Low background germanium planar detector for gamma-ray spectrometry

A new ultra-low background planar germanium spectrometer has been developed. The planar geometry improves the sensitivity and energy resolution below 600 keV. The integral background counting rate in the Laboratoire Souterrain de Modane (4800 m water equivalent) in the energy range from 20 to 1500 keV for the planar Ge (mass=800 g) is 140 count/day. After 40 days of statistics, the background counting rates for all expected single lines are below 0.5 count/day with the exception of 210Pb(46-keV line) which was measured to be (1.76±0.25) count/day. Monte Carlo simulations have been performed to explain the origin of the remaining background and to calculate the detection efficiencies. Sensitivities around 1 mBq/kg are obtained within few days of statistics for 226Ra and 228Th. The main achievement is the high sensitivities for 210Pb (46-keV line) and 238U (234Th: 63 and 93 keV lines). For an aluminium sample (mass=1 kg) the limits obtained in 15 days are 210Pb<9mBq/kg and 238U<3mBq/kg.