM and 19% relative efficiency, and anticompton NaI(Tl) detectors. The latter is composed of an annulus NaI(Tl) crystal, 8in.dia. x 8in.long, and a plug NaI(Tl) crystal, 3in.dia. x 3in.thick. For point sources of single gamma-ray emitters such as 137Cs and 54Mn, the Compton suppression factor greater than 4 is obtained at maximum. Continuum natural background levels measured in the anticompton mode can be reduced to 43% of that measured in the normal mode. The minimum detectable disintegration rates of the spectrometer for 137Cs, 60Co and 144Ce are 5.8, 17.1 and 33.5 dpm, respectively, in 1000 min counting under threshold condition of 50% error. The lowest detectable specific activities per gram of 137Cs, 60Co and 144Ce in 250g soil sample counting are 74 fCi/g, 240 fCi/g and 820 fCi/g, respectively, with precision of 10%. (auth.)