arXiv subjects

K. Ni

Publications and source records attributed to K. Ni.

At least 19 recordsLinked to original sources

arXiv2026

Background decomposition of the CONUS+ run 1 data

The CONUS+ experiment is measuring the coherent elastic neutrino nucleus scattering (CE$\nu$NS) process using reactor anti-neutrinos as a source and four low energy threshold point-contact high-purity germanium spectrometers for their detection. It achieved the first measurement of coherent neutrino scattering at a nuclear reactor in run 1 of the experiment with a detection energy threshold of 160 eV$_{ee}$. This work presents the decomposition of the background spectra of the three detectors used in the run 1 analysis and the development of the corresponding background model with Geant4-based Monte Carlo simulations. The background model is used as the underlying input for the likelihood fit of the analysis. It is shown that reactor-correlated backgrounds are subdominant in all energy regions, specifically in the region of interest for CE$\nu$NS searches below 350 eV$_{ee}$ where their contribution is one order of magnitude below the expected CE$\nu$NS signal. Furthermore, cosmic ray muons and neutrons are identified as the dominant background source below 1 keV$_{ee}$ contributing approximately 75 - 90 \% of the recorded background rate. The final background model predicts an average rate of (47.5 $\pm$ 3.2) d$^{-1}$ kg$^{-1}$ in reactor on measurement in the energy region between [0.4, 1.0] keV$_{ee}$, which is in excellent agreement with the average measured value of (48.0 $\pm$ 0.6) d$^{-1}$ kg$^{-1}$. Similar agreement is found in all energy regions of both reactor on and off measurements.

physics.ins-det↗

arXiv2026

New constraints on physics within and beyond the standard model from the latest CONUS datasets

Its detections with pion-decay-at-rest, solar and recently with reactor antineutrinos by the CONUS collaboration render coherent elastic neutrino-nucleus scattering (CE$\nu$NS) an established tool for investigations within and beyond the Standard Model (SM). The CONUS experiment located at the nuclear power plants in Brokdorf (Germany) and Leibstadt (Switzerland) operates Germanium semiconductor detectors in a compact shield at close distance to the reactor core. An observation with $3.7 \sigma$ significance is reported at the Leibstadt site, showing good agreement with its SM prediction. Physics investigations performed with the last datasets collected at the Brokdorf reactor and with the first data obtained at the Leibstadt site are summarized. By using the experimental analysis framework, the presented results contain the full systematics that underlie the experiment. Previously determined limits with neutrino-electron scattering on the neutrino magnetic moment and a neutrino millicharge are improved to $\mu_{\nu} <5.18\cdot 10^{-11}\mu_\mathrm{B}$ and $q_{\nu}<1.76\cdot 10^{-12} e_0$ (90% C.L). Further, the scale of new physics related to NSIs is improved to $\Lambda_{\rm NSI}$=145 GeV and limits on the coupling of light new mediators are lowered down to $4 \cdot 10^{-7}$ (90% C.L.) with the new data. Finally, the determination of the Weinberg angle with CE$\nu$NS and reactor antineutrinos yields $\sin^{2}\theta_W= 0.28^{+0.03}_{-0.04}$ at a momentum transfer of $\sim 10 \ \mathrm{MeV}$.

hep-ex↗

arXiv2026

A New Concept of Liquid Xenon Time Projection Chamber for Medical Imaging

Liquid xenon time projection chambers offer a homogeneous detection medium with excellent intrinsic energy resolution, fast scintillation, and true three-dimensional position sensitivity, making them an attractive alternative to crystal-based detectors for positron emission tomography (PET). In this work, we present a new single-phase liquid xenon time projection chamber (TPC) concept optimized for medical imaging, employing combined scintillation and electroluminescence-based ionization readout to enable low-noise signal amplification and intrinsic depth-of-interaction measurement. We evaluate the system-level performance of this detector concept using Monte Carlo simulations based on OpenGATE and Geant4, with direct comparison to conventional LYSO-based PET systems. The study focuses on detection sensitivity, energy-based event selection efficiency, and reconstructed spatial resolution. While LYSO detectors provide higher absolute stopping efficiency due to their higher density, liquid xenon detectors exhibit improved photopeak purity as a result of superior intrinsic energy resolution, leading to enhanced rejection of scattered events. Point-source reconstruction studies demonstrate that the intrinsic three-dimensional position sensitivity of the liquid xenon TPC translates into a reconstructed spatial resolution of approximately 1~mm full width at half maximum (FWHM) at the system level, compared to approximately 4~mm for LYSO-based systems under comparable conditions. These results indicate that liquid-xenon-based PET detectors can achieve competitive or superior imaging performance, particularly for applications requiring high spatial resolution, large axial acceptance, and scalable detector geometries.

