arXiv subjects

Kai Zuber

Publications and source records attributed to Kai Zuber.

At least 19 recordsLinked to original sources

arXiv2026

Measurement of the Electron capture of $^{76}$As into the first excited state of $^{76}$Ge

The neutrinoless double beta decay of $^{76}$Ge is searched for in the large-scale experiment LEGEND. The measurement of the half-life of this process would give access to the neutrino mass using the nuclear matrix element. Experimentally the contribution of the $^{76}$As ground state to the nuclear matrix element can be investigated via the branching ratios of its $\beta^-$ and electron capture decay. While energetically, the electron capture of $^{76}$As into the first excited state of $^{76}$Ge is possible and was measured once before this work, the electron capture into the $^{76}$Ge ground state was not observed yet. The present study investigates the branching of $^{76}$As that is produced via $^{75}$As(n,$\gamma$) on a thin As$_2$O$_3$ sample. A silicon drift detector measures characteristic X-rays emitted by the germanium atoms caused by an inner vacancy after the electron capture. A high-purity germanium detector is used to measure the 562.9$\,$keV $\gamma$-rays emitted after electron capture into the excited state. Investigation of coincident signals in both detectors leads to the branching ratio of the $^{76}$As electron capture into the first excited state of $^{76}$Ge of $\nu_{\mathrm{EC}^\ast} = (0.0572 \pm 0.0029 (\mathrm{stat.}) \pm 0.0074(\mathrm{syst.}))\%$. This is the first measurement with the full uncertainty budget quantified.

nucl-ex↗

arXiv2025

Underground nuclear astrophysics: Status and recent results from Felsenkeller laboratory

For almost three decades it has been known that the study of astro-physically important nuclear reactions between stable nuclei requires the use of low-background, underground accelerator laboratories. The Felsenkeller shallow-underground laboratory in Dresden, shielded by a 45 m thick rock cover, hosts a 5 MV Pelletron ion accelerator with an external sputter ion source (mainly able to provide carbon and oxygen beams) and an internal radio-frequency ion source (providing proton and alpha beams). The reduced muon, neutron and gamma-ray background achieved both with natural and active shielding situate the laboratory well in line with deep underground accelerator labs worldwide and allows highly sensitive nuclear reaction experiments. Currently, measurements affecting the solar fusion and Big Bang nucleosynthesis are ongoing. In addition to in-house research by HZDR and TU Dresden, the lab is an open facility for scientific users worldwide, with beam time applications reviewed by an independent science advisory board. Furthermore, EU-supported transnational access is available via the ChETEC- INFRA network for nuclear astrophysics. A brief introduction to underground nuclear astrophysics, status of the Felsenkeller shallow-underground laboratory and some preliminary results are discussed.

nucl-ex↗

arXiv2025

Gas-jet target with online interferometric thickness measurement for nuclear astrophysics

A new jet gas target system has been developed for the Felsenkeller 5 MV underground ion accelerator for nuclear astrophysics. It provides either a 1.5$\times10^{18}$ cm$^{-2}$ thick cylindrical jet or a 7$\times10^{17}$ cm$^{-2}$ thick wall of nitrogen gas, with a surface of 10$\times$10 mm$^2$ to be seen by the ion beam. The system includes a de Laval type nozzle and altogether five pumping stages: In addition to the jet catcher and the jet chamber surrounding it, there are three stages connecting the jet to the ion accelerator. Behind the jet chamber, as seen from the ion beam, a windowless static-type gas target and, subsequently, a beam calorimeter have been installed. This work describes the offline tests of the gas target system prior to its installation on the beam line of the Felsenkeller accelerator. The thickness of the jet has been determined using three different methods: By computational fluid dynamics simulations, with a Mach-Zehnder interferometer, and by $\alpha$-energy loss using a mixed $\alpha$ source. The three methods were shown to be in agreement. For 0-6 bar inlet gas pressure, a linear relationship between inlet pressure and jet thickness has been found. Different shapes of de Laval type inlet nozzles, both circular and slit-type, have been manufactured from fused silica glass or stainless steel and tested using measurements and simulations. The power and stability of the beam calorimeter have been tested. The interferometry has been shown to work reliably and to give two-dimensional projections of the gas jet with sub-mm resolution.

