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Point-contact p-type high-purity germanium detectors (PPC HPGe) are particularly suited for detection of sub-keV nuclear recoils from coherent elastic scattering of neutrinos or light dark matter particles. While these particles are expected to interact homogeneously in the entire detector volume, specific classes of external background radiation preferably deposit their energy close to the semi-active detector surface, in which diffusion processes dominate that subsequently lead to slower rising pulses compared to the ones from the fully active bulk volume. Dedicated studies of their shape are therefore highly beneficial for the understanding and the rejection of these unwanted events. This article reports about the development of a data-driven pulse shape discrimination (PSD) method for the four 1 kg size PPC HPGe detectors of the Conus experiment in the keV and sub-keV regime down to 210 eV ee. The impact of the electronic noise at such low energies is carefully examined. It is shown that for an acceptance of 90% of the faster signal-like pulses from the bulk volume, approx. 50% of the surface events can be rejected at the energy threshold and that their contribution is fully suppressed above 800 eV ee. Applied to the Conus background data, such a PSD rejection cut allows to achieve an overall (15 - 25) % reduction of the total background budget. The new method allows to improve the sensitivity of future Conus analyses and to refine the corresponding background model in the sub-keV energy region. [ABSTRACT FROM AUTHOR]

The future Ricochet experiment aims to search for new physics in the electroweak sector by measuring the Coherent Elastic Neutrino-Nucleus Scattering process from reactor antineutrinos with high precision down to the sub-100 eV nuclear recoil energy range. While the Ricochet collaboration is currently building the experimental setup at the reactor site, it is also finalizing the cryogenic detector arrays that will be integrated into the cryostat at the Institut Laue Langevin in early 2024. In this paper, we report on recent progress from the Ge cryogenic detector technology, called the CryoCube. More specifically, we present the first demonstration of a 30 eVee (electron equivalent) baseline ionization resolution (RMS) achieved with an early design of the detector assembly and its dedicated High Electron Mobility Transistor (HEMT) based front-end electronics with a total input capacitance of about 40 pF. This represents an order of magnitude improvement over the best ionization resolutions obtained on similar phonon-and-ionization germanium cryogenic detectors from the EDELWEISS and SuperCDMS dark matter experiments, and a factor of three improvement compared to the first fully-cryogenic HEMT-based preamplifier coupled to a CDMS-II germanium detector with a total input capacitance of 250 pF. Additionally, we discuss the implications of these results in the context of the future Ricochet experiment and its expected background mitigation performance. [ABSTRACT FROM AUTHOR]

The GERmanium Detector Array (Gerda) experiment operated enriched high-purity germanium detectors in a liquid argon cryostat, which contains 0.33% of 36 Ar, a candidate isotope for the two-neutrino double-electron capture (2 ν ECEC) and therefore for the neutrinoless double-electron capture (0 ν ECEC). If detected, this process would give evidence of lepton number violation and the Majorana nature of neutrinos. In the radiative 0 ν ECEC of 36 Ar, a monochromatic photon is emitted with an energy of 429.88 keV, which may be detected by the Gerda germanium detectors. We searched for the 36 Ar 0 ν ECEC with Gerda data, with a total live time of 4.34 year (3.08 year accumulated during Gerda Phase II and 1.26 year during Gerda Phase I). No signal was found and a 90% CL lower limit on the half-life of this process was established T 1 / 2 > 1.5 · 10 22 year. [ABSTRACT FROM AUTHOR]

A first search for rare decays of gadolinium isotopes was performed with an ultra-low background high-purity germanium detector at Gran Sasso Underground Laboratory (Italy). A 198 g Gd 2 O 3 powder sample was measured for 63.8 d with a total Gd exposure of 12.6 kg × d. 152 Gd is the most promising isotope for resonant enhanced neutrinoless double electron capture which could significantly increase the decay rate over other neutrinoless double beta decay processes. The half-life for this decay was constrained to > 4.2 × 10 12 year (90% credibility). This limit is still orders of magnitude away from theoretical predictions but it is the first established limit on the transition paving the way for future experiments. In addition, other rare alpha and double beta decay modes were investigated in 152 Gd, 154 Gd, and 160 Gd with half-life limits in the range of 10 17 - 20 year. [ABSTRACT FROM AUTHOR]

