Production and characterization of 228Th calibration sources with low neutron emission for GERDA

The GERDA experiment at the Laboratori Nazionali del Gran Sasso (LNGS) searches for the neutrinoless double beta decay of Ge-76. In view of the GERDA Phase II data collection, four new Th-228 radioactive sources for the calibration of the germanium detectors enriched in Ge-76 have been produced with a new technique, leading to a reduced neutron emission rate from (alpha, n) reactions. The gamma activities of the sources were determined with a total uncertainty of similar to 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 similar to 10(-6) n/(s.Bq), with a reduction of about one order of magnitude with respect to commercially available Th-228 sources. Additionally, a specific leak test with a sensitivity to leaks down to similar to 10 mBq was developed to investigate the tightness of the stainless steel capsules housing the sources after their use in cryogenic environment.