Germanium Detector Array

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.

After completing the GERDA experiment, the GERDA collaboration will merge with MAJORANA-collaboration to build a new experiment LEGEND.

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:

. 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 of magnitude.

Phase II (7 strings, 35.8 kg of enriched detectors) was started in Dec 2015.[1]:10

Preliminary results of Phase II have been published in Nature.[2] The background index for BEGe detectors is 0.7·10−3 counts/(keV·kg·yr), which translates to less than one count in the signal region after an exposure of 100 kg·yr. The present limit on the half life is T1/2=5.3·1025 yr (90% C.L.).

As of 2018, the Phase II data-taking continues.

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References
  1. GERDA collaboration; M.Agostini; et al. (8 July 2016). First results from GERDA Phase II (PDF). XXVII International Conference on Neutrino Physics and Astrophysics (Neutrino 2016). London.
  2. GERDA collaboration; M.Agostini; et al. (2017-04-05), "Background-free search for neutrinoless double-β decay of 76Ge with GERDA", Nature (in German), 544 (7648), pp. 47–52, arXiv:1703.00570, Bibcode:2017Natur.544...47A, doi:10.1038/nature21717, PMID 28382980

Publications
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