arXiv subjects

Stefan Schönert

Publications and source records attributed to Stefan Schönert.

16 recordsLinked to original sources

arXiv2026

Procurement and Purification of Liquid Argon for the LEGEND-200 Experiment

LEGEND-200 requires high-purity liquid argon for effective background discrimination. In this paper, we present the design, construction, and performance of a dedicated liquid argon purification system, along with the procurement and purification of liquid argon for filling the LEGEND-200 cryostat to its total capacity of 91 t. The purifier is based on copper catalyst and molecular sieve to remove oxygen and water. Starting with liquid argon of 5.5 quality, featuring an effective scintillation light triplet lifetime $τ_t$ of about 0.9 $μ$s, we achieved a final purity corresponding to $τ_t$ = 1.3 $μ$s. After complete filling of the LEGEND-200 cryostat, the measured effective triplet lifetime was 1.16 $μ$s. The notable reduction is caused by a residual nitrogen impurity introduced by an accidentally spoiled liquid argon delivery. An excessive nitrogen influx was prevented by the LEGEND Liquid Argon Monitoring Apparatus (LLAMA), which served as one of the three independent purity monitors during the filling campaign.

physics.ins-det↗

arXiv2026

Epitaxy of strained, nuclear-spin free $^{76}$Ge quantum wells from solid source materials

Germanium quantum well heterostructures have rapidly emerged as a leading platform for solid-state quantum information processing; however, material quality limits scalability, and higher structural quality, higher purity, as well as zero nuclear spin, are required. Here, we address these problems by employing the heaviest of Ge isotopes, by evaporating high-purity $^{76}$Ge radiation detector material, as utilized in fundamental neutrino particle physics experiments, to fabricate $^{76}$Ge/$^{28}$Si$^{76}$Ge quantum wells for quantum applications and explore the respective challenges. Specifically, we demonstrate improved results on strain-relaxed virtual Si$_{0.2}$Ge$_{0.8}$ substrates, forward graded from Si, with a dislocation density below 3.7$\cdot$10$^{5}$ cm$^{-2}$, explore nuclear spin-free solid-source molecular beam epitaxy, and demonstrate first quantum transport in $^{76}$Ge quantum wells. We demonstrate a record-level quantum well interface width of 0.3 nm by X-ray reflectivity, and quantitatively compare it to atom probe tomography and scanning transmission electron microscopy. The grown layer reveals nuclear-spin-bearing impurity concentrations below 10$^{19}$ cm$^{-3}$ and chemical impurity levels below 10$^{18}$ cm$^{-3}$, except for residual carbon attributed to the graphite crucible of the Ge source, which may reach up to 10$^{19}$ cm$^{-3}$. Low-temperature magneto-transport measurements yield electron mobilities of 6.1$\cdot$10$^4$ cm$^2$V$^{-1}$s$^{-1}$ at 15 mK with a carrier density of 2.2$\cdot$10$^{11}$ cm$^{-2}$, indicating that residual carbon is the dominant scattering mechanism.

physics.app-ph↗

arXiv2025

First operation of poly(ethylene naphthalate) enclosures for high-purity germanium detectors in liquid argon for $^{42}$K/$^{42}$Ar mitigation

Commercial argon contains cosmogenic $^{42}$Ar whose progeny $^{42}$K is a critical background component for the Large Enriched Germanium Experiment for Neutrinoless $ββ$ Decay (LEGEND). LEGEND operates High-Purity Germanium (HPGe) detectors bare in liquid argon. $^{42}$K is attracted by the HPGe detectors' electric fields, and drifts toward the germanium surface, where it undergoes beta decay. LEGEND-1000 will mitigate $^{42}$K-induced background by using underground-sourced argon, depleted in cosmogenic isotopes. If underground argon is not available, mitigation techniques must be employed. Poly(ethylene naphthalate) (PEN) enclosures were proposed to hinder the ion drift, decrease the beta-particle's energy, and produce scintillation light. In this paper, we report on operating two HPGe detectors, both bare and PEN-enclosed, in $^{42}$Ar-enriched liquid argon, and find no evidence for deterioration of energy stability or resolution due to the enclosures. We monitor the beta and gamma rates of $^{42}$K, find complex time-dependencies extending to roughly 30 days after applying the HPGe detectors' high-voltage, and qualitatively demonstrate the $^{42}$K suppression capabilities of enclosures.

physics.ins-det↗

arXiv2025

$^{42}$Ar Production and Injection to a Liquid Argon Environment for Background Mitigation Studies