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

A new study of double beta decay processes in natural Zr isotopes using Zr metal samples and an ultra-low-background high-purity germanium detectors was performed at Laboratori Nazionali del Gran Sasso of INFN (Italy). Two data sets were collected with a 736.35 g and a 129.94 g Zr metal over 59.7 days and 37.3 days, respectively, and were used for a detailed analysis. A new limit on the double beta decay of 94 Zr to the 1st excited level of 94 Mo was set as T 1 / 2 β β > 2.1 × 10 20 year , which is a factor of three better than the previous result. It is shown that the current radiopurity of Zr metal (tens of Bq/kg), produced via two methods, do not comply with the radiopurity requirements for low-background experiments. Hence, we propose to use a purified ZrCl 4 powder in an optimized geometry as a new approach for future experiments with extended sensitivity focused on studies of rare nuclear processes in Zr isotopes. [ABSTRACT FROM AUTHOR]

A 510 day long-term measurement of a 45.3 g platinum foil acting as the sample and high voltage contact in an ultra-low-background high purity germanium detector was performed at Laboratori Nazionali del Gran Sasso (Italy). The data was used for a detailed study of double beta decay modes in natural platinum isotopes. Limits are set in the range O (10 14 - 10 19) years (90% C.L.) for several double beta decay transitions to excited states confirming, and partially extending existing limits. The highest sensitivity of the measurement, greater than 10 19 years, was achieved for the two neutrino and neutrinoless double beta decay modes of the isotope 198 Pt. Additionally, novel limits for inelastic dark matter scattering on 195 Pt are placed up to mass splittings of approximately 500 keV. We analyze several techniques to extend the sensitivity and propose a few approaches for future medium-scale experiments with platinum-group elements. [ABSTRACT FROM AUTHOR]

The impurity density in high-purity germanium detectors is crucial to understand and simulate such detectors. However, the information about the impurities provided by the manufacturer, based on Hall effect measurements, is typically limited to a few locations and comes with a large uncertainty. As the voltage dependence of the capacitance matrix of a detector strongly depends on the impurity density distribution, capacitance measurements can provide a path to improve the knowledge on the impurities. The novel method presented here uses a machine-learned surrogate model, trained on precise GPU-accelerated capacitance calculations, to perform full Bayesian inference of impurity distribution parameters from capacitance measurements. All steps use open-source Julia software packages. Capacitances are calculated with SolidStateDetectors.jl, machine learning is done with Flux.jl and Bayesian inference performed using BAT.jl. The capacitance matrix of a detector and its dependence on the impurity density is explained and a capacitance bias-voltage scan of an n-type true-coaxial test detector is presented. The study indicates that the impurity density of the test detector also has a radial dependence. [ABSTRACT FROM AUTHOR]

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 76 Ge 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 228 Th 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 56 Co 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. [ABSTRACT FROM AUTHOR]

Events from alpha interactions on the surfaces of germanium detectors are a major contribution to the background in germanium-based searches for neutrinoless double-beta decay. Surface events are subject to charge trapping, affecting their pulse shape and reconstructed energy. A study of alpha events on the passivated end-plate of a segmented true-coaxial n-type high-purity germanium detector is presented. Charge trapping is analysed in detail and an existing pulse-shape analysis technique to identify alpha events is verified with mirror pulses observed in the non-collecting channels of the segmented test detector. The observed radial dependence of charge trapping confirms previous results. A dependence of the probability of charge trapping on the crystal axes is observed for the first time. A first model to describe charge trapping effects within the framework of the simulation software SolidStateDetectors.jl is introduced. The influence of metalisation on events from low-energy gamma interactions close to the passivated surface is also presented. [ABSTRACT FROM AUTHOR]

