Agostini M. et al. (GERDA Collaboration) M. Wójcik, G. Zuzel, K. Pelczar, K. Panas, M. Misiaszek, N. Frodyma, D. Borowicz

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http://dx.doi.org/10.1140/epjc/s10052-015-3409-6

Abstract

An optimized digital shaping filter has been developed for the Gerda experiment which searches for neutrinoless double beta decay in 76Ge. The Gerda Phase I energy calibration data have been reprocessed and an average improvement of 0.3 keV in energy resolution (FWHM) corresponding to 10 % at the Q value for 0νββ decay in 76Ge is obtained. This is possible thanks to the enhanced low-frequency noise rejection of this Zero Area Cusp (ZAC) signal shaping filter.

Agostini M. et al. (GERDA Collaboration) M. Wójcik, G. Zuzel, K. Pelczar, M. Misiaszek, N. Frodyma, D. Borowicz

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http://dx.doi.org/10.1140/epjc/s10052-014-3253-0

Abstract

The GERmanium Detector Array (Gerda) at the Gran Sasso Underground Laboratory (LNGS) searches for the neutrinoless double beta decay (0νββ) of 76Ge. Germanium detectors made of material with an enriched 76Ge fraction act simultaneously as sources and detectors for this decay. During Phase I of theexperiment mainly refurbished semi-coaxial Ge detectors from former experiments were used. For the upcoming Phase II, 30 new 76Ge enriched detectors of broad energy germanium (BEGe)-type were produced. A subgroup of these detectors has already been deployed in Gerda during Phase I. The present paper reviews the complete production chain of these BEGe detectors including isotopic enrichment, purification, crystal growth and diode production. The efforts in optimizing the mass yield and in minimizing the exposure of the 76Ge enriched germanium to cosmic radiation during processing are described. Furthermore, characterization measurements in vacuum cryostats of the first subgroup of seven BEGe detectors and their long-term behavior in liquid argon are discussed. The detector performance fulfills the requirements needed for the physics goals of Gerda Phase II.

M Agostini et al., (GERDA Collaboration), N. Frodyma, M. Misiaszek, K. Pelczar, M. Wojcik, G. Zuzel

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http://arxiv.org/abs/1306.5084

http://link.springer.com/article/10.1140/epjc/s10052-013-2583-7

Abstract

The GERDA experiment located at the LNGS searches for neutrinoless double beta (0\nu\beta\beta) decay of ^{76}Ge using germanium diodes as source and detector. In Phase I of the experiment eight semi-coaxial and five BEGe type detectors have been deployed. The latter type is used in this field of research for the first time. All detectors are made from material with enriched ^{76}Ge fraction. The experimental sensitivity can be improved by analyzing the pulse shape of the detector signals with the aim to reject background events. This paper documents the algorithms developed before the data of Phase I were unblinded. The double escape peak (DEP) and Compton edge events of 2.615 MeV \gamma\ rays from ^{208}Tl decays as well as 2\nu\beta\beta\ decays of ^{76}Ge are used as proxies for 0\nu\beta\beta\ decay. For BEGe detectors the chosen selection is based on a single pulse shape parameter. It accepts 0.92$\pm$0.02 of signal-like events while about 80% of the background events at Q_{\beta\beta}=2039 keV are rejected.
For semi-coaxial detectors three analyses are developed. The one based on an artificial neural network is used for the search of 0\nu\beta\beta\ decay. It retains 90% of DEP events and rejects about half of the events around Q_{\beta\beta}. The 2\nu\beta\beta\ events have an efficiency of 0.85\pm0.02 and the one for 0\nu\beta\beta\ decays is estimated to be 0.90^{+0.05}_{-0.09}. A second analysis uses a likelihood approach trained on Compton edge events. The third approach uses two pulse shape parameters. The latter two methods confirm the classification of the neural network since about 90% of the data events rejected by the neural network are also removed by both of them. In general, the selection efficiency extracted from DEP events agrees well with those determined from Compton edge events or from 2\nu\beta\beta\ decays.

M Agostini et al., (GERDA Collaboration), N. Frodyma, K. Pelczar, M. Wojcik, G. Zuzel

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http://arxiv.org/abs/1306.5084

http://link.springer.com/article/10.1140/epjc/s10052-014-2764-z

Abstract

The GERmanium Detector Array (GERDA) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double beta decay of 76Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the Q-value of the decay, Q_bb. 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_bb. The main parameters needed for the neutrinoless double beta decay 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_bb with a background index ranging from 17.6 to 23.8*10^{-3} counts/(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-bb is dominated by close sources, mainly due to 42K, 214Bi, 228Th, 60Co and alpha emitting isotopes from the 226Ra 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_bb with a constant background. This gives a background index consistent with the full model and uncertainties of the same size.

M Agostini et al., (GERDA Collaboration), N. Frodyma, K. Pelczar, M. Wojcik, G. Zuzel

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http://dx.doi.org/10.1088/0954-3899/40/3/035110

Abstract

The primary goal of the GERmanium Detector Array (GERDA) experiment at the Laboratori Nazionali del Gran Sasso of INFN is the search for the neutrinoless double beta decay of 76Ge. High-purity germanium detectors made from material enriched in 76Ge are operated directly immersed in liquid argon, allowing for a substantial reduction of the background with respect to predecessor experiments. The first 5.04 kg yr of data collected in Phase I of the experiment have been analyzed to measure the half-life of the neutrino-accompanied double beta decay of 76Ge. The observed spectrum in the energy range between 600 and 1800 keV is dominated by the double beta decay of 76Ge. The half-life extracted from GERDA data is T2ν1/2 = (1.84+0.14−0.10) × 1021 yr.

K.H. Ackermann et al., (GERDA Collaboration), N. Frodyma, K. Pelczar, M. Wojcik, G. Zuzel

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http://arxiv.org/abs/1212.4067
http://link.springer.com/article/10.1140%2Fepjc%2Fs10052-013-2330-0

Abstract

The GERDA collaboration is performing a search for neutrinoless double beta decay of ⁷⁶Ge with the eponymous detector. The experiment has been installed and commissioned at the Laboratori Nazionali del Gran Sasso and has started operation in November 2011. The design, construction and first operational results are described, along with detailed information from the R&D phase.