The GErmanium Detector Array (Gerda) experiment, located at the Laboratori Nazionali del Gran Sass15, searches for the 0νββ decay of 76Ge. Gerda Phase II was started in December 2015, aiming to reach a sensitivity on the 0νββ decay half-life larger than 1026 yr in three yr of data taking with about 100 kg·yr exposure with background index of about 10-3 cts/(keV·kg·yr). The major upgrade of Phase II is the deployment of thirty newly produced Broad Energy Germanium detectors. They contribute to the background reduction with better energy resolution and enhanced pulse shape discrimination. To achieve the necessary background reduction, the setup was complemented with LAr veto. After 6 months of operation a first data release with 10.8 kg·yr exposure was performed, showing that the background goal has been achieved. A new lower limit on the 0νββ decay half life of 76Ge of 5.3·1025 yr (90% C.L.) has been set.

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 Qββ = 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 0νββ decay. It retains 90 % of DEP events and rejects about half of the events around Qββ. The 2νββ events have an efficiency of 0.85±0.02 and the one for 0νββ 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.

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.

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 kgyear). It will reach thereby a half-life sensitivity of more than 10 26 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.

The GERmanium Detector Array (Gerda) experiment, located at the Gran Sasso underground laboratory in Italy, is built for the search of 0vββ decay in 76Ge. Gerda operates bare high purity germanium detectors submersed in liquid Argon (LAr). Phase I of the experiment was completed reaching an exposure of about 21 kg.yr with a background level of 10-2 cts/(keV.kg.yr). Gerda Phase I set a limit on the 0vββ decay of 76Ge of T1/20v > 2.1.1025 yr. In Phase II 35 kg of germanium detectors enriched in 76Ge are operated to reach an exposure of 100 kg.yr. The design goal is to reduce the background by one order of magnitude to reach the sensitivity for T1/20v = (1026) yr. The Phase II setup comprises thirty newly produced Broad Energy Germanium (BEGe) detectors. They contribute to the background reduction with better energy resolution and enhanced pulse shape discrimination. To achieve the necessary background reduction, the setup was complemented with LAr veto. The hardware upgrade for Phase II was finished and all detectors were deployed in December 2015. We present the first results of Phase II with 10.8 kg.yr exposure reached in June 2016.

An optimized digital shaping filter has been developed for the Gerda experiment which searches for neutrinoless double beta decay in $$^{76}$$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 $$Q$$Q value for $$0 u \beta \beta $$0νββ decay in $$^{76}$$⁷⁶Ge is obtained. This is possible thanks to the enhanced low-frequency noise rejection of this Zero Area Cusp (ZAC) signal shaping filter.

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. © 2013 The Author(s).