Performance of a NaI(Tl)-Ge Compton-suppression spectrometer and its application to the measurement of 137Cs and 7Be in the surface soil

A Compton suppression spectrometer composed of a 526 cm3 Ge and a NaI(Tl) detector for the low-background γ-ray spectrometry has been assembled and its performance has been investigated. The Compton suppression factor was 3 for the 662 keV γ-ray from 137Cs and 10 for the 1332 keV γ-ray from 60Co. Background counting rates in the γ-ray energy region of 200-2000 keV were reduced to 1/70-1/20 in the normal mode and to 1/145-1/53 in the Compton-suppression mode, respectively. An application to the measurement of the 137Cs and 7Be concentration in the surface soil collected in the Higashi-Hiroshima campus of Hiroshima University has been performed. The activity level of 7Be was 20±2 mBq g-1 and uniformly distributed. Whereas, the activity level of 137Cs was ranged from 0.23-14.1 mBq g-1 reflecting whether the soil was cultivated or not. (author)