physics.ins-det↗

arXiv2025

Evidence for neutron-induced $\boldsymbol{\gamma}$-ray emissions in the vicinity of the $\boldsymbol{Q}$ value of $^{76}\text{Ge}$ $0\nu\beta\beta$ decay

Neutrinoless double-beta decay of nuclei represents one of the most promising methods for uncovering physics beyond the Standard Model. In this context, $^{76}$Ge stands out as a particularly attractive candidate, as it can serve as an intrinsic component in semiconductor detectors. If the neutrinoless process occurs in $^{76}$Ge, its signature would appear as a distinct peak at the $Q$ value of 2039 keV. A neutron activation measurement was performed on a germanium sample isotopically enriched in $^{76}$Ge at the DT neutron generator of TU Dresden. The measurement confirmed the presence of $\gamma$ rays with energies of 2033.1$\pm$0.5 keV, 2035.5$\pm$0.4 keV, and 2040.22$\pm$0.26 keV originating from the decays of $^{74}$Ga and $^{76}$Ga. These $\gamma$ rays lie in close proximity to the expected neutrinoless double-beta decay signal of $^{76}$Ge.

nucl-ex↗

arXiv2024

The Felsenkeller shallow-underground laboratory for nuclear astrophysics

In the Felsenkeller shallow-underground site, protected from cosmic muons by a 45 m thick rock overburden, a research laboratory including a 5 MV Pelletron ion accelerator and a number of radioactivity-measurement setups is located. The laboratory and its installations are described in detail. The background radiation has been studied, finding suppression factors of 40 for cosmic-ray muons, 200 for ambient neutrons, and 100 for the background in germanium $\gamma$-ray detectors. Using an additional active muon veto, typically the background is just twice as high as in very deep underground laboratories. The properties of the accelerator including its external and internal ion sources and beam line are given. For the radioactivity counting setup, detection limits in the 10$^{-4}$ Bq range have been obtained. Practical aspects for the usage of the laboratory by outside scientific users are discussed.

physics.ins-det↗

arXiv2022

Study of a possible silicon photomultiplier based readout of the large plastic scintillator neutron detector NeuLAND

The NeuLAND (New Large-Area Neutron Detector) plastic-scintillator-based time-of-flight detector for 0.1-1.6 GeV neutrons is currently under construction at the Facility for Antiproton and Ion Research (FAIR), Darmstadt, Germany. In its final configuration, NeuLAND will consist of 3000 2.7 m $\times$ 5 cm $\times$ 5 cm big plastic scintillator bars that are read out on each end by fast timing photomultipliers. Here, data from a comprehensive study of an alternative light readout scheme using silicon photomultipliers (SiPM) are reported. For this purpose, a NeuLAND bar was instrumented on each end with a SiPM-based prototype of the same geometry as a 1'' photomultiplier tube, including four 6 $\times$ 6 mm$^2$ SiPMs, amplifiers, high voltage supply, and microcontroller. Tests were carried out using the 35 MeV electron beam from the superconducting Electron Linac for beams with high Brilliance and low Emittance (ELBE) with its picosecond-level time jitter in two different modes of operation, namely parasitic mode with one electron per bunch and single-user mode with 1-60 electrons per bunch. Acqiris fast digitisers were used for data acquisition. In addition, off-beam tests using cosmic rays and the NeuLAND data acquisition scheme have been carried out. Typical time resolutions of $\sigma_t\leq$ 120 ps were found for $\geq$95% efficiency for minimum ionising particles, improving on previous work at ELBE and exceeding the NeuLAND timing goal of $\sigma_t$ < 150 ps. Over a range of 10-300 MeV deposited energy in the NeuLAND bar, the gain was found to deviate by $\leq$10% ($\leq$20%) from linearity for 35 mm (75 mm) SiPM pitch, respectively, satisfactory for calorimetric use of the full NeuLAND detector. The dark rate of the prototype studied was found to be lower than the expected cosmic-ray induced background in NeuLAND.

