1. Phase I: Use available 76 Ge 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 76 Ge. Segmentation of detectors. Long-term future (Phase III): World-wide collaboration: O(500 kg) experiment. The GERDA experiment GERDA collaboration The GERDA collaboration Max-Planck-Institut für Kernphysik / Heidelberg 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 forbidden in the standard model (Lepton- number violating process). Only possible if is massive Majorana particle. Signature is peak at Q-value of decay. 4.2  evidence for  -decay reported for 76 Ge (Q-value: 2039 keV): Klapdor-Kleingrothaus et al., NIM A 522 (2004) 371-406. GERDA design and sensitivity Water tank (Muon-veto) Cryostat Cleanroom Ge-detector array Cryogenic liquid Neutrinoless double beta decay Background reduction and suppression techniques (  also see dedicated posters) Material screening:Cryogenic gas purification  -screening with ultralow background Ge-spectrometers. GeMPI at Gran Sasso reaches few ten  Bq/kg level. Large scale N 2 purification plant at Gran Sasso: 222 Rn in N 2 <0.5  Bq/m 3. Recent result: Ar can be purified to same purity level as N 2. Detector segmentation  -decay is single-side event. Double-side background events can be discriminated by detector segmenation. Phase I activities Prototype detector operationsOpening of enriched detectors Phase I detector array Goal: Spectroscopic performance of the detector assembly in a radon-free test bench: Same resolution achieved for bare crystal as measured in the cryostat. Heidelberg-Moscow detector ANG I IGEX detector RG III 2 76 Ge-diodes have been removed from their cryostat and measured. Both diodes have been refurbished and are stored underground again. R&D project for low background detector operation in liquid argon. Lock Lead Steel Poly- ethylene Copper Liquid argon PMTs Phase II detectors Example: 6  3z segmentation and achieved background reduction for a 60 Co source. The LArGe facility 37 kg of enriched Ge for new 76 Ge 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. Muon-induced background Prompt background (without  -veto) Energy (keV) Anticoincidence (phase I): 10 -3 Segmentation (phase II): 3·10 -4 Counts/kg/keV/y goal no cut: 10 -2 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. Background in LN 2 [cts/(kg·keV·y)] Background in LAr [cts/(kg·keV·y)] 77,77m Ge1.0 · 10 -5 1.1 · 10 -4 Others5 · 10 -6 5 · 10 -5 Delayed background Background goal 10 -4 cts/(kg·keV·y) can be achieved for LN 2. More neutrons produced in LAr  Background above 10 -4 cts/(kg·keV·y). Goal can be met by delayed coinci- dence cut (muon,  -rays,  -decay). MC simulation Operation of bare 76 Ge diodes in ultrapure cryogenic liquid (LAr/LN 2 ). 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. Assumption:  E=4 keV