Atmosphere-sourced argon contains traces of $^{42}$Ar, whose $β^-$-decaying progeny $^{42}$K represents a significant intrinsic background for rare-event experiments using liquid argon (LAr) as detector or shielding medium. Understanding and mitigating this background is crucial for current and future large-scale detectors in neutrino and dark-matter physics. To enable controlled studies of $^{42}$K behavior and suppression techniques, $^{42}$Ar was produced by irradiating natural argon with 34 MeV $^{7}$Li$^{3+}$ ions at the Maier-Leibnitz-Laboratorium tandem accelerator, using beam currents of $101 \pm 5$ nA and $140 \pm 5$ nA, yielding $476 \pm 9$ Bq within two weeks, corresponding to a production rate of $\sim 1 \times 10^{6}$ atoms$\,$s$^{-1}$. The activated argon was injected into the one-ton SCARF cryostat, where two HPGe detectors monitored the subsequent $^{42}$K activity build-up. A time-dependent model describing $^{42}$Ar mixing and $^{42}$K equilibration in LAr yielded characteristic mixing time constants between one and two days. The established production and injection capability provides a reproducible platform for high-statistics $^{42}$K background studies, essential for developing and validating suppression strategies for next-generation LAr-based rare-event experiments such as LEGEND-1000.

physics.ins-det↗

arXiv2024

A liquid-phase loop-mode argon purification system

Noble gas and liquid detectors rely on high chemical purity for successful operation. While gaseous purification has emerged as a reliable method of producing high-purity noble fluids, the requirement for large mass flows drives the development of liquid-phase purification. We constructed a medium-scale liquid argon (LAr) purification system based on a copper catalyst and 4 A molecular sieve capable of purifying 1 t of commercial LAr 5.0 to a long effective triplet lifetime of $τ_3 \sim 1.3 μ$s. We further demonstrate that a quenched effective triplet lifetime of $τ_3 \sim 1 μ$s, due to contamination by air, can be recovered in loop-mode purification to $τ_3 \sim 1.3 μ$s after > 20 volume exchanges.

physics.ins-det↗

arXiv2023

17O enrichment of CaWO4 crystals for spin-dependent DM search

For many years, various experiments have attempted to shed light on the nature of dark matter (DM). This work investigates the possibility of using CaWO4 crystals for the direct search of spin-dependent DM interactions using the isotope 17O with a nuclear spin of 5/2. Due to the low natural abundance of 0.038%, an enrichment of the CaWO4 crystals with 17O is developed during the crystal production process at the Technical University of Munich. Three CaWO4 crystals were enriched, and their 17O content was measured by nuclear magnetic resonance spectroscopy at the University of Leipzig. This paper presents the concept and first results of the 17O enrichment and discusses the possibility of using enriched crystals to increase the sensitivity for the spin-dependent DM search with CRESST.

astro-ph.IM↗

arXiv2023

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 $^{76}$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.

hep-ex↗

arXiv2021

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.

physics.ins-det↗

arXiv2020

Hydrogen reduction of enriched germanium dioxide and zone-refining for the LEGEND experiment

The LEGEND experiment, now under construction, will operate a large array of Ge detectors for the search of neutrinoless double-beta decay of $^{76}$Ge. In this paper we report on the process development for the hydrogen reduction of germanium dioxide enriched in $^{76}$Ge as part of the effort to manufacture detectors for the LEGEND experiment. The process was optimized via a kinetic un-reacted shrinking model and tested with a batch of natural GeO$_2$. We completed the reduction of a batch of 23 kg isotopically enriched Ge with an average yield of 99.85%. Subsequently, the Ge was purified to intrinsic purity by zone-refining and an overall Ge yield of 99.05% was achieved. Using an intermediate underground storage, an average cosmogenic exposure of 156 h was accumulated. Special care was taken to avoid and recycle losses during the process.

physics.ins-det↗

arXiv2020

Digital Contact Tracing Service: An improved decentralized design for privacy and effectiveness

We propose a decentralized digital contact tracing service that preserves the users' privacy by design while complying to the highest security standards. Our approach is based on Bluetooth and measures actual encounters of people, the contact time period, and estimates the proximity of the contact. We trace the users' contacts and the possible spread of infectious diseases while preventing location tracking of users, protecting their data and identity. We verify and improve the impact of tracking based on epidemiological models. We compare a centralized and decentralized approach on a legal perspective and find a decentralized approach preferable considering proportionality and data minimization.

cs.CY↗

arXiv2019

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 ($0νββ$) decay of $^{76}$Ge. The technological challenge of GERDA is to operate in a "background-free" regime in the region of interest (ROI) after analysis cuts for the full 100$\,$kg$\cdot$yr 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 $Q_{ββ}$ for the $0νββ$ search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos ($2νββ$) 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 $16.04^{+0.78}_{-0.85} \cdot 10^{-3}\,$cts/(kg$\cdot$keV$\cdot$yr) for the enriched BEGe data set and $14.68^{+0.47}_{-0.52} \cdot 10^{-3}\,$cts/(kg$\cdot$keV$\cdot$yr) for the enriched coaxial data set. These values are similar to the one of Gerda Phase I despite a much larger number of detectors and hence radioactive hardware components.

