Abstract
The Germanium Detector Array (GERDA) experiment is searching for neutrinoless double beta (0νββ ) decay of 76 Ge, a lepton number violating nuclear process predicted by extensions of the Standard Model. GERDA is an array of bare germanium diodes immersed in liquid argon located at the Gran Sasso National Laboratory (LNGS) in Italy. The results of the GERDA Phase I data taking with a total exposure of 21.6 kg yr and a background index of 0.01 cts/(keV kg yr) are presented in this paper. No signal was observed and a lower limit of T 1/2 0ν > 2.1×1025 yr (90% C.L.) was derived for the half-life of the 0νββ decay of 76 Ge. Phase II of the experiment aims to reduce the background around the region of interest by a factor of ten.
Highlights
For some even-even nuclei, decay via single β-emission is energetically forbidden
The experimental signature of 0νββ decay is a peak at the Q-value of the decay, Qββ = 2039 keV, which lies above the continuous energy spectrum of the 2νββ decay
The observed spectrum in the energy range between 600 and 1800 keV is dominated by the double beta decay of 76Ge
Summary
For some even-even nuclei, decay via single β-emission is energetically forbidden. their neutrino accompanied double beta (2νββ) decay is allowed in the framework of the Standard Model. Eight p-type high purity germanium (HPGe) semi-coaxial detectors from the HdM [3] and IGEX [4] experiments were refurbished and used as the main GERDA Phase I detectors. They are enriched to ∼86% in 76Ge and have a total mass of 17 kg. The golden coaxial dataset with an exposure of 17.9 kg yr contains all data taken with the enriched semi-coaxial detectors with the exception of a short period of approximately 30 days This was due to increased activity after the insertion of the five BEGe detectors. The mean exposure-weighted energy resolutions for the GERDA detectors are 4.8 ± 0.2 keV for the semi-coaxial detectors and 3.2 ± 0.2 keV for the BEGe detectors
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