[en] The GERmanium Detector Array, GERDA, is designed to search for neutrinoless double beta (0νββ) decay of "7"6Ge and it is installed in the Laboratori Nazionali del Gran Sasso (LNGS) of INFN, Italy. The GERDA experiment has completed the Phase I with a total collected exposure of 21.6 kg yr and a background index (BI) of the order of BI ≃ 10"−"2 cts/(keVkg yr). No excess of events from 0νββ decay has been observed and a lower limit on the half-life on the 0νββ decay for "7"6Ge has been estimated: T"0"ν_1/2 > 2.1·10"2"5 yr at 90% CL. The goal of GERDA Phase II is to reach the target sensitivity of T"0"ν_1/2 ≃ 1.4 · 10"2"6 yr, with an increased total mass of the enriched material and a reduced background level. In this paper the results from GERDA Phase I and the major improvements planned for Phase II are discussed.

[en] High Purity Germanium detectors are used in many applications, from nuclear and astro-particle physics, to homeland security or environment protection. Although quite standard configurations are often used, with cryostats, charge sensitive amplifiers and analog or digital acquisition systems all commercially available, it might be the case that a few specific applications, e.g. satellites, portable devices, cryogenic physics experiments, etc. also require the development of a few additional or complementary techniques. An interesting case is for sure GERDA, the Germanium Detector Array experiment, searching for neutrino-less double beta decay of "7"6Ge at the Gran Sasso National Laboratory of INFN - Italy. In GERDA the entire detector array, composed of semi-coaxial and BEGe naked crystals, is operated suspended inside a cryostat filled with liquid argon, that acts not only as cooling medium and but also as an active shield, thanks to its scintillation properties. These peculiar circumstances, together with the additional requirement of a very low radioactive background from all the materials adjacent to the detectors, clearly introduce significant constraints on the design of the Ge front-end readout electronics. All the Ge readout solutions developed within the framework of the GERDA collaboration, for both Phase I and Phase II, will be briefly reviewed, with their relative strength and weakness compared together and with respect to ideal Ge readout. Finally, the digital processing techniques developed by the GERDA collaboration for energy estimation of Ge detector signals will be recalled. (authors)

[en] Using data from the NEMO-3 experiment, we have measured the two-neutrino double beta decay (2νββ) half-life of ⁸²Se as T_1/2^2ν = [9.39 ± 0.17(stat) ± 0.58(syst) ] x 10¹⁹ y under the single-state dominance hypothesis for this nuclear transition. The corresponding nuclear matrix element is vertical stroke M^2ν vertical stroke = 0.0498 ± 0.0016. In addition, a search for neutrinoless double beta decay (0νββ) using 0.93 kg of ⁸²Se observed for a total of 5.25 y has been conducted and no evidence for a signal has been found. The resulting half-life limit of T_1/2^0ν > 2.5 x 10²³ y (90% C.L.) for the light neutrino exchange mechanism leads to a constraint on the effective Majorana neutrino mass of left angle m_ν right angle < (1.2-3.0) eV, where the range reflects 0νββ nuclear matrix element values from different calculations. Furthermore, constraints on lepton number violating parameters for other 0νββ mechanisms, such as right-handed currents, majoron emission and R-parity violating supersymmetry modes have been set. (orig.)

[en] 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 the experiment 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. (orig.)

[en] The Gerda collaboration is performing a sensitive search for neutrinoless double beta decay of ⁷⁶Ge at the INFN Laboratori Nazionali del Gran Sasso, Italy. The upgrade of the Gerda experiment from Phase I to Phase II has been concluded in December 2015. The first Phase II data release shows that the goal to suppress the background by one order of magnitude compared to Phase I has been achieved. Gerda is thus the first experiment that will remain ''background-free'' up to its design exposure (100 kg year). It will reach thereby a half-life sensitivity of more than 10²⁶ year within 3 years of data collection. This paper describes in detail the modifications and improvements of the experimental setup for Phase II and discusses the performance of individual detector components. (orig.)

[en] 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. (orig.)

[en] The Gerda experiment located at the Laboratori Nazionali del Gran Sasso of INFN searches for neutrinoless double beta (0νββ) decay of ⁷⁶Ge 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 ⁷⁶Ge 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 γ rays from ²⁰⁸Tl decays as well as two-neutrino double beta (2νββ) decays of ⁷⁶Ge are used as proxies for 0νββ decay. For BEGe detectors the chosen selection is based on a single pulse shape parameter. It accepts 0.92±0.02 of signal-like events while about 80 % of the background events at Q_ββ =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 νββ decay. It retains 90 % of DEP events and rejects about half of the events around Q_ββ. The 2 νββ events have an efficiency of 0.85 ±0.02 and the one for 0 νββ 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 2νββ decays. (orig.)

[en] 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 signature of the signal is a monoenergetic peak at 2039 keV, the Q_ββ 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_ββ. The main parameters needed for the 0νββ 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_ββ with a background index ranging from 17.6 to 23.8 x 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_ββ is dominated by close sources,mainly due to ⁴²K, ²¹⁴Bi, ²²⁸Th, ⁶⁰Co and α 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 γ peaks, the energy spectrum can be fitted in an energy range of 200 keV around Q_ββ with a constant background. This gives a background index consistent with the full model and uncertainties of the same size. (orig.)

[en] A search for neutrinoless ββ decay processes accompanied with Majoron emission has been performed using data collected during Phase I of the GERmanium Detector Array (GERDA) experiment at the Laboratori Nazionali del Gran Sasso of INFN (Italy). Processes with spectral indices n = 1, 2, 3, 7 were searched for. No signals were found and lower limits of the order of 10²³ yr on their half-lives were derived, yielding substantially improved results compared to previous experiments with ⁷⁶Ge. A new result for the half-life of the neutrino-accompanied ββ decay of ⁷⁶Ge with significantly reduced uncertainties is also given, resulting in T_1/2^2ν = (1.926 ± 0.094) @ x 10²¹ yr. (orig.)

[en] 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 "3"6Ar 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 "3"6Ar was established: T_1_/_2 > 3.6 x 10"2"1 years at 90% CI. (orig.)