physics.ins-det↗

arXiv2022

The first large-scale shell-model calculation of the two-neutrino double beta decay of $^{76}$Ge to the excited states in $^{76}$Se

Large-scale shell-model calculations were carried out for the half-lives and branching ratios of the $2\nu\beta\beta$ decay of $^{76}$Ge to the ground state and the lowest three excited states $2_1^+$, $0_2^+$ and $2_2^+$ in $^{76}$Se. In total, the wave functions of more than 10,000 intermediate $1^+$ states in $^{76}$As were calculated in a three-step procedure allowing an efficient use of the available computer resources. In the first step, 250 lowest states, below some 5 MeV of excitation energy, were calculated without truncations within a full major shell $0f_{5/2}-1p-0g_{9/2}$ for both protons and neutrons. The wave functions of the rest of the states, up to some 30 MeV, were computed in two more steps by introducing two consecutive stages of truncation. The computed magnitudes of the $2\nu\beta\beta$ nuclear matrix elements (including the value of the axial-vector coupling $g_{\rm A}$), $\vert M_{2\nu}\vert g_{\rm A}^2$, converged to the values 0.168$g_{\rm A}^2$, $1.2\times10^{-3}$$g_{\rm A}^2$, 0.121$g_{\rm A}^2$, and $3.1\times10^{-3}$$g_{\rm A}^2$ for the $0^+_{\rm g.s.}$, $2^+_1$, $0^+_2$, and $2^+_2$ states, respectively. Using up-to-date phase-space integrals, the corresponding branching ratios were derived to be 99.926\%, 4.4$\times10^{-5}$\%, 0.074\% and 2.5$\times10^{-7}$\%. The experimental half-life $(1.926\pm0.094)\times10^{21}$ yr of the ground-state transition was used to derive the value $g_{\rm A}=0.80\pm0.01$ for the axial-vector coupling, which is consistent with other shell-model calculations suggesting a quenched value of $g_{\rm A}$. Using this value of $g_{\rm A}$, predictions for the transition half-lives were derived.

nucl-th↗

arXiv2021

Confirmation of $g_{\rm A}$ quenching using the revised spectrum-shape method for the analysis of the $^{113}$Cd $β$-decay as measured with the COBRA demonstrator

In this article we present an updated spectrum-shape analysis of the $^{113}$Cd fourfold forbidden non-unique $β$-decay transition in order to address the quenching of the weak axial-vector coupling $g_{\rm A}$ in low-momentum exchange nuclear processes. The experimental data were collected in a dedicated low-threshold run with the COBRA demonstrator at the LNGS and resulted in 44 individual $^{113}$Cd spectra. These data are evaluated in the context of three nuclear model frameworks based on a revised version of the spectrum-shape method and the conserved vector current hypothesis. The novel idea devised in the present work is to fit the value of the small relativistic nuclear matrix element (s-NME) driving the nuclear model calculations, which remained essentially as a free parameter in previous studies. This is done by tuning the nuclear structure calculations and making use of the interplay of $g_{\rm A}$ and the s-NME such that the experimentally known $^{113}$Cd half-life gets reproducible by the different frameworks. In this way, a best fit s-NME value can be derived for each of the considered nuclear models, which finally enters the template calculations used to perform the spectrum-shape analysis for each of the obtained $^{113}$Cd spectra. The primary analysis strategy results in significantly quenched values of the axial-vector coupling for all three nuclear models: $\overline{g}_{\rm A}(\text{ISM}) = 0.907 \pm 0.064$, $\overline{g}_{\rm A}(\text{MQPM}) = 0.993 \pm 0.063$ and $\overline{g}_{\rm A}(\text{IBFM-2}) = 0.828 \pm 0.140$. Moreover, with our data-driven approach one of the main shortcomings of the spectrum-shape method has been resolved. This achievement is a milestone in the description of strongly forbidden $β$-decays and adds to the indications for the existence of a quenching of $g_{\rm A}$ in low-momentum exchange nuclear processes.