nucl-ex↗

arXiv2009

Pulse shape discrimination studies with a Broad-Energy Germanium detector for signal identification and background suppression in the GERDA double beta decay experiment

First studies of event discrimination with a Broad-Energy Germanium (BEGe) detector are presented. A novel pulse shape method, exploiting the characteristic electrical field distribution inside BEGe detectors, allows to identify efficiently single-site events and to reject multi-site events. The first are typical for neutrinoless double beta decays (0-nu-2-beta) and the latter for backgrounds from gamma-ray interactions. The obtained survival probabilities of backgrounds at energies close to Q(76Ge) = 2039 keV are 0.93% for events from 60Co, 21% from 226Ra and 40% from 228Th. This background suppression is achieved with 89% acceptance of 228Th double escape events, which are dominated by single site interactions. Approximately equal acceptance is expected for 0-nu-2-beta-decay events. Collimated beam and Compton coincidence measurements demonstrate that the discrimination is largely independent of the interaction location inside the crystal and validate the pulse-shape cut in the energy range of Q(76Ge). The application of BEGe detectors in the GERDA and the Majorana double beta decay experiments is under study.

nucl-ex↗

arXiv2008

Vetoing atmospheric neutrinos in a high energy neutrino telescope

We discuss the possibility to suppress downward atmospheric neutrinos in a high energy neutrino telescope. This can be achieved by vetoing the muon which is produced by the same parent meson decaying in the atmosphere. In principle, atmospheric neutrinos with energies $E_ν> 10$ TeV and zenith angle up to 60 degree can be vetoed with an efficiency of > 99%. Practical realization will depend on the depth of the neutrino telescope, on the muon veto efficiency and on the ability to identify downward moving neutrinos with a good energy estimation.

astro-ph↗

arXiv2008

Performance of bare high-purity germanium detectors in liquid argon for the GERDA experiment

The GERmanium Detector Array, GERDA, will search for neutrinoless double beta decay in 76Ge at the National Gran Sasso Laboratory of the INFN. Bare high-purity germanium detectors enriched in 76Ge will be submerged in liquid argon serving simultaneously as a shield against external radioactivity and as a cooling medium. In GERDA Phase-I, reprocessed enriched-Ge detectors, which were previously operated by the Heidelberg-Moscow and IGEX collaborations, will be redeployed. Before operating the enriched detectors, tests are performed with non-enriched bare HPGe detectors in the GERDA underground Detector Laboratory to test the Phase-I detector assembly, the detector handling protocols, the refurbishment technology and to study the long-term stability in liquid argon. The leakage currents in liquid argon and liquid nitrogen have been extensively studied under varying gamma irradiation conditions. In total three non-enriched high-purity p-type prototype germanium detectors have been operated successfully. The detector performance is stable over the long-term measurements. For the first time, performance of bare high-purity germanium detectors in liquid argon is reported.

physics.ins-det↗

arXiv2008

Pulse Shape Analysis with a Broad-Energy Germanium Detector for the GERDA experiment

To reduce background in experiments looking for rare events, such as the GERDA double beta decay experiment, it is necessary to employ active background-suppression techniques. One of such techniques is the pulse shape analysis of signals induced by the interaction of radiation with the detector. Analysis of the time-development of the impulses can distinguish between an interaction of an electron and an interaction of a multiple-scattered photon inside the detector. This information can be used to eliminate background events from the recorded data. Results of pulse-shape analysis of signals from a commercially available broad-energy germanium detector are presented and the pulse-shape discrimination capability of such detector configuration for use in low-background experiments is discussed.

nucl-ex↗

arXiv2005

Prototype scintillator cell for an In-based solar neutrino detector

We describe the work carried out at MPIK to design, model, build and characterize a prototype cell filled with a novel indium-loaded scintillator of interest for real-time low energy solar neutrino spectroscopy. First, light propagation in optical modules was studied with experiments and Monte Carlo simulations. Subsequently a 5 cm x 5 cm x 100 cm prototype detector was set up and the optical performances of several samples were measured. We first tested a benchmark PXE-based scintillator, which performed an attenuation length of ~ 4.2 m and a photo-electron yield of ~ 730 pe/MeV. Then we measured three In-loaded samples. At an In-loading of 44 g/l, an energy resolution of ~ 11.6 % and a spatial resolution of ~ 7 cm were attained for 477 keV recoil electrons. The long-range attenuation length in the cell was ~1.3 m and the estimated photo-electron yield ~ 200 pe/MeV. Light attenuation and relative light output of all tested samples could be reproduced reasonably well by MC. All optical properties of this system have remained stable over a period of > 1 y.

physics.ins-det↗