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 ⁷⁶Ge. In view of the GERDA Phase II data collection, four new ²²⁸Th radioactive sources for the calibration of the germanium detectors enriched in ⁷⁶Ge 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 ³He 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 ²²⁸Th 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