nucl-ex↗

arXiv2020

Presupernova neutrinos: directional sensitivity and prospects for progenitor identification

We explore the potential of current and future liquid scintillator neutrino detectors of O (10) kt mass to localize a pre-supernova neutrino signal in the sky. In the hours preceding the core collapse of a nearby star (at distance D < 1 kpc), tens to hundreds of inverse beta decay events will be recorded, and their reconstructed topology in the detector can be used to estimate the direction to the star. Although the directionality of inverse beta decay is weak (~8% forward-backward asymmetry for currently available liquid scintillators), we find that for a fiducial signal of 200 events (which is realistic for Betelgeuse), a positional error of ~60 degrees can be achieved, resulting in the possibility to narrow the list of potential stellar candidates to less than ten, typically. For a configuration with improved forward-backward asymmetry (~40%, as expected for a lithium-loaded liquid scintillator), the angular sensitivity improves to ~15 degrees, and - when a distance upper limit is obtained from the overall event rate - it is in principle possible to uniquely identify the progenitor star. Any localization information accompanying an early supernova alert will be useful to multi-messenger observations and to particle physics tests using collapsing stars.

astro-ph.HE↗

arXiv2020

Examining the $N$ = 28 shell closure through high-precision mass measurements of $^{46-48}$Ar

The strength of the $N$ = 28 magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of $^{46-48}$Ar, performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. The new mass values are up to 90 times more precise than previous measurements. While they suggest the persistence of the $N$ = 28 shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth of spectroscopic data and theoretical investigations performed with the \emph{SDPF-U} phenomenological shell model interaction. Our results are also compared with \emph{ab initio} calculations using the Valence Space In-Medium Similarity Renormalization Group and the Self-Consistent Green's Function approaches. Both calculations provide a very good account of mass systematics at and around $Z$ = 18 and, generally, a consistent description of the physics in this region. This combined analysis indicates that $^{46}$Ar is the transition between the closed-shell $^{48}$Ca and collective $^{44}$S.

nucl-ex↗

arXiv2020

$Q_{\textrm{EC}}$-value determination for $^{21}$Na$\rightarrow^{21}$Ne and $^{23}$Mg$\rightarrow^{23}$Na mirror-nuclei decays using high-precision mass spectrometry with ISOLTRAP at ISOLDE/CERN

We report on high-precision $Q_{\textrm{EC}}$ values of the $^{21}$Na$\rightarrow^{21}$Ne and $^{23}$Mg$\rightarrow^{23}$Na mirror $β$-transitions from mass measurements with ISOLTRAP at ISOLDE/CERN. A precision of $δm/m = 9 \cdot 10^{-10}$ and $δm/m = 1.5 \cdot 10^{-9}$ was reached for the masses of $^{21}$Na and $^{23}$Mg, respectively. We reduce the uncertainty of the $Q_{\textrm{EC}}$ values by a factor five, making them the most precise experimental input data for the calculation of the corrected $\mathcal{F} t$-value of these mixed Fermi/Gamow-Teller transitions. For the $^{21}$Na$\rightarrow^{21}$Ne $Q_{\textrm{EC}}$ value, a $2.3 σ$ deviation from the literature $Q_{\textrm{EC}}$-value was found.

nucl-ex↗

arXiv2019

Calculated solar-neutrino capture rate for a radiochemical 205 Tl-based solar-neutrino detector

