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キーワード:
Physics, Instrumentation and Detectors, physics.ins-det,Nuclear Experiment, nucl-ex
要旨:
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.
