Characterization of BEGe detectors in the HADES underground laboratory Erica Andreotti n Joint Research Centre, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium on behalf of the GERDA Collaboration article info Available online 20 November 2012 Keywords: Detectors of radiation HPGe detectors BEGe detectors GERDA abstract This paper describes the characterization of newly produced Broad Energy Germanium (BEGe) detectors, enriched in the isotope 76 Ge. These detectors have been produced in the frame of the GERDA experiment. The aim of the characterization campaign consists in the determination of all the important operational parameters (active volume, dead layer thickness and uniformity, energy resolution, detector stability in time, quality of pulse shape discrimination). A protocol test procedure and devoted set-ups, partially automated, have been developed in view of the large number ð 25Þ of BEGe’s detectors to be tested. The characterization is carried out in the HADES underground laboratory, located 225 m below ground (  500 m water equivalent) in Mol, Belgium. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Broad Energy Germanium (BEGe) detectors are known for their enhanced pulse shape properties, which allow discriminating multiple site from single site events [1]. This feature can be exploited for background reduction in experiments looking for extremely rare events, such as neutrinoless double beta decays ð0nbbÞ. The GERDA experiment will use BEGe detectors for its second phase, where the use of pulse shape discrimination techniques will additionally suppress the background. GERDA [2] is an experiment designed to investigate the 0nbb decay of 76 Ge ðQ bb ¼ 2039 keVÞ. If observed, this process would imply the violation of total lepton number conservation and it would be the proof that the neutrino is its own anti-particle, i.e. a Majorana fermion. Possibly, it will also allow the determination of the absolute neutrino mass scale. The background reduction is the main difficulty in this search, due to the extremely long half-life of the decay. For this reason, all the necessary precautions have to be taken in order to prevent radioactive contamination and cosmogenic activation in the detectors. The experiment itself is located at the Laboratori Nazionali del Gran Sasso (LNGS), Italy, where a natural shielding from cosmic radiation of 3800 m water equivalent (w.e.) is guaranteed. In its second phase, GERDA will use BEGe detectors made from Ge enriched to 86% in 76 Ge. The 35 kg starting material is being processed by Canberra in Oak Ridge, USA [3]. A first batch of seven crystals was shipped by sea to Belgium in January 2012, where they have been successfully turned into working BEGe detectors. The diode production is being performed by Canberra in Olen, Belgium. The exposure to cosmic radiation was minimized by storing the crystals in a container equipped with shielding layers of steel and water during the transport. The diodes are always stored in underground locations in the vicinity of the plants during the various production phases. The specifications given by Canberra are the following: crystal diameter 75 75 mm, crystal length 30 5 þ 10 mm, energy resolution equal or better than 1.64–1.79 keV at the 1332 keV 60 Co line, depletion voltage below 4 kV. In order to verify that the necessary requirements are met, a complete characterization of the detectors’ properties is needed prior to their installation in the GERDA experimental set-up. Due to the large number of detectors ð 25Þ to be tested in a relatively short time, automated data acquisition and analysis tools have been developed as described in Section 3. A dedicated full set of radioactive sources for fast screening has also been provided. 2. The HADES underground laboratory The characterization is being performed in the HADES (High Activity Disposal Experimental Site) underground laboratory [4], located on the premises of the Belgian Nuclear Research Center SCKCEN in the town of Mol, Belgium. The  500 m w.e. over- burden guarantees a muon flux reduction of the order of 10 4 with respect to the ground level. This location was chosen because of Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.11.053 n Tel.: þ 32 499721476. E-mail addresses: Erica.ANDREOTTI@ec.europa.eu, ericaand@libero.it Nuclear Instruments and Methods in Physics Research A 718 (2013) 475–477