physics.ins-det↗

arXiv2026

Design of a high voltage delivery system for noble liquid time projection chambers

Noble liquid time projection chambers (TPCs) are a leading technology in the detection of ionizing radiation, particularly in applications such as accelerator neutrino physics, dark matter detection, and neutrinoless double beta decay. This paper addresses the design considerations for implementing stable high voltage (HV) systems within large noble liquid TPCs, with a focus on the nEXO experiment. Utilizing insights from prior HV research and experimental investigations, we outline factors influencing HV stability and discuss design choices to improve stability and prevent electrical discharges. A novel HV delivery system concept is presented, tailored for the nEXO TPC, which incorporates these design considerations while also meeting the stringent radiopurity requirements of the nEXO neutrinoless double beta decay search. These design considerations and their specific implementation towards a HV delivery system offer guidance to future experiments applying high voltage in noble liquid environments.

physics.ins-det↗

arXiv2025

Characterization of CRYO ASIC for charge readout in the nEXO experiment

nEXO is a proposed next-generation experiment searching for the neutrinoless double beta decay of $^{136}$Xe using a tonne-scale liquid xenon (LXe) time projection chamber (TPC). To image the ionization signals from events in the liquid xenon, the detector will employ metallized fused-silica charge collection tiles instrumented with cryogenic application-specific integrated circuits (ASICs), referred to as CRYO ASIC, which are designed to operate directly in LXe to minimize input capacitance and pick-up noise. Here we present the performance of the CRYO ASIC mounted on an auxiliary printed circuit board and evaluated both in a cryogenic environmental chamber and in a dedicated LXe test stand. We demonstrate that the ASICs achieve the desired performance at liquid xenon temperatures, showing a gain stability better than 0.2% over 24-hour operation and reliable in-situ calibration using an on-chip pulser. In the LXe test stand, we show that boiling caused by the chip heat dissipation can be mitigated by operating the system above ~0.1 MPa. The in-LXe noise measured agrees with simulation, which indicates it the $150~e^-$ design requirement can be satisfied. These results establish CRYO ASIC as a viable low-noise in-LXe charge readout solution for nEXO.

physics.ins-det↗

arXiv2025

Development & Characterization of Electrodes for large-scale Xenon Time Projection Chambers

Dual-phase liquid xenon time projection chambers are the core detector elements of many experiments that conduct searches for Dark Matter and rare events, as well as in neutrino and high-energy physics. As part of this detector technology, high-voltage electrodes are instrumental for the generation of observable signals and their physical interpretation. Thus, electrode design and manufacturing has to fulfill stringent requirements, and their production is associated with significant engineering challenges. In this work we describe the successful development of electrodes on the 1.5 m-scale, from their design and simulation to subsequent assembly and high-voltage testing in a gaseous argon environment. The produced electrodes were recently installed as an anode and a cathode during an upgrade to the XENONnT experiment.

physics.ins-det↗

arXiv2025

Ultra-pure Nickel for Structural Components of Low-Radioactivity Instruments

The next generation of rare-event search experiments in nuclear and particle physics demand structural materials combining exceptional mechanical strength with ultra-low levels of radioactive contamination. This study evaluates chemical vapor deposition (CVD) nickel as a candidate structural material for such applications. Manufacturer-supplied CVD Ni grown on aluminum substrates underwent tensile testing before and after welding alongside standard Ni samples. CVD Ni exhibited a planar tensile strength of ~600 MPa, significantly surpassing standard nickel. However, welding and heat treatment were found to reduce the tensile strength to levels comparable to standard Ni, with observed porosity in the welds likely contributing to this reduction. Material assay via inductively coupled plasma mass spectrometry (ICP-MS) employing isotope-dilution produced measured bulk concentration of 232-Th, 238-U, and nat-K at the levels of ~70 ppq, <100 ppq, and ~900 ppt, respectively, which is the lowest reported in nickel. Surface-etch profiling uncovered higher concentrations of these contaminants extending ~10 micrometer beneath the surface, likely associated with the aluminum growth substrate. The results reported are compared to the one other well documented usage of CVD Ni in a low radioactive background physics research experiment and a discussion is provided on how the currently reported results may arise from changes in CVD fabrication or testing process. These results establish CVD Ni as a promising low-radioactivity structural material, while outlining the need for further development in welding and surface cleaning techniques to fully realize its potential in large-scale, low radioactive background rare-event search experiments.

