A low background gamma-ray spectrometer with a large well HPGe detector.

A low background gamma-ray spectrometer was established in a ground-based laboratory. It consists of a large well (diameter: 28 mm, depth: 40 mm) HPGe detector and an anti-cosmic shielding system. The photon background of the present system was measured with and without the anti-cosmic mode, and compared with each other, and the detection capability with the detection efficiency calibration was estimated. The background with passive and active shields for the energy region between 40 keV and 3000 keV was reduced by 78% compared to one with just passive shield. The full energy peak efficiency for cylindrical samples with three different heights and the minimum detectable activity of the present system were tabulated for main radionuclides to be measured in environmental monitoring.