52 research outputs found
Topologies of 76 Ge double-beta decay events and calibration procedure biases
The analysis of the time profile of electrical signals produced by energy depositions in germanium detectors allows discrimination of events with different topologies. This is especially relevant for experiments searching for the neutrinoless double beta decay of 76Ge to distinguish the sought-after signal from other background sources. The standard calibration procedures used to tune the selection criteria for double-beta decay events use a 228Th source, because it provides samples of signal-like events. These samples exhibit energy spatial distributions with subtle different topologies compared to neutrinoless double-beta decay events. In this work, we will characterize these topological differences and, with the support of a 56Co source, evaluate biases and precision of calibration techniques which use such event samples. Our results will be particularly relevant for future experiments in which a solid estimation of the efficiency is required
Charge-carrier collective motion in germanium detectors for ββ-decay searches
The time analysis of the signal induced by the drift of charge carriers in high purity germanium detectors provides information on the event topology. Millions of charge carriers are produced in a typical event. Their initial distribution, stochastic diffusion and Coulomb self-repulsion affect the time structure of the signal. We present a comprehensive study of these effects and evaluate their impact on the event discrimination capabilities for the three geometries which will be used in the LEGEND experiment for neutrinoless double-beta decay
The first search for bosonic super-WIMPs with masses up to 1 MeV/c with GERDA
We present the first search for bosonic super-WIMPs as keV-scale dark matter
candidates performed with the GERDA experiment. GERDA is a neutrinoless
double-beta decay experiment which operates high-purity germanium detectors
enriched in Ge in an ultra-low background environment at the Laboratori
Nazionali del Gran Sasso (LNGS) of INFN in Italy. Searches were performed for
pseudoscalar and vector particles in the mass region from 60 keV/c to 1
MeV/c. No evidence for a dark matter signal was observed, and the most
stringent constraints on the couplings of super-WIMPs with masses above 120
keV/c have been set. As an example, at a mass of 150 keV/c the most
stringent direct limits on the dimensionless couplings of axion-like particles
and dark photons to electrons of and
at 90% credible interval,
respectively, were obtained.Comment: 6 pages, 3 figures, submitted to Physical Review Letters, added list
of authors, updated ref. [21
Search for tri-nucleon decays of Ge76 in GERDA
We search for tri-nucleon decays of Ge in the dataset from the GERmanium Detector Array (GERDA) experiment. Decays that populate excited levels of the daughter nucleus above the threshold for particle emission lead to disintegration and are not considered. The ppp-, ppn-, and pnn-decays lead to Cu, Zn, and Ga nuclei, respectively. These nuclei are unstable and eventually proceed by the beta decay of Ga to Ge (stable). We search for the Ga decay exploiting the fact that it dominantly populates the 66.7 keV Ga state with half-life of 0.5 s. The nnn-decays of Ge that proceed via Ge are also included in our analysis. We find no signal candidate and place a limit on the sum of the decay widths of the inclusive tri-nucleon decays that corresponds to a lower lifetime limit of 1.210 yr (90% credible interval). This result improves previous limits for tri-nucleon decays by one to three orders of magnitude
Pulse shape analysis in Gerda Phase II
The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double- decay in Ge using isotopically enriched high purity germanium detectors at the Laboratori Nazionali del Gran Sasso of INFN. After Phase I (2011–2013), the experiment benefited from several upgrades, including an additional active veto based on LAr instrumentation and a significant increase of mass by point-contact germanium detectors that improved the half-life sensitivity of Phase II (2015–2019) by an order of magnitude. At the core of the background mitigation strategy, the analysis of the time profile of individual pulses provides a powerful topological discrimination of signal-like and background-like events. Data from regular Th calibrations and physics data were both considered in the evaluation of the pulse shape discrimination performance. In this work, we describe the various methods applied to the data collected in Gerda Phase II corresponding to an exposure of 103.7 kg year. These methods suppress the background by a factor of about 5 in the region of interest around keV, while preserving % of the signal. In addition, an exhaustive list of parameters is provided which were used in the final data analysis
Final Results of GERDA on the Search for Neutrinoless Double- Decay
The GERmanium Detector Array (GERDA) experiment searched for the
lepton-number-violating neutrinoless double- () decay of
Ge, whose discovery would have far-reaching implications in cosmology
and particle physics. By operating bare germanium diodes, enriched in
Ge, in an active liquid argon shield, GERDA achieved an unprecedently
low background index of counts/(keVkgyr) in