We present a convolutional autoencoder to denoise pulses from a p-type point contact high-purity germanium detector similar to those used in several rare event searches. While we focus on training procedures that rely on detailed detector physics simulations, we also present implementations requiring only noisy detector pulses to train the model. We validate our autoencoder on both simulated data and calibration data from an 241 Am source, the latter of which is used to show that the denoised pulses are statistically compatible with data pulses. We demonstrate that our denoising method is able to preserve the underlying shapes of the pulses well, offering improvement over traditional denoising methods. We also show that the shaping time used to calculate energy with a trapezoidal filter can be significantly reduced while maintaining a comparable energy resolution. Under certain circumstances, our denoising method can improve the overall energy resolution. The methods we developed to remove electronic noise are straightforward to extend to other detector technologies. Furthermore, the latent representation from the encoder is also of use in quantifying shape-based characteristics of the signals. Our work has great potential to be used in particle physics experiments and beyond. [ABSTRACT FROM AUTHOR]

A novel Compton Scanner setup has been built, commissioned and operated at the Max-Planck-Institute for Physics in Munich to collect pulses from bulk events in high-purity germanium detectors for pulse shape studies. In this fully automated setup, the detector under test is irradiated from the top with 661.660 keV gammas, some of which Compton scatter inside the detector. The interaction points in the detector can be reconstructed when the scattered gammas are detected with a pixelated camera placed at the side of the detector. The wide range of accepted Compton angles results in shorter measurement times in comparison to similar setups where only perpendicularly scattered gammas are selected by slit collimators. In this paper, the construction of the Compton Scanner, its alignment and the procedure to reconstruct interaction points in the germanium detector are described in detail. The creation of a first pulse shape library for an n-type segmented point-contact germanium detector is described. The spatial reconstruction along the beam axis is validated by a comparison to measured surface pulses. A first comparison of Compton Scanner pulses to simulated pulses is presented to demonstrate the power of the Compton Scanner to test simulation inputs and models. [ABSTRACT FROM AUTHOR]

We report first constraints on electromagnetic properties of neutrinos from neutrino-electron scattering using data obtained from the CONUS germanium detectors, i.e. an upper limit on the effective neutrino magnetic moment and an upper limit on the effective neutrino millicharge. The electron antineutrinos are emitted from the 3.9 GW th reactor core of the Brokdorf Nuclear Power Plant in Germany. The CONUS low-background detectors are positioned at a distance of 17.1 m from the reactor core center. The analyzed data set includes 689.1 kg d collected during reactor ON periods and 131.0 kg d collected during reactor OFF periods in the energy range of. With the current statistics, we are able to determine an upper limit on the effective neutrino magnetic moment of μ ν < 7.5 · 10 - 11 μ B at 90% confidence level. No neutrino signal in this channel or in the CE ν NS channel has been observed at a nuclear power plant so far. From this first magnetic moment limit we can derive an upper bound on the neutrino millicharge of | q ν | < 3.3 · 10 - 12 e 0 . [ABSTRACT FROM AUTHOR]

The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double- β decay in 76 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 228 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 Q β β = 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. [ABSTRACT FROM AUTHOR]

The detection of low-energy deposition in the range of sub-eV through ionization using germanium (Ge) with a bandgap of ∼ 0.7 eV requires internal amplification of the charge signal. This can be achieved through high electric field that accelerates charge carriers, which can then generate more charge carriers. The minimum electric field required to generate internal charge amplification is derived for different temperatures. We report the development of a planar point contact Ge detector in terms of its fabrication and the measurements of its leakage current and capacitance as a function of applied bias voltage. With the determination of the measured depletion voltage, the field distribution is calculated using GeFiCa, which predicts that the required electric field for internal charge amplification can be achieved in proximity to the point contact. The energy response to an Am-241 source is characterized and discussed. We conclude that such a detector with internal charge amplification can be used to search for low-mass dark matter. [ABSTRACT FROM AUTHOR]

