Phase I: Use available 76 Ge diodes from Heidelberg-Moscow and IGEX experiments (~18 kg).

1. The GERDA experiment GERDA collaboration The GERDA collaboration Background reduction and suppression techniques ( also see dedicated posters) • European collaboration of the following institutes: • INFN Laboratori Nazionali del Gran Sasso, Assergi/Italy • Jagellonian University, Cracow/Poland • Joint Institute for Nuclear Research, Dubna/Russia • Institute for Reference Materials and Measurements (IRMM), Geel/Belgium • Max-Planck-Institut für Kernphysik, Heidelberg/Germany • Institut für Kernphysik, Universität Köln/Germany • Università di Milano Bicocca e INFN Milano, Milano/Italy • Institute for Theoretical and Experimental Physics, Moscow/Russia • Institute for Nuclear Research of the Russian Academy of Sciences, Moscow/Russia • Russian Research Centre Kurchatov Institute, Moscow/Russia • Max-Planck-Institut für Physik, München/Germany • Physikalisches Institut, Universität Tübingen/Germany Cryogenic gas purification Material screening: • Large scale N2 purification plant at Gran Sasso: 222Rn in N2 <0.5 mBq/m3. • Recent result: • Ar can be purified to same purity level as N2. • g-screening with ultralow background Ge-spectrometers. • GeMPI at Gran Sasso reaches few ten mBq/kg level. Neutrinoless double beta decay Detector segmentation The LArGe facility • 0nbb-decay is single-side event. • Double-side background events can be discriminated by detector segmenation. • forbidden in the standard model (Lepton-number violating process). • Only possible if n is massive Majorana particle. • Signature is peak at Q-value of decay. • R&D project for low background detector operation in liquid argon. Lock Poly-ethylene PMTs 4.2s evidence for 0nbb-decay reported for 76Ge (Q-value: 2039 keV): Klapdor-Kleingrothaus et al., NIM A 522 (2004) 371-406. Liquid argon Copper Example: 6f/3z segmentation and achieved background reduction for a 60Co source. Lead Steel GERDA design and sensitivity Operation of bare 76Ge diodes in ultrapure cryogenic liquid (LAr/LN2). Contaminations from cryostat/ crystal holder can be avoided. Low mass detector suspension and holder made out of carefully selected materials only. Experiment will be performed in the Gran Sasso underground lab. Phase I activities Cleanroom Prototype detector operations Opening of enriched detectors Water tank (Muon-veto) Goal: Spectroscopic performance of the detector assembly in a radon-free test bench: Same resolution achieved for bare crystal as measured in the cryostat. 2 76Ge-diodes have been removed from their cryostat and measured. Cryogenic liquid Ge-detector array Cryostat • Phase I: • Use available 76Ge diodes from Heidelberg-Moscow and IGEX experiments (~18 kg). • Scrutinize with high siginificance current evidence. • Phase II: • Add new diodes up to 40 kg mass of 76Ge. • Segmentation of detectors. • Long-term future (Phase III): • World-wide collaboration: O(500 kg) experiment. Heidelberg-Moscow detector ANG I Assumption: DE=4 keV IGEX detector RG III Phase I detector array Both diodes have been refurbished and are stored underground again. Muon-induced background Prompt background (without m-veto) Delayed background Phase II detectors MC simulation • 37 kg of enriched Ge for new 76Ge diodes already produced in Russia. • Segmented, true coaxial n-type detectors. • Signal from each segment and core signal are read out separately. • Extremely low mass support structure. • Special suspension system. • First segmented prototype detector successfully operated. Counts/kg/keV/y no cut: 10-2 Anticoincidence (phase I): 10-3 Segmentation (phase II): 3·10-4 goal • Background goal 10-4cts/(kg·keV·y) can be achieved for LN2. • More neutrons produced in LAr  Background above 10-4 cts/(kg·keV·y). • Goal can be met by delayed coinci-dence cut (muon, g-rays, b-decay). Energy (keV) • Goal is not achievable without muon-veto. • But 75% efficient muon-veto is sufficient. • Water Čerenkov veto with light reflector foil (VM2000) is expected to be more efficient. Max-Planck-Institut für Kernphysik / Heidelberg