Background components of Ge detectors in Ogoya underground laboratory.

Ogoya underground laboratory, (OUL; 270 m water equivalent), has 5 well, 4 planar, and 1 coaxial-type ultra-low background Ge detectors with passive shield of old lead. An anticoincidence system with a plastic scintillator (PS) has been tested to reduce cosmic-ray-induced background. Energy spectra of PSs, coincidence counting rate, and angular distribution of cosmic rays were studied. Based on this study, we propose a new index of Ge surface area-normalized background counting rate (d(-1) cm(-2)) to compare shielding efficiency between different types of Ge detectors.