A search for alpha and double beta decays of ytterbium isotopes was performed with an ultra low-background high purity germanium detector at Gran Sasso Underground Laboratory (Italy). A 194.7 g Yb 2 (C 2 O 4 ) 3 powder sample was measured for 11.3 days with a total Yb exposure of 1.25 kg × day. Half-life limits for α -decay modes of 168 Y b , 170 Y b , 171 Y b , 172 Y b , 173 Y b , 174 Y b and 176 Y b into the first excited states have been obtained between 6 × 10 14 years and 2 × 10 16 years. These are the first experimental constraints of these decay modes. Double electron capture of 168 Y b and double beta decay of 176 Y b into the first excited 2 + and 0 + states could be excluded with limits between 1 × 10 14 years to 8 × 10 16 years. This improves the experimental information on some of the decay modes compared to previous constraints. [ABSTRACT FROM AUTHOR]

Rare event search experiments using germanium detectors are performed in underground laboratories to minimize the background induced by cosmic rays. However, the cosmogenic activation of cupreous detector components on the ground generates long half-life radioisotopes and contributes to the background level. We measured cosmogenic activation with 142.50 kg of copper bricks after 504 days of exposure at an altitude of 2469.4 m outside the China Jinping Underground Laboratory (CJPL). The specific activities of the cosmogenic nuclides produced in the copper bricks were measured using a low-background germanium gamma-ray spectrometer at CJPL. The production rates at sea level, in units of nuclei/kg/day, were 18.6 ± 2.0 for 54 Mn, 9.9 ± 1.3 for 56 Co, 48.3 ± 5.5 for 57 Co, 51.8 ± 2.5 for 58 Co, and 39.7 ± 5.7 for 60 Co. The measurement will help to constrain cosmogenic background estimation for rare event searches using copper as a detector structure and shielding material. Based on the measured production rates, the impact of the cosmogenic background in cupreous components of germanium detectors on the next generation CDEX-100 experiment was assessed with the expected exposure history above ground. [ABSTRACT FROM AUTHOR]

Intense fluxes of reactor antineutrinos offer a unique possibility to probe the fully coherent character of elastic neutrino scattering off atomic nuclei. In this regard, detectors face the challenge to register tiny recoil energies of a few keV at the maximum. The Conus experiment was installed in 17.1 m distance from the reactor core of the nuclear power plant in Brokdorf, Germany, and was designed to detect this neutrino interaction channel by using four 1 kg-sized point contact germanium detectors with sub-keV energy thresholds. This report describes the unique specifications addressed to the design, the research and development, and the final production of these detectors. It demonstrates their excellent electronic performance obtained during commissioning under laboratory conditions as well as during the first 2 years of operation at the reactor site which started on April 1, 2018. It highlights the long-term stability of different detector parameters and the achieved background levels of the germanium detectors inside the Conus shield setup. [ABSTRACT FROM AUTHOR]

A review of tools and methods to calculate electrostatic potentials and fields inside high-purity germanium detectors in various configurations is given. The methods are illustrated concretely with a new educational program named GeFiCa - Germanium detector Field Calculator. Demonstrated in GeFiCa are generic numerical calculations based on the successive over-relaxation method as well as analytic ones whenever simplification is possible due to highly symmetric detector geometries. GeFiCa is written in C + + and provided as an extension to the CERN ROOT libraries widely used in the particle physics community. Calculation codes for individual detectors, provided as ROOT macros and python scripts, are distributed along with the GeFiCa core library, serving as both examples showing the usage of GeFiCa and starting points for customized calculations. They can be run without compilation in a ROOT interactive session or directly from a Linux shell. The numerical results are saved in a ROOT tree, making full use of the I/O optimization and plotting functionalities in ROOT. The speed and precision of the calculation are comparable to other commonly used packages, which qualifies GeFiCa as a scientific research tool. However, the main focus of GeFiCa is to clearly explain and demonstrate the analytic and numeric methods to solve Poisson's equation, practical coding considerations and visualization methods, with intensive documentation and example macros. It serves as a one-stop resource for people who want to understand the operating mechanism of such a package under the hood. [ABSTRACT FROM AUTHOR]

