Eur. Phys. J. C (2013) 73: 2583
https://doi.org/10.1140/epjc/s10052-013-2583-7

Regular Article - Experimental Physics

Pulse shape discrimination for Gerda Phase I data

¹ INFN Laboratori Nazionali del Gran Sasso, LNGS, Assergi, Italy
² Institute of Physics, Jagiellonian University, Cracow, Poland
³ Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden, Germany
⁴ Joint Institute for Nuclear Research, Dubna, Russia
⁵ Institute for Reference Materials and Measurements, Geel, Belgium
⁶ Max-Planck-Institut für Kernphysik, Heidelberg, Germany
⁷ Dipartimento di Fisica, Università Milano Bicocca, Milano, Italy
⁸ INFN Milano Bicocca, Milano, Italy
⁹ Dipartimento di Fisica, Università degli Studi di Milano e INFN Milano, Milano, Italy
¹⁰ Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia
¹¹ Institute for Theoretical and Experimental Physics, Moscow, Russia
¹² National Research Centre “Kurchatov Institute”, Moscow, Russia
¹³ Max-Planck-Institut für Physik, München, Germany
¹⁴ Physik Department and Excellence Cluster Universe, Technische Universität München, München, Germany
¹⁵ Dipartimento di Fisica e Astronomia, Università di Padova, Padova, Italy
¹⁶ INFN Padova, Padova, Italy
¹⁷ Physikalisches Institut, Eberhard Karls Universität Tübingen, Tübingen, Germany
¹⁸ Physik Institut der Universität Zürich, Zürich, Switzerland

^* e-mail: gerda-eb@mpi-hd.mpg.de

Received: 9 July 2013
Published online: 9 October 2013

Abstract

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 0νββ decay. It retains 90 % of DEP events and rejects about half of the events around Q _ββ . The 2νββ events have an efficiency of 0.85±0.02 and the one for 0νββ decays is estimated to be . 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.