Low background gamma ray spectrometer using the anticoincidence shield technique at KAERI

We develop a ultra-low background gamma ray spectrometer, using active and passive shielding technique at the same time. Cosmic ray induced background is suppressed by means of active shield devices consisting of plastic scintillating plates of 50 mm thick and anti-coincidence electronic system. The shield is made of 150 mm thick walls of very low activity lead, especially 20 mm with activity of <10 Bq / kg. The observed background was 1.2 s-1 and 0.36 s-1 with and without active shield, respectively, on the regions from 50 keV to 3 MeV. The detection efficiency curve has been precisely measured for regions from 80 keV to 2 MeV with a 10 3 ml marinelli beaker sample, made with calibrated mixed-sources consists of 109Cd, 57Co, 139Ce, 203Hg, 113Sn, 85Sr, 137Cs, 60Co and 88 Y. The virtues of the method are demonstrated by applying on experiment that requires the lowest detection limit