Radiochemical experiments for low-energy solar-neutrino detection have been making headlines by exploiting the isotopes \iso{Cl}{37} and \iso{Ga}{71}. Such a very low-threshold measurement of this type can also be performed using \iso{Tl}{205}, which has been considered for decades for this purpose. A unique feature of this detector nucleus is the integration is the solar-neutrino flux over millions of years owing to its long-living daughter \iso{Pb}{205}. In this study we have calculated for the first time the cross section for the charged-current solar-neutrino scattering off \iso{Tl}{205}. Taking into account the solar-model-predicted neutrino fluxes and the electron-neutrino survival probabilities, a solar-neutrino capture rate of 62.2 $\pm 8.6$ SNU is determined, a value significantly smaller than in previous estimates.

nucl-th↗

arXiv2019

Search for dark matter induced de-excitation of $^{180}$Ta$\rm ^m$

Weak-scale dark matter particles, in collisions with nuclei, can mediate transitions between different nuclear energy levels. In particular, owing to sizeable momentum exchange, dark matter particles can enable de-excitation of nuclear isomers that are extremely long lived with respect to regular radioactive decays. In this paper, we utilize data from a past experiment with $^{180}$Ta$\rm ^m$ to search for $γ$-lines that would accompany dark matter induced de-excitation of this isomer. Non-observation of such transitions above background yields the first direct constraint on the lifetime of $^{180}$Ta$\rm ^m$ against DM-initiated transitions: $T_{1/2}>1.3\times 10^{14}$~a at 90\% C.I. Using this result, we derive novel constraints on dark matter models with strongly interacting relics, and on models with inelastic dark matter particles. Existing constraints are strengthened by this independent new method. The obtained limits are also valid for the Standard Model $γ$-decay of $^{180}$Ta$\rm ^m$.

astro-ph.CO↗

arXiv2019

Quenching of $g_{\rm A}$ deduced from the $β$-spectrum shape of $^{113}$Cd measured with the COBRA experiment

A dedicated study of the quenching of the weak axial-vector coupling strength $g_{\rm A}$ in nuclear processes has been performed by the COBRA collaboration. This investigation is driven by nuclear model calculations which show that the $β$-spectrum shape of the fourfold forbidden non-unique decay of $^{113}$Cd strongly depends on the effective value of $g_{\rm A}$. Using an array of CdZnTe semiconductor detectors, 45 independent $^{113}$Cd spectra were obtained and interpreted in the context of three nuclear models. The resulting effective mean values are $\bar{g}_{\rm A}(\text{ISM}) = 0.915 \pm 0.007$, $\bar{g}_{\rm A}(\text{MQPM}) = 0.911 \pm 0.013$ and $\bar{g}_{\rm A}(\text{IBFM-2}) = 0.955 \pm 0.022$. These values agree well within the determined uncertainties and deviate significantly from the free value of $g_{\rm A}$. This can be seen as a first step towards answering the long-standing question regarding quenching effects related to $g_{\rm A}$ in low-energy nuclear processes.

nucl-ex↗

arXiv2019

In-situ measurement of the scintillation light attenuation in liquid argon in the GERDA experiment

The GERDA experiment searches for the neutrinoless double beta ($0νββ$) decay in $^{76}$Ge in order to probe whether the neutrino is a Majorana particle and to shed light on the neutrino mass ordering. For investigating such a rare decay it is necessary to minimize the background of the experiment. In Phase II of the GERDA experiment the scintillation light of liquid argon (LAr) is used as an additional background veto. In order to estimate the efficiency of such a LAr veto it has to be known how far the scintillation light, which peaks at 128 nm, can travel within the LAr. A dedicated setup was built to measure the attenuation length of the scintillation light in the LAr in-situ within the cryostat of GERDA. The setup is composed of a stainless steel housing with a photomultiplier tube (PMT) at one side and a moveable $^{90}$Sr source at the other side to measure the light intensity at different distances between source and PMT. Furthermore, a sophisticated simulation was developed in order to determine the solid angle correction as well as the background for this measurement. The analysis results in an absorption length of $15.8 \pm 0.7$ (stat) ${}^{+1.5}_{-3.2}$ (syst) cm under the assumption of a scattering length of 70 cm at 128 nm. The obtained value of the absorption length is specific for the LAr in GERDA at the time of the measurement.

