Spectroscopic Performances of the GERDA Cryogenic Charge Sensitive Amplifier based on JFET-CMOS ASIC, coupled to Germanium Detectors Alessio D’Andragora, Carla M. Cattadori, Assunta di Vacri, Matthias Junker, Luciano Pandola, Alberto Pullia, Stefano Riboldi, Francesca Zocca, Marik Barnabé Heider, Dusan Budjáš, Jürgen Kiko, Stefan Schönert, Konstantin Gusev, Béla Majorovits Abstract–In the GERDA (GERmanium Detector Array) double-beta-decay experiment, it is planned to operate in liquid Argon (LAr) germanium detectors, organized in three fold strings. In this application the use of cryogenic front-end (FE) electronics is mandatory. Two versions of Charge Sensitive Amplifier (CSA), namely a 1-channel (1-ch) and a 3-channel (3- ch), based on JFET-CMOS circuits, have been realized and tested. The 3-chs CSA are designed to serve the detector string. While in the reference test the 1-ch circuit and a custom encapsulated germanium (Ge) detector (SUB) were operated both submerged in liquid Nitrogen (LN 2 ), in the naked detector test both the 1-ch circuit and the naked unsegmented Ge detector were submerged in liquid Argon (LAr). A resolution of 3.2 keV FWHM at 1.332 MeV 60 Co has been obtained in the latter configuration to be compared to 2.2 keV obtained in the reference test. The 3-ch CSA, based on three JFETs connected to three channels of the CMOS ASIC, mounted on a Cuflon PCB, has been tested both coupled to the reference SUB and to a naked prototype detector. The obtained resolution for the 1.332 MeV line of 60 Co was 2.4 keV with the CSA coupled to the SUB, and 2.9 keV with the naked Ge detector. The spectroscopic performances have been measured connecting the CSA output to an acquisition system through 10 m long cryogenic cables to simulate the real FE connection in the GERDA environment. I. INTRODUCTION HE GERDA experiment [1,2] at Laboratori Nazionali del Gran Sasso (LNGS) searches for neutrinoless double beta decay of 76 Ge adopting a novel technique; germanium diodes, isotopically enriched up to 86% in 76 Ge, acting both as the source and the detector, are deployed in three detector strings and operated bare in a 64 m 3 liquid Argon (LAr) bath. LAr is A. D’Andragora is with INFN Laboratori Nazionali del Gran Sasso (LNGS) Assergi (AQ), Italy, and University of L’Aquila, Department of Electrical and Information Engineering, L’Aquila, Italy (telephone: +39 0862437704, e-mail: alessio.dandragora@lngs.infn.it) C. M. Cattadori is with INFN Milano Bicocca, Milano, Italy. A. di Vacri, L. Pandola and M. Junker are with INFN LNGS, Assergi (AQ), Italy. A. Pullia, S. Riboldi and F. Zocca are with University of Milano, Department of Physics and INFN Sezione di Milano, Milano, Italy. M. Barnabé Heider, D. Budjáš, J. Kiko and S. Schönert are with MPIK, Heidelberg, Germany K. Gusev is with RRC Kurchatov Institute, Moscow, Russia, and JINR, Dubna, Russia. B. Majorovits is with MPI, Munich, Germany. both the cooling medium and the shield against external radioactivity. The length of cabling between the detectors and the room temperature environment is around 10 m; the front-end (FE) CSA must work at cryogenic temperature. Fig. 1 shows the GERDA setup. Fig. 1. A schematic view of the GERDA setup. The Ge diodes hang free inside a stainless steal cryostat of 64 m 3 of LAr in order to provide the necessary cooling and to avoid massive shielding with heavy materials. Materials made from light elements are preferred in the vicinity of the detectors. Two versions, 1-ch and 3-ch, of a JFET-CMOS CSA, both based on the same architecture and ASIC circuit, have been developed, manufactured and tested coupled with prototype detectors. The 1-ch CSA has been coupled to a GERDA prototype Ge detector in LAr (setup #1) and to an encapsulated Ge detector (SUB) in liquid Nitrogen (LN 2 ) (setup #2). The 3-ch CSA has been coupled to the setup #2 and to a Ge detector in LAr in the final GERDA cabling and mounting configuration (setup #3). Both detectors are p-type coaxial close-end High Purity Germanium (HPGe) Detectors. The outer contact is biased at positive high voltage, and the read-out electrode is DC- coupled to the preamplifier. II. CIRCUIT ARCHITECTURE AND FEATURES OF THE JFET- CMOS In Fig. 2 the structure of the CSA is shown. It is optimized for negative output voltage swings. It consists of an external T 2009 IEEE Nuclear Science Symposium Conference Record N13-43 9781-4244-3962-1/09/$25.00 ©2009 IEEE 396