nucl-ex↗

arXiv2025

Sensitivity of nEXO to $^{136}$Xe Charged-Current Interactions: Background-free Searches for Solar Neutrinos and Fermionic Dark Matter

We study the sensitivity of nEXO to solar neutrino charged-current interactions, $\nu_e + ^{136}$Xe$\rightarrow ^{136}$Cs$^* + e^-$, as well as analogous interactions predicted by models of fermionic dark matter. Due to the recently observed low-lying isomeric states of $^{136}$Cs, these interactions will create a time-delayed coincident signal observable in the scintillation channel. Here we develop a detailed Monte Carlo of scintillation emission, propagation, and detection in the nEXO detector to model these signals under different assumptions about the timing resolution of the photosensor readout. We show this correlated signal can be used to achieve background discrimination on the order of $10^{-9}$, enabling nEXO to make background-free measurements of solar neutrinos above the reaction threshold of 0.668 MeV. We project that nEXO could measure the flux of CNO solar neutrinos with a statistical uncertainty of 25%, thus contributing a novel and competitive measurement towards addressing the solar metallicity problem. Additionally, nEXO could measure the mean energy of the $^7$Be neutrinos with a precision of $\sigma \leq 1.5$ keV and could determine the survival probability of $^{7}$Be and $pep$ solar $\nu_e$ with precision comparable to state-of-the-art. These quantities are sensitive to the Sun's core temperature and to non-standard neutrino interactions, respectively. Furthermore, the strong background suppression would allow nEXO to search for for charged-current interactions of fermionic dark matter in the mass range $m_\chi$ = $0.668$-$7$ MeV with a sensitivity up to three orders of magnitude better than current limits.

nucl-ex↗

arXiv2025

Ultra-sensitive radon assay using an electrostatic chamber in a recirculating system

Rare event searches such as neutrinoless double beta decay and Weakly Interacting Massive Particle detection require ultra-low background detectors. Radon contamination is a significant challenge for these experiments, which employ highly sensitive radon assay techniques to identify and select low-emission materials. This work presents the development of ultra-sensitive electrostatic chamber (ESC) instruments designed to measure radon emanation in a recirculating gas loop, for future lower background experiments. Unlike traditional methods that separate emanation and detection steps, this system allows continuous radon transport and detection. This is made possible with a custom-built recirculation pump. A Python-based analysis framework, PyDAn, was developed to process and fit time-dependent radon decay data. Radon emanation rates are given for various materials measured with this instrument. A radon source of known activity provides an absolute calibration, enabling statistically-limited minimal detectable activities of 20 $\mu$Bq. These devices are powerful tools for screening materials in the development of low-background particle physics experiments.

physics.ins-det↗

arXiv2025

Direct observation of coherent elastic antineutrino-nucleus scattering

Neutrinos are elementary particles that interact only very weakly with matter. Neutrino experiments are, therefore, usually big, with masses in the multi-tonne range. The thresholdless interaction of coherent elastic scattering of neutrinos on atomic nuclei leads to greatly enhanced interaction rates, which allows for much smaller detectors. The study of this process gives insights into physics beyond the Standard Model of particle physics. The CONUS+ experiment was designed to first detect elastic neutrino-nucleus scattering in the fully coherent regime with low-energy neutrinos produced in nuclear reactors. For this purpose, semi-conductor detectors based on high-purity germanium crystals with extremely low-energy thresholds were developed. Here we report the first observation of a neutrino signal with a statistical significance of 3.7 sigma from the CONUS+ experiment, operated at the nuclear power plant in Leibstadt, Switzerland. In 119 days of reactor operation (395$\pm$106) neutrinos were measured compared with a predicted number from calculations assuming Standard Model physics of (347$\pm$59) events. With increased precision, there is potential for fundamental discoveries in the future. The CONUS+ results in combination with other measurements of this interaction channel might therefore mark a starting point for a new era in neutrino physics.

