Flux Modulations seen by the Muon Veto of the GERDA Experiment

The GERDA experiment at LNGS of INFN is equipped with an active muon veto. The main part of the system is a water Cherenkov veto with 66~PMTs in the water tank surrounding the GERDA cryostat. The muon flux recorded by this veto shows a seasonal modulation. Two effects have been identified which are caused by secondary muons from the CNGS neutrino beam (2.2 %) and a temperature modulation of the atmosphere (1.4 %). A mean cosmic muon rate of $I^0_{\mu} = (3.477 \pm 0.002_{\textrm{stat}} \pm 0.067_{\textrm{sys}}) \times 10^{-4}$/(s$\cdot$m$^2$) was found in good agreement with other experiments at LNGS at a depth of 3500~meter water equivalent.Comment: 7 pages, 6 figure

Limit on the Radiative Neutrinoless Double Electron Capture of 36^{36}Ar from GERDA Phase I

Neutrinoless double electron capture is a process that, if detected, would give evidence of lepton number violation and the Majorana nature of neutrinos. A search for neutrinoless double electron capture of $^{36}$Ar has been performed with germanium detectors installed in liquid argon using data from Phase I of the GERmanium Detector Array (GERDA) experiment at the Gran Sasso Laboratory of INFN, Italy. No signal was observed and an experimental lower limit on the half-life of the radiative neutrinoless double electron capture of $^{36}$Ar was established: $T_{1/2} >$ 3.6 $\times$ 10$^{21}$ yr at 90 % C.I.Comment: 7 pages, 3 figure

Determination of dead-layer variation in HPGe detectors

The dead-layer uniformity of the top surface of two high purity germanium detectors has been studied using a novel automated scanning set-up that allows a fine-grained topography of a detector's top and lateral surfaces. Comparisons between measurements and Monte Carlo simulations allowed implementation of a dead-layer variation into the detector model, which reproduces the measurements results. The effect of the non-uniform dead-layer on activity determinations based on low-energy gamma-rays (i.e. below 100 key) has been determined to be of the order of 10% or more. (C) 2013 Elsevier Ltd. All rights reserved

A Compton scattering setup for pulse shape discrimination studies in germanium detectors

Pulse shape discrimination is an important handle to improve sensitivity in low background experiments. A dedicated setup was built to investigate the response of high-purity germanium detectors to single Compton scattered events. Using properly collimated \u3b3-ray sources, it is possible to select events with known interaction location. The aim is to correlate the position dependent signal shape with geometrical and electrical properties of the detector. We report on design and performance of the setup with a first look on data

Isotopically modified Ge detectors for GERDA: from production to operation

The GERDA experiment searches for the neutrinoless double beta (0v beta beta) decay of Ge-76 using high-purity germanium detectors made of material enriched in Ge-76. For Phase II of the experiment a sensitivity for the half life T-1/2(0v) similar to 2.10(26) yr is envisioned. Modified Broad Energy Germanium detectors (BEGe) with thick n(+) electrodes provide the capability to efficiently identify and reject background events, while keeping a large acceptance for the 0v beta beta-decay signal through novel pulse-shape discrimination (PSD) techniques. The viability of producing thick-window BEGe-type detectors for the GERDA experiment is demonstrated by testing all the production steps from the procurement of isotopically modified germanium up to working BEGe detectors. Comprehensive testing of the spectroscopic as well as PSD performance of the GERDA Phase II prototype BEGe detectors proved that the properties of these detectors are identical to those produced previously from natural germanium material following the standard production line of the manufacturer. Furthermore, the production of BEGe detectors from a limited amount of isotopically modified germanium served to optimize the production, in order to maximize the overall detector mass yield. The results of this test campaign provided direct input for the subsequent production of the enriched germanium detectors

The background in the 0 nu beta beta experiment GERDA

The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double beta ( 0\u3bd\u3b2\u3b2 ) decay of 76 Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the Q\u3b2\u3b2 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\u3b2\u3b2 . The main parameters needed for the 0\u3bd\u3b2\u3b2 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\u3b2\u3b2 with a background index ranging from 17.6 to 23.8 7 10 123 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\u3b2\u3b2 is dominated by close sources, mainly due to 42 K, 214 Bi, 228 Th, 60 Co and \u3b1 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 \u3b3 peaks, the energy spectrum can be fitted in an energy range of 200 keV around Q\u3b2\u3b2 with a constant background. This gives a background index consistent with the full model and uncertainties of the same size

Limit on Neutrinoless Double Beta Decay of 76Ge by GERDA

The Gerda experiment at the Laboratori Nazionali del Gran Sasso in Italy uses germanium detectors made from material with an enriched Ge-76 isotope fraction to search for neutrinoless double beta decay of this nucleus. Applying a blind analysis we find no signal after an exposure of 21.6 kg.yr and a background of about 0.01 cts/(keV.kg.yr). A half-life limit of T-1/20(v) > 2.1 . 10(25) yr (90% C.L.) is extracted. The previous claim of a signal for Ge-76 is excluded with 99% probability in a model independent way

Search of Neutrinoless Double Beta Decay with the GERDA Experiment

The GERDA (GERmanium Detector Array) is an experiment for the search of neutrinoless double beta decay (0 nu beta beta) in Ge-76, located at Laboratori Nazionali del Gran Sasso of INFN (Italy). In the first phase of the experiment, a 90% confidence level (C.L.) sensitivity of 2.4 . 10(25) yr on the 0 nu beta beta decay half-life was achieved with a 21.6 kg.yr exposure and an unprecedented background index in the region of interest of 10(-2) counts/(keV.kg.yr). No excess of signal events was found, and an experimental lower limit on the half-life of 2.1 . 10(25) yr (90% C.L.) was established. Correspondingly, the limit on the effective Majorana neutrino mass is m(ee) < 0.2-0.4 eV, depending on the considered nuclear matrix element. The previous claim for evidence of a 0 nu beta beta decay signal is strongly disfavored, and the field of research is open again