Abstract
The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double-beta (0νββ) decay of 76Ge. The technological challenge of Gerda is to operate in a “background-free” regime in the region of interest (ROI) after analysis cuts for the full 100 kg·yr target exposure of the experiment. A careful modeling and decomposition of the full-range energy spectrum is essential to predict the shape and composition of events in the ROI around Qββ for the 0νββ search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos (2νββ) and in order to identify the location of residual impurities. The latter will permit future experiments to build strategies in order to further lower the background and achieve even better sensitivities. In this article the background decomposition prior to analysis cuts is presented for Gerda Phase II. The background model fit yields a flat spectrum in the ROI with a background index (BI) of {16.04}_{-0.85}^{+0.78}cdotp {10}^{-3} cts/(keV·kg·yr) for the enriched BEGe data set and {14.68}_{-0.52}^{+0.47}cdotp {10}^{-3} cts/(keV·kg·yr) for the enriched coaxial data set. These values are similar to the one of Phase I despite a much larger number of detectors and hence radioactive hardware components.
Highlights
Background expectationThe event energy distribution of the three data sets is displayed in figure 2; the sum spectrum of M1-enriched Broad Energy Germanium (BEGe) (enrBEGe) and M1-enriched coaxial (enrCoax) in the top panel and M2-enrGe in the bottom panel
The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of Istituto Nazionale di Fisica Nucleare (INFN) is searching for neutrinoless double-beta (0νββ) decay of 76Ge
We find for the M1-enrBEGe data set 66.4 %, 94.5 % and 99.6 % of points in the 1σ, 2σand 3σ-bands, for the M1-enrCoax data set 66.0 %, 94.7 % and 99.8 % and for the M2-enrGe data set 70.0 %, 96.1 % and 99.7 %, respectively
Summary
Background expectationThe event energy distribution of the three data sets is displayed in figure 2; the sum spectrum of M1-enrBEGe and M1-enrCoax in the top panel and M2-enrGe in the bottom panel. The highest energies displayed are dominated by a peak like structure emerging at 5.3 MeV with a pronounced low energy tail. This is a typical spectral feature of α particles and can, here, be attributed to 210Po decay on the thin detector p+ surfaces [14]. Events above the 210Po peak belong to α decays emerging from the 226Ra sub-chain on the detector p+ surfaces. All these components contribute to M2-enrGe except for 39Ar, 2νββ and high energy α components. This is due to the short range of α (tens of μm) and β particles (typically smaller than 1.5 cm) in LAr and germanium with respect to the distance between detectors which is of the order of several cm
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