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 flux 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 emitted neutron flux 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.