The GERmanium Detector Array (Gerda) is a low background experiment located at the Laboratori Nazionali del Gran Sasso in Italy, which searches for neutrinoless double-beta decay of 76 Ge into 76 Se+2e - . Gerda has been conceived in two phases. Phase II, which started in December 2015, features several novelties including 30 new 76Ge enriched detectors. These were manufactured according to the Broad Energy Germanium (BEGe) detector design that has a better background discrimination capability and energy resolution compared to formerly widely-used types. Prior to their installation, the new BEGe detectors were mounted in vacuum cryostats and characterized in detail in the Hades underground laboratory in Belgium. This paper describes the properties and the overall performance of these detectors during operation in vacuum. The characterization campaign provided not only direct input for Gerda Phase II data collection and analyses, but also allowed to study detector phenomena, detector correlations as well as to test the accuracy of pulse shape simulation codes. [ABSTRACT FROM AUTHOR]

The GERmanium Detector Array ( Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double beta ( $$0\nu \beta \beta $$ ) decay of $$^{76}$$ Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the $$Q_{\beta \beta }$$ value of the decay. To avoid bias in the signal search, the present analysis does not consider all those events, that fall in a 40 keV wide region centered around $$Q_{\beta \beta }$$ . The main parameters needed for the $$0\nu \beta \beta $$ analysis are described. A background model was developed to describe the observed energy spectrum. The model contains several contributions, that are expected on the basis of material screening or that are established by the observation of characteristic structures in the energy spectrum. The model predicts a flat energy spectrum for the blinding window around $$Q_{\beta \beta }$$ with a background index ranging from 17.6 to 23.8 $$\times $$ $$10^{-3}$$ cts/(keV kg yr). A part of the data not considered before has been used to test if the predictions of the background model are consistent. The observed number of events in this energy region is consistent with the background model. The background at $$Q_{\beta \beta }$$ is dominated by close sources, mainly due to $$^{42}$$ K, $$^{214}$$ Bi, $$^{228}$$ Th, $$^{60}$$ Co and $$\alpha $$ emitting isotopes from the $$^{226}$$ Ra decay chain. The individual fractions depend on the assumed locations of the contaminants. It is shown, that after removal of the known $$\gamma $$ peaks, the energy spectrum can be fitted in an energy range of 200 keV around $$Q_{\beta \beta }$$ with a constant background. This gives a background index consistent with the full model and uncertainties of the same size. [ABSTRACT FROM AUTHOR]

4 b Simulated cosmogenic background spectra in 0-2.8 MeV region from copper components of germanium detectors for CDEX-100 experiment The publisher apologizes for the inconvenience caused. J. C (2021) 81:1041 https://doi.org/10.1140/epjc/s10052-021-09827-w In this article the caption of figure 4 contained a typesetting mistake. [Extracted from the article]

For the first time, planar high-purity germanium detectors with thin amorphous germanium contacts were successfully operated directly in liquid nitrogen and liquid argon in a cryostat at the Max-Planck-Institut für Physics in Munich. The detectors were fabricated at the Lawrence Berkeley National Laboratory and the University of South Dakota, using crystals grown at the University of South Dakota. They survived long-distance transportation and multiple thermal cycles in both cryogenic liquids and showed reasonable leakage currents and spectroscopic performance. Also discussed are the pros and cons of using thin amorphous semiconductor materials as an alternative contact technology in large-scale germanium experiments searching for physics beyond the Standard Model. [ABSTRACT FROM AUTHOR]

Experiments searching for rare processes like neutrinoless double beta decay heavily rely on the identification of background events to reduce their background level and increase their sensitivity. We present a novel machine learning based method to recognize one of the most abundant classes of background events in these experiments. By combining a neural network for feature extraction with a smaller classification network, our method can be trained with only a small number of labeled events. To validate our method, we use signals from a broad-energy germanium detector irradiated with a 228 Th gamma source. We find that it matches the performance of state-of-the-art algorithms commonly used for this detector type. However, it requires less tuning and calibration and shows potential to identify certain types of background events missed by other methods. [ABSTRACT FROM AUTHOR]