Development of a low neutron emission 228Th source for the calibration of GERDA

GERDA (GERmanium Detector Array) is a very-low background experiment under construction at LNGS (Laboratori Nazionali del Gran Sasso, 3400 m.w.e.) in Italy. It is designed to search for the neutrinoless double-β decay using an array of enriched 76Ge detectors. A limit of T1/2=15.1025 y in Phase II of GERDA can be achieved by limiting the background rate to B ≤ 1.10-3 cts/(keV.kg.y). This requires a good understanding and suppression of the background produced by cosmic rays and natural radioactivity. Furthermore neutrons from spontaneous fission and (α,n) reactions in adjacent materials can contribute considerably to the background rate. 228Th has been established as a good calibration source candidate for GERDA due to its γ-emission in the region of interest around Qββ=2.04 MeV. The calibration setup requires that the 228Th source is permanently installed in the setup during data taking in a parking position at 3.5 m above the germanium detector array. Monte Carlo simulations have shown that the γ-background from the calibration source located in its parking position can be suppressed by a Ta absorber. This work investigates the significance of the neutron background induced by (α,n) reactions within the intrinsic components of a commercialy available 228Th calibration source. Furthermore a new type of a 228Th source has been developed in order to reduce the n-rate approximately by two orders of magnitude compared to a commercial 228Th source.