Low-level germanium gamma-ray spectrometry at the μBq/kg level and future developments towards higher sensitivity

摘要

GeMPI, a highly sensitive germanium gamma-spectrometer operated at the Gran Sasso underground laboratory is described. Its complete cryostat system and shield was made from highly selected low activity materials. Radon suppression, also during sample insertion is achieved by an air-lock system combined with an airtight steel casing around the shield, which is pressurised with nitrogen gas. The achieved background level in combination with a large sample capacity of up to 15 1 around the 2.2-kg Ge crystal allows measuring concentrations of the gamma-active chain members down to 10~(12) g/g of U/Th equivalent (12.3 μBq/kg ~(238)U, 4.06 μBq/kg ~(232)Th) and 10~(-9) g/g of K (31 μBq/kg ~(40)K). This is demonstrated with some key measurements of Cu and Pb shielding materials. Cosmogenic production rates for 8 radioisotopes have been determined in exposed Cu to range from 50 μBq/kg for ~(46)Sc to 2.1 mBq/kg for ~(60)Co. A possible further background reduction in Ge-spectrometry by operating naked crystals in liquid nitrogen is discussed.