the signal region and met the design goal to collect an exposure of 100
kgyr in a background-free regime. When combined with the result of Phase
I, no signal is observed after 127.2 kgyr of total exposure. A limit on
the half-life of decay in Ge is set at
yr at 90% C.L., which coincides with the sensitivity
assuming no signal.Comment: 7 pages, 3 figures, submitted to Physical Review Letter
Calibration of the Gerda experiment
The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double-β decay in 76Ge with an array of about 40 high-purity isotopically-enriched germanium detectors. The experimental signature of the decay is a monoenergetic signal at Qββ= 2039.061 (7) keV in the measured summed energy spectrum of the two emitted electrons. Both the energy reconstruction and resolution of the germanium detectors are crucial to separate a potential signal from various backgrounds, such as neutrino-accompanied double-β decays allowed by the Standard Model. The energy resolution and stability were determined and monitored as a function of time using data from regular 228Th calibrations. In this work, we describe the calibration process and associated data analysis of the full Gerda dataset, tailored to preserve the excellent resolution of the individual germanium detectors when combining data over several years
Pulse shape analysis in Gerda Phase II
The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double-β decay in 76Ge using isotopically enriched high purity germanium detectors at the Laboratori Nazionali del Gran Sasso of INFN. After Phase I (2011–2013), the experiment benefited from several upgrades, including an additional active veto based on LAr instrumentation and a significant increase of mass by point-contact germanium detectors that improved the half-life sensitivity of Phase II (2015–2019) by an order of magnitude. At the core of the background mitigation strategy, the analysis of the time profile of individual pulses provides a powerful topological discrimination of signal-like and background-like events. Data from regular 228Th calibrations and physics data were both considered in the evaluation of the pulse shape discrimination performance. In this work, we describe the various methods applied to the data collected in Gerda Phase II corresponding to an exposure of 103.7 kg year. These methods suppress the background by a factor of about 5 in the region of interest around Qββ=2039 keV, while preserving (81±3)% of the signal. In addition, an exhaustive list of parameters is provided which were used in the final data analysis
Modeling of GERDA Phase II data
The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double-beta (0\u3bd\u3b2\u3b2) decay of 76Ge. The technological challenge of Gerda is to operate in a \u201cbackground-free\u201d regime in the region of interest (ROI) after analysis cuts for the full 100 kg\ub7yr target exposure of the experiment. A careful modeling and decomposition of the full-range energy spectrum is essential to predict the shape and composition of events in the ROI around Q\u3b2\u3b2 for the 0\u3bd\u3b2\u3b2 search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos (2\u3bd\u3b2\u3b2) and in order to identify the location of residual impurities. The latter will permit future experiments to build strategies in order to further lower the background and achieve even better sensitivities. In this article the background decomposition prior to analysis cuts is presented for Gerda Phase II. The background model fit yields a flat spectrum in the ROI with a background index (BI) of 16.04 120.85+0.78\ub710 123 cts/(keV\ub7kg\ub7yr) for the enriched BEGe data set and 14.68 120.52+0.47\ub710 123 cts/(keV\ub7kg\ub7yr) for the enriched coaxial data set. These values are similar to the one of Phase I despite a much larger number of detectors and hence radioactive hardware components
Characterization of inverted coaxial Ge detectors in GERDA for future double- decay experiments
Neutrinoless double- decay of Ge is searched for with germanium
detectors where source and detector of the decay are identical. For the success
of future experiments it is important to increase the mass of the detectors. We
report here on the characterization and testing of five prototype detectors
manufactured in inverted coaxial (IC) geometry from material enriched to 88% in
Ge. IC detectors combine the large mass of the traditional semi-coaxial
Ge detectors with the superior resolution and pulse shape discrimination power
of point contact detectors which exhibited so far much lower mass. Their
performance has been found to be satisfactory both when operated in vacuum
cryostat and bare in liquid argon within the GERDA setup. The measured
resolutions at the Q-value for double- decay of Ge
(Q = 2039 keV) are about 2.1 keV full width at half maximum in
vacuum cryostat. After 18 months of operation within the ultra-low background
environment of the GERmanium Detector Array (GERDA) experiment and an
accumulated exposure of 8.5 kgyr, the background index after analysis
cuts is measured to be counts
/(keVkgyr) around Q. This work confirms the
feasibility of IC detectors for the next-generation experiment LEGEND.Comment: 13 pages, 12 figures, submitted to EPJ
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