Abstract
The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double beta (\(0\nu \beta \beta \)) decay of \(^{76}\)Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the \(Q_{\beta \beta }\) value of the decay. To avoid bias in the signal search, the present analysis does not consider all those events, that fall in a 40 keV wide region centered around \(Q_{\beta \beta }\). The main parameters needed for the \(0\nu \beta \beta \) analysis are described. A background model was developed to describe the observed energy spectrum. The model contains several contributions, that are expected on the basis of material screening or that are established by the observation of characteristic structures in the energy spectrum. The model predicts a flat energy spectrum for the blinding window around \(Q_{\beta \beta }\) with a background index ranging from 17.6 to 23.8 \(\times \) \(10^{-3}\) cts/(keV kg yr). A part of the data not considered before has been used to test if the predictions of the background model are consistent. The observed number of events in this energy region is consistent with the background model. The background at \(Q_{\beta \beta }\) is dominated by close sources, mainly due to \(^{42}\)K, \(^{214}\)Bi, \(^{228}\)Th, \(^{60}\)Co and \(\alpha \) emitting isotopes from the \(^{226}\)Ra decay chain. The individual fractions depend on the assumed locations of the contaminants. It is shown, that after removal of the known \(\gamma \) peaks, the energy spectrum can be fitted in an energy range of 200 keV around \(Q_{\beta \beta }\) with a constant background. This gives a background index consistent with the full model and uncertainties of the same size.
Highlights
Some even–even nuclei are energetically forbidden to decay via single β emission, while the decay via emission of two electrons and two neutrinos is energetically allowed.The experimentally observed neutrino accompanied double beta (2νββ) decay is a second order weak process with half lives of the order of 1018−24 yr [1]
GERmanium Detector Array (Gerda) operates high purity germanium (HPGe) detectors made from material enriched to about 86 % in 76Ge in liquid argon (LAr) which serves both as coolant and as shielding
The analysis presented here considers data taken until 3 March 2013, corresponding to a live time of 417.19 days and an exposure of E = 16.70 kg yr for the coaxial detectors; the four Broad Energy Germanium (BEGe) detectors acquired between 205 and 230 days of live time each, yielding a total exposure of E = 1.80 kg yr
Summary
Some even–even nuclei are energetically forbidden to decay via single β emission, while the decay via emission of two electrons and two neutrinos is energetically allowed.The experimentally observed neutrino accompanied double beta (2νββ) decay is a second order weak process with half lives of the order of 1018−24 yr [1]. The Gerda collaboration has blinded a region of Qββ ± 20 keV during the data taking period [2]. During this time, analysis methods and background models have been developed and tested. Analysis methods and background models have been developed and tested The latter is described in this paper together with other parameters demonstrating the data quality. If the energies of individual events fall within a range Qββ ± 20 keV, these events are stored during the blinding mode in the backup files only They are not converted to the data file that is available for analysis. After cross checks of the background model (Sect. 7) the paper concludes with the prediction for the background at Qββ and the prospective sensitivity of Gerda Phase I (Sect. 8)
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