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Germanium Detector Array

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The Germanium Detector Array (or GERDA) experiment is searching for neutrinoless double beta decay (0νββ) in Ge-76 at the underground Laboratori Nazionali del Gran Sasso (LNGS). Neutrinoless beta decay is expected to be a very rare process if it occurs. The collaboration predicts less than one event each year per kilogram of material, appearing as a narrow spike around the 0νββ Q-value (Qββ = 2039 keV) in the observed energy spectrum. This means background shielding is required to detect any rare decays. The LNGS facility has 1400 meters of rock overburden, equivalent to 3000 meters of water shielding, reducing cosmic radiation background.

Contents

Design

The experiment uses high purity enriched Ge crystal diodes (HPGe) as a beta decay source and particle detector. The detectors from the HdM and Igex experiments were reprocessed and used in phase 1. The detector array is suspended in a liquid argon cryostat lined with copper and surrounded by an ultra-pure water tank. PMTs in the water tank and plastic scintillators above detect and exclude background muons. Pulse-shape discrimination (PSD) is applied as a cut to discriminate between particle types.

Phase 2 will increase the active mass to 38 kg using 30 new broad energy germanium (BEGe) detectors. A magnitude reduction in background is planned to 10−3 counts/(keV·kg·yr) using cleaner materials. This will increase the half-life sensitivity to 1026 years once 100 kg·yr of data is taken and enable evaluation of possible ton-scale expansion.

Results

Phase I collected data November 2011 to May 2013, with 21.6 kg·yr exposure, obtaining a 0νββ 90% CL half-life limit of:

T 0 ν β β > 2.1 ⋅ 10 25 y r . This limit can be combined with previous results, increasing it to 3·1025 yr, disfavoring the Heidelberg-Moscow detection claim. A bound on the effective neutrino mass was also reported: mν < 400 meV.

The double beta decay half-life was also measured: T2νββ = 1.84·1021 yr.

Phase II will have additional enriched Ge detectors and reduced background, raising the sensitivity about one order.

Phase II (7 strings, 35.8 kg of enriched detectors) was deployed in Dec 2015.

Mid 2016 : First results from phase II released. confirming reduced background and (with only 10.8 kg·yr exposure) raising the lower bound on half life.

Publications

  • Agostini, M.; et al. (GERDA Collaboration) (19 September 2013). "Results on Neutrinoless Double-β Decay of 76Ge from Phase I of the GERDA Experiment". Physical Review Letters. 111 (12): 122503. arXiv:1307.4720. Bibcode:2013PhRvL.111l2503A. doi:10.1103/PhysRevLett.111.122503. PMID 24093254. 
  • Agostini, M.; et al. (GERDA Collaboration) (12 February 2013). "Measurement of the half-life of the two-neutrino double beta decay of 76Ge with the GERDA experiment". Journal of Physics G. 40 (3): 035110. arXiv:1212.3210. Bibcode:2013JPhG...40c5110T. doi:10.1088/0954-3899/40/3/035110. 
  • Ackermann, K.-H.; et al. (March 2013). "The GERDA experiment for the search of 0νββ decay in 76Ge". European Physical Journal C. 73 (3): 2330. arXiv:1212.4067. Bibcode:2013EPJC...73.2330A. doi:10.1140/epjc/s10052-013-2330-0. 

    • References

    Germanium Detector Array Wikipedia
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