Background of Ge detectors in Ogoya underground laboratory

From 1995, ultra low background (BG) counting system has been set up at Ogoya underground laboratory, OUL (270m water equivalent) in the old mining tunnel. There are 5 well detectors, 4 planar detectors, and a coaxial detector with low-level background count-rate, with high detector efficiency, and with pure massive shield. Here, we propose new BG value normalized in Ge surface area (cpd/cm2Ge) to compare BG level between different types of Ge detectors. In this BG value, 0.52 is best one for planer type detector (38 cm2 x 3 cm) and 0.54 for coaxial detector (93% efficiency) in OUL. Anticoincidence system with plastic scintillator (PS) is tested to reduce cosmic-ray (CR) induced components. Energy spectra of PS with 5 cm thickness were measured at 0 and 270 MWe. It is not clear to set discrimination voltage to cut off gamma-ray events, because energy spectrum of CR (muon) was extremely broad. Coincidence count rate of Ge-PS is not depend on thickness of PS and time. Angular distribution of CR is cos(θ) or cos2(θ) as a function of altitude angle of θ. At OUL, BG count rate of Ge-PS coincidence events is 0.4 cpd/cm2Ge for coaxial and planer Ge with effective PS solid angle of 0.28 and 0.54 sr, respectively. (author)