physics.ins-det↗

arXiv2019

Constraint on the axion-electron coupling constant and the neutrino magnetic dipole moment by using the tip-RGB luminosity of fifty globular clusters

The current constraints in the neutrino magnetic dipole moment and axion-electron coupling constant ($\mathrm{μ_ν\leq2.2\times10^{-12}μ_{B}}$ and $\mathrm{α_{ae}\leq0.5\times10^{-26}}$) are tested against the observational calibration on the tip-RGB of fifty galactic globular clusters, covering $\mathrm{-1.95\leq[M/H]\leq+0.04}$. When each energy sink is introduced into stellar models separately, the current known constraints predict a tip-RGB bolometric luminosity that does not conflict the calibration, while when they are assumed to occur simultaneously, these constraints can be lowered down by 50\% and 75\%.

astro-ph.SR↗

arXiv2019

Direct decay-energy measurement as a route to the neutrino mass

A high-precision measurement of the $^{131}$Cs$ \rightarrow ^{131}$Xe ground-to-ground-state electron-capture $Q_{\textrm{EC}}$-value was performed using the ISOLTRAP mass spectrometer at ISOLDE/CERN. The novel PI-ICR technique allowed to reach a relative mass precision $δm/m$ of $1.4\cdot10^{-9}$. A mass resolving power $m/Δm$ exceeding $1\cdot10^7$ was obtained in only $1\,$s trapping time. Allowed electron-capture transitions with sub-keV or lower decay energies are of high interest for the direct determination of the $ν_e$ mass. The new measurement improves the uncertainty on the ground-to-ground-state $Q_{\textrm{EC}}$-value by a factor 25 precluding the $^{131}$Cs$ \rightarrow ^{131}$Xe pair as a feasible candidate for the direct determination of the $ν_e$ mass.

nucl-ex↗

arXiv2018

The new Felsenkeller 5 MV underground accelerator

The field of nuclear astrophysics is devoted to the study of the creation of the chemical elements. By nature, it is deeply intertwined with the physics of the Sun. The nuclear reactions of the proton-proton cycle of hydrogen burning, including the 3He(α,γ)7Be reaction, provide the necessary nuclear energy to prevent the gravitational collapse of the Sun and give rise to the by now well-studied pp, 7Be, and 8B solar neutrinos. The not yet measured flux of 13N, 15O, and 17F neutrinos from the carbon-nitrogen-oxygen cycle is affected in rate by the 14N(p,γ)15O reaction and in emission profile by the 12C(p,γ)13N reaction. The nucleosynthetic output of the subsequent phase in stellar evolution, helium burning, is controlled by the 12C(α,γ)16O reaction. In order to properly interpret the existing and upcoming solar neutrino data, precise nuclear physics information is needed. For nuclear reactions between light, stable nuclei, the best available technique are experiments with small ion accelerators in underground, low-background settings. The pioneering work in this regard has been done by the LUNA collaboration at Gran Sasso/Italy, using a 0.4 MV accelerator. The present contribution reports on a higher-energy, 5.0 MV, underground accelerator in the Felsenkeller underground site in Dresden/Germany. Results from γ-ray, neutron, and muon background measurements in the Felsenkeller underground site in Dresden, Germany, show that the background conditions are satisfactory for nuclear astrophysics purposes. The accelerator is in the commissioning phase and will provide intense, up to 50μA, beams of 1H+, 4He+ , and 12C+ ions, enabling research on astrophysically relevant nuclear reactions with unprecedented sensitivity.

physics.acc-ph↗