Production and Characterization of 228Th Calibration Sources with Low Neutron Emission for GERDA

TL;DR

This paper reports the development of new 228-Th calibration sources with significantly reduced neutron emission for the GERDA experiment, enhancing detector calibration while minimizing background noise.

Contribution

A novel production technique for 228-Th sources that reduces neutron flux by an order of magnitude compared to commercial sources.

Findings

Neutron flux reduced by about ten times.

Gamma activity measured with 4% uncertainty.

Leak test sensitivity down to 10 mBq.

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 flux from ( alpha; n ) reactions. The gamma activities of the sources were determined with a total uncertainty of 4 percent 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, a reduction of about one order of magnitude with respect to commercially available 228-Th sources was obtained. 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.