Production and characterization of $^{228}$Th calibration sources with low neutron emission for GERDA

Abstract

The GERDA experiment at the Laboratori Nazionali del Gran Sasso (LNGS) searches for the neutrinoless double beta decay of (76)Ge. In view of the GERDA Phase II data collection, four new (228)Th radioactive sources for the calibration of the germanium detectors enriched in (76)Ge have been produced with a new technique, leading to a reduced neutron emission rate from (α, n) reactions. The gamma activities of the sources were determined with a total uncertainty of ~4% using an ultra-low background HPGe detector operated underground at LNGS. The neutron emission rate was determined using a low background LiI(Eu) detector and a (3)He counter at LNGS. In both cases, the measured neutron activity is ~10(−)(6) n/(s⋅Bq), with a reduction of about one order of magnitude with respect to commercially available (228)Th sources. Additionally, a specific leak test with a sensitivity to leaks down to ~10 mBq was developed to investigate the tightness of the stainless steel capsules housing the sources after their use in cryogenic environment.