hep-ex↗

arXiv2024

Background characterization of the CONUS+ experimental location

CONUS+ is an experiment aiming at detecting coherent elastic neutrino-nucleus scattering (CE$\nu$NS) of reactor antineutrinos on germanium nuclei in the fully coherent regime, continuing the CONUS physics program conducted at the Brokdorf nuclear power plant (KBR), Germany. The CONUS+ experiment is installed in the Leibstadt nuclear power plant (KKL), Switzerland, at a distance of 20.7 m from the 3.6 GW reactor core, where the antineutrino flux is $1.5\cdot 10^{13}$~s$^{-1}$cm$^{-2}$. The CE$\nu$NS signature will be measured with four point-contact high-purity low energy threshold germanium (HPGe) detectors. A good understanding of the background is crucial, especially events correlated with the reactor thermal power are troublesome. A large background characterization campaign was conducted during reactor on and off times to find the best location for the CONUS+ setup. On-site measurements revealed a correlated, highly thermalized neutron field with a maximum fluence rate of $(2.3\pm0.1)\cdot 10^{4}$~neutrons~d$^{-1}$cm$^{-2}$ during reactor operation. The $\gamma$-ray background was studied with a HPGe detector without shield. The muon flux was examined using a liquid scintillator detector measuring (107$\pm$3)~muons~s$^{-1}$m$^{-2}$, which corresponds to an average overburden of 7.4~m of water equivalent. The new background conditions in CONUS+ are compared to the previous CONUS ones, showing a 30 times higher flux of neutrons, but a 26 times lower component of reactor thermal power correlated $\gamma$-rays over 2.7 MeV. The lower CONUS+ overburden increases the number of muon-induced neutrons by 2.3 times and the flux of cosmogenic neutrons. Finally, all the measured rates are discussed in the context of the CONUS+ background, together with the CONUS+ modifications performed to reduce the impact of the new background conditions at KKL.

physics.ins-det↗

arXiv2024

The CONUS+ experiment

The CONUS+ experiment aims to detect coherent elastic neutrino-nucleus scattering (CEvNS) of reactor antineutrinos on germanium nuclei in the fully coherent regime, continuing on this way the CONUS physics program started at the Brokdorf nuclear power plant, Germany. The CONUS+ setup is installed in the nuclear power plant in Leibstadt, Switzerland, at a distance of 20.7 m from the 3.6 GW thermal power reactor core. The CEvNS signature will be measured with the same four point-contact high-purity germanium (HPGe) detectors produced for the former experiment, however refurbished and with optimized low energy thresholds. To suppress the background in the CONUS+ detectors, the passive and active layers of the original CONUS shield were modified such to fit better to the significantly changed background conditions at the new experimental location. New data acquisition and monitoring systems were developed. A direct network connection between the experiment and the Max-Planck-Institut fur Kernphysik (MPIK) makes it possible to control and monitor data acquisition in real time. The impact of all these modifications is discussed with particular emphasis on the resulting CEvNS signal prediction for the first data collection phase of CONUS+. Prospects of the planned upgrade in a second phase integrating new larger HPGe detectors are also discussed.

hep-ex↗

arXiv2024

Imaging of single barium atoms in a second matrix site in solid xenon for barium tagging in a $^{136}$Xe double beta decay experiment

Neutrinoless double beta decay is one of the most sensitive probes for new physics beyond the Standard Model of particle physics. One of the isotopes under investigation is $^{136}$Xe, which would double beta decay into $^{136}$Ba. Detecting the single $^{136}$Ba daughter provides a sort of ultimate tool in the discrimination against backgrounds. Previous work demonstrated the ability to perform single atom imaging of Ba atoms in a single-vacancy site of a solid xenon matrix. In this paper, the effort to identify signal from individual barium atoms is extended to Ba atoms in a hexa-vacancy site in the matrix and is achieved despite increased photobleaching in this site. Abrupt fluorescence turn-off of a single Ba atom is also observed. Significant recovery of fluorescence signal lost through photobleaching is demonstrated upon annealing of Ba deposits in the Xe ice. Following annealing, it is observed that Ba atoms in the hexa-vacancy site exhibit antibleaching while Ba atoms in the tetra-vacancy site exhibit bleaching. This may be evidence for a matrix site transfer upon laser excitation. Our findings offer a path of continued research toward tagging of Ba daughters in all significant sites in solid xenon.

