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The search for neutrinoless double-beta decay remains today one of the most important areas in particle and nuclear physics. Germanium detectors are an excellent technology for this search because of their state-of-the-art energy resolution, but a dead layer in a germanium crystal can reduce the active volume, which can affect both exposure and half-life sensitivity. In this work, we used machine learning methods to study the dead layer in enriched germanium crystals. 1000 sets of events were simulated with various combinations of dead layer parameters. A fully connected neural network was used to determine these parameters from the energy spectra of a germanium detector exposed to a gamma calibration source Barium-133.

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.

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.

A search for full energy depositions from bosonic keV-scale dark matter candidates of masses between 65 and 1021 keV has been performed with data collected during Phase II of the GERmanium Detector Array ( Gerda ) experiment. Our analysis includes direct dark matter absorption as well as dark Compton scattering. With a total exposure of 105.5 kg years, no evidence for a signal above the background has been observed. The resulting exclusion limits deduced with either Bayesian or Frequentist statistics are the most stringent direct constraints in the major part of the 140–1021 keV mass range. As an example, at a mass of 150 keV the dimensionless coupling of dark photons and axion-like particles to electrons has been constrained to α ′ / α < 8.7 × 10 - 24 and g ae < 3.3 × 10 - 12 at 90% credible interval (CI), respectively. Additionally, a search for peak-like signals from beyond the Standard Model decays of nucleons and electrons is performed. We find for the inclusive decay of a single neutron in 76 Ge a lower lifetime limit of τ n > 1.5 × 10 24 years and for a proton τ p > 1.3 × 10 24 years at 90% CI. For the electron decay e - → ν e γ a lower limit of τ e > 5.4 × 10 25 years at 90% CI has been determined.

The ability to detect liquid argon scintillation light from within a densely packed high-purity germanium detector array allowed the Gerda experiment to reach an exceptionally low background rate in the search for neutrinoless double beta decay of 76 Ge. Proper modeling of the light propagation throughout the experimental setup, from any origin in the liquid argon volume to its eventual detection by the novel light read-out system, provides insight into the rejection capability and is a necessary ingredient to obtain robust background predictions. In this paper, we present a model of the Gerda liquid argon veto, as obtained by Monte Carlo simulations and constrained by calibration data, and highlight its application for background decomposition.

The radioactive isotope 85 Kr is found in significant quantities in the atmosphere largely due to nuclear industry. Its β -decay with a half-life of 10.7 years and a Q-value of 687 keV is a dangerous background source for low-threshold noble gas and liquid detectors, which distill their detector medium from air. The Gerda experiment was operating high-purity germanium detectors immersed in a clean liquid argon bath deep underground to search for neutrinoless double beta decay with unprecedented sensitivity. The 85 Kr specific activity in the liquid argon at the start of the second phase of the experiment has been determined to be ( 0.36 ± 0.03 )  mBq/kg through an analysis of the full subsequent data set that exploits the excellent γ -ray spectroscopic capabilities of Gerda .

The beta decay of 77 Ge and 77 m Ge, both produced by neutron capture on 76 Ge, is a potential background for Germanium based neutrinoless double-beta decay search experiments such as GERDA or the LEGEND experiment. In this work we present a search for 77 Ge decays in the full GERDA Phase II data set. A delayed coincidence method was employed to identify the decay of 77 Ge via the isomeric state of 77 As ( 9 / 2 + , 475 keV , T 1 / 2 = 114 μ s , 77 m As). New digital signal processing methods were employed to select and analyze pile-up signals. No signal was observed, and an upper limit on the production rate of 77 Ge was set at < 0.216 nuc/(kg · yr) (90% CL). This corresponds to a total production rate of 77 Ge and 77 m Ge of < 0.38 nuc/(kg ·  yr) (90% CL), assuming equal production rates. A previous Monte Carlo study predicted a value for in-situ 77 Ge and 77 m Ge production of (0.21 ± 0.07) nuc/(kg.yr), a prediction that is now further corroborated by our experimental limit. Moreover, tagging the isomeric state of 77 m As can be utilised to further suppress the 77 Ge background. Considering the similar experimental configurations of LEGEND-1000 and GERDA, the cosmogenic background in LEGEND-1000 at LNGS is estimated to remain at a sub-dominant level.

The beta decay of$$^{77}$$77 Ge and$$^{77\\textrm{m}}$$77 m Ge, both produced by neutron capture on$$^{76}$$76 Ge, is a potential background for Germanium based neutrinoless double-beta decay search experiments such as GERDA or the LEGEND experiment. In this work we present a search for$$^{77}$$77 Ge decays in the full GERDA Phase II data set. A delayed coincidence method was employed to identify the decay of$$^{77}$$77 Ge via the isomeric state of$$^{77}$$77 As ($$9/2^+$$9 / 2 + ,$${475}\\,\\hbox {keV}$$475 keV ,$${T_{1/2} = {114}\\,{\\upmu }\\hbox {s}}$$T 1 / 2 = 114 μ s ,$$^{77\\textrm{m}}$$77 m As). New digital signal processing methods were employed to select and analyze pile-up signals. No signal was observed, and an upper limit on the production rate of$$^{77}$$77 Ge was set at$$<0.216$$< 0.216 nuc/(kg$$\\cdot $$· yr) (90% CL). This corresponds to a total production rate of$$^{77}$$77 Ge and$$^{77\\textrm{m}}$$77 m Ge of$$<{0.38}$$< 0.38 nuc/(kg$$\\cdot $$·  yr) (90% CL), assuming equal production rates. A previous Monte Carlo study predicted a value for in-situ$$^{77}$$77 Ge and$$^{77\\textrm{m}}$$77 m Ge production of (0.21 ± 0.07) nuc/(kg.yr), a prediction that is now further corroborated by our experimental limit. Moreover, tagging the isomeric state of$$^{77\\textrm{m}}$$77 m As can be utilised to further suppress the$$^{77}$$77 Ge background. Considering the similar experimental configurations of LEGEND-1000 and GERDA, the cosmogenic background in LEGEND-1000 at LNGS is estimated to remain at a sub-dominant level.

We search for tri-nucleon decays of$$^{76}$$76 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$$^{73}$$73 Cu,$$^{73}$$73 Zn, and$$^{73}$$73 Ga nuclei, respectively. These nuclei are unstable and eventually proceed by the beta decay of$$^{73}$$73 Ga to$$^{73}$$73 Ge (stable). We search for the$$^{73}$$73 Ga decay exploiting the fact that it dominantly populates the 66.7 keV$$^{73m}$$73 m Ga state with half-life of 0.5 s. The nnn-decays of$$^{76}$$76 Ge that proceed via$$^{73m}$$73 m 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 1.2$$\\times $$× 10$$^{26}$$26  yr  (90% credible interval). This result improves previous limits for tri-nucleon decays by one to three orders of magnitude.

The radioactive isotope Kr is found in significant quantities in the atmosphere largely due to nuclear industry. Its -decay with a half-life of 10.7 years and a Q-value of 687 keV is a dangerous background source for low-threshold noble gas and liquid detectors, which distill their detector medium from air. The Gerda experiment was operating high-purity germanium detectors immersed in a clean liquid argon bath deep underground to search for neutrinoless double beta decay with unprecedented sensitivity. The Kr specific activity in the liquid argon at the start of the second phase of the experiment has been determined to be  mBq/kg through an analysis of the full subsequent data set that exploits the excellent -ray spectroscopic capabilities of Gerda.