physics.atom-ph↗

arXiv2024

Supernova electron-neutrino interactions with xenon in the nEXO detector

Electron-neutrino charged-current interactions with xenon nuclei were modeled in the nEXO neutrinoless double-$\beta$ decay detector (~5 metric ton, 90% ${}^{136}$Xe, 10% ${}^{134}$Xe) to evaluate its sensitivity to supernova neutrinos. Predictions for event rates and detectable signatures were modeled using the Model of Argon Reaction Low Energy Yields (MARLEY) event generator. We find good agreement between MARLEY's predictions and existing theoretical calculations of the inclusive cross sections at supernova neutrino energies. The interactions modeled by MARLEY were simulated within the nEXO simulation framework and were run through an example reconstruction algorithm to determine the detector's efficiency for reconstructing these events. The simulated data, incorporating the detector response, were used to study the ability of nEXO to reconstruct the incident electron-neutrino spectrum and these results were extended to a larger xenon detector of the same isotope enrichment. We estimate that nEXO will be able to observe electron-neutrino interactions with xenon from supernovae as far as 5-8 kpc from Earth, while the ability to reconstruct incident electron-neutrino spectrum parameters from observed interactions in nEXO is limited to closer supernovae.

hep-ph↗

arXiv2023

An integrated online radioassay data storage and analytics tool for nEXO

Large-scale low-background detectors are increasingly used in rare-event searches as experimental collaborations push for enhanced sensitivity. However, building such detectors, in practice, creates an abundance of radioassay data especially during the conceptual phase of an experiment when hundreds of materials are screened for radiopurity. A tool is needed to manage and make use of the radioassay screening data to quantitatively assess detector design options. We have developed a Materials Database Application for the nEXO experiment to serve this purpose. This paper describes this database, explains how it functions, and discusses how it streamlines the design of the experiment.

physics.ins-det↗

arXiv2022

Snowmass 2021 Underground Facilities for the Cosmic Frontier Topical Report

Detecting dark matter directly using sensitive underground detectors is one of the most promising ways to address the question of what is dark matter. The next generation underground experiments will search for dark matter in two directions. One is to use larger target mass detectors to further probe smaller cross sections in the WIMP parameter space and the other is to probe lower mass dark matter which requires new technologies with extremely low threshold. Once dark matter is detected, experiments with directional sensitivities and experiments with longer exposure and larger target masses are needed for the time-dependent signatures, such as annual modulations, to verify their cosmic origin and to further study their properties. This report covers the underground facilities need for the next generation dark matter search experiments with noble liquids, cryogenic bolometers and other technologies.

hep-ex↗

arXiv2022

A Review of NEST Models for Liquid Xenon and Exhaustive Comparison to Other Approaches

This paper will discuss the microphysical simulation of interactions in liquid xenon, the active detector medium in many leading rare-event searches for new physics, and describe experimental observables useful for understanding detector performance. The scintillation and ionization yield distributions for signal and background will be presented using the Noble Element Simulation Technique (NEST), which is a toolkit based on experimental data and simple, empirical formulae, which mimic previous microphysics modeling, but are guided by data. The NEST models for light and charge production as a function of the particle type, energy, and electric field will be reviewed, as well as models for energy resolution and final pulse areas. NEST will be compared to other models or sets of models, and vetted against real data, with several specific examples pulled from XENON, ZEPLIN, LUX, LZ, PandaX, and table-top experiments used for calibrations.

hep-ex↗

arXiv2022

An approximate likelihood for nuclear recoil searches with XENON1T data

The XENON collaboration has published stringent limits on specific dark matter -nucleon recoil spectra from dark matter recoiling on the liquid xenon detector target. In this paper, we present an approximate likelihood for the XENON1T 1 tonne-year nuclear recoil search applicable to any nuclear recoil spectrum. Alongside this paper, we publish data and code to compute upper limits using the method we present. The approximate likelihood is constructed in bins of reconstructed energy, profiled along the signal expectation in each bin. This approach can be used to compute an approximate likelihood and therefore most statistical results for any nuclear recoil spectrum. Computing approximate results with this method is approximately three orders of magnitude faster than the likelihood used in the original publications of XENON1T, where limits were set for specific families of recoil spectra. Using this same method, we include toy Monte Carlo simulation-derived binwise likelihoods for the upcoming XENONnT experiment that can similarly be used to assess the sensitivity to arbitrary nuclear recoil signatures in its eventual 20 tonne-year exposure.

hep-ex↗