Electron Recoil Research Articles

CYGν\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u $$\\end{document}S: detecting solar neutrinos with directional gas time projection chambers

Cygnus is a proposed global network of large-scale gas time projection chambers (TPCs) with the capability of directionally detecting nuclear and electron recoils at ≳\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gtrsim $$\\end{document}keV energies. The primary focus of Cygnus so far has been the detection of dark matter, with directional sensitivity providing a means of circumventing the so-called “neutrino fog”. However, the excellent background rejection and electron/nuclear recoil discrimination provided by the 3-dimensional reconstruction of ionisation tracks could turn the solar neutrino background into an interesting signal in its own right. For example, directionality would facilitate the simultaneous spectroscopy of multiple different flux sources. Here, we evaluate the possibility of measuring solar neutrinos using the same network of gas TPCs built from 10 m3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}-scale modules operating under conditions that enable simultaneous sensitivity to both dark matter and neutrinos. We focus in particular on electron recoils, which provide access to low-energy neutrino fluxes like pp, pep, 7\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^7$$\\end{document}Be, and CNO. An appreciable event rate is already detectable in experiments consisting of a single 10 m3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document} module, assuming standard fill gases such as CF4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_4$$\\end{document} mixed with helium at atmospheric pressure. With total volumes around 1000 m3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document} or higher, the TPC network could be complementary to dedicated neutrino observatories, whilst entering the dark-matter neutrino fog via the nuclear recoil channel. We evaluate the required directional performance and background conditions to observe, discriminate, and perform spectroscopy on neutrino events. We find that, under reasonable projections for planned technology that will enable 10–30-degree angular resolution and ∼10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 10$$\\end{document}% fractional energy resolution, Cygnus could be a competitive directional neutrino experiment.

Comprehensive constraints on fermionic dark matter-quark tensor interactions in direct detection experiments* *Supported in part by the Major Project of Basic and Applied Basic Research of Guangdong Province, China(2020B0301030008) and the National Natural Science Foundation of China(12035008, 12247151, 12305110, 12347121)

Effective field theory (EFT) provides a model-independent framework for interpreting the results of dark matter (DM) direct detection experiments. In this study, we demonstrate that the two fermionic DM-quark tensor operators and can contribute to the DM electric and magnetic dipole moments via nonperturbative QCD effects, in addition to the well-studied contact DM-nucleon operators. We then investigate the constraints on these two operators by considering both the contact and dipole contributions using the XENON1T nuclear recoil and Migdal effect data. We also recast other existing bounds on the DM dipole operators, derived from electron and nuclear recoil measurements in various direct detection experiments, as constraints on the two tensor operators. For , our results significantly extend the reach of constraints on the DM-quark tensor operators to masses as low as , with the bound exceeding that obtained by the Migdal effect with only contact interactions by approximately an order of magnitude. In particular, for the operator with DM mass , the latest PandaX constraint on the DM electric dipole moment puts more stringent bounds than the previous direct detection limit. We also briefly discuss the constraints obtained from experiments other than direct detection.

Search for Cosmic-Ray Boosted Sub-MeV Dark-Matter–Electron Scattering in PandaX-4T

We report the first search for the elastic scatterings between cosmic-ray boosted sub-MeV dark matter (DM) and electrons in the PandaX-4T liquid xenon experiment. Sub-MeV DM particles can be accelerated by scattering with electrons in the cosmic rays and produce detectable electron recoil signals in the detector. Using the commissioning data from PandaX-4T of 0.63 tonne·year exposure, we set new constraints on DM-electron scattering cross sections for DM masses ranging from 10 eV/c2 to 3 keV/c2. Published by the American Physical Society 2024

Prospects for measuring the time variation of astrophysical neutrino sources at dark matter detectors

We study the prospects for measuring the time variation of solar and atmospheric neutrino fluxes at future large-scale xenon and argon dark matter detectors. For solar neutrinos, a yearly time variation arises from the eccentricity of Earth’s orbit and, for charged current interactions, from a smaller energy-dependent day-night variation due to flavor regeneration as neutrinos travel through Earth. For a 100-ton xenon detector running for ten years with a xenon-136 fraction of ≲0.1%, in the electron recoil channel a time-variation amplitude of about 0.8% is detectable with a power of 90% and the level of significance of 10%. This is sufficient to detect time variation due to eccentricity, which has amplitude of ∼3%. In the nuclear recoil channel, the detectable amplitude is about 10% under current detector resolution and efficiency conditions, and this generally reduces to about 1% for improved detector resolution and efficiency, the latter of which is sufficient to detect time variation due to eccentricity. Our analysis assumes both known and unknown periods. We provide scalings to determine the sensitivity to an arbitrary time-varying amplitude as a function of detector parameters. Identifying the time variation of the neutrino fluxes will be important for distinguishing neutrinos from dark matter signals and other detector-related backgrounds and extracting properties of neutrinos that can be uniquely studied in dark matter experiments. Published by the American Physical Society 2024

A systematic investigation on dark matter-electron scattering in effective field theories

In this paper, we systematically investigate the general dark matter-electron interactions within the framework of effective field theories (EFTs). We consider both the non-relativistic (NR) EFT and the relativistic EFT descriptions of the interactions with the spin of dark matter (DM) up to one, i.e., the scalar (ϕ), fermion (χ), and vector (X) DM scenarios. We first collect the leading-order NR EFT operators describing the DM-electron interactions, and construct especially the NR operators for the vector DM case. Next, we consider all possible leading-order relativistic EFT operators including those with a photon field and perform the NR reduction to match them onto the NR EFT. Then we rederive the DM-bound-electron scattering rate within the NR EFT framework and find that the matrix element squared, which is the key input that encodes the DM and atomic information, can be compactly decomposed into three terms. Each term is a product of a DM response function (a0,1,2), which is essentially a factor of Wilson coefficients squared, and its corresponding generalized atomic response function W~0,1,2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\left({\\overset{\\sim }{W}}_{0,1,2}\\right) $$\\end{document}. Lastly, we employ the electron recoil data from the DM direct detection experiments (including XENON10, XENON1T, and PandaX-4T) to constrain all the non-relativistic and relativistic operators in all three DM scenarios. We set strong bounds on the DM-electron interactions in the sub-GeV region. Particularly, we find that the latest PandaX-4T S2-only data provide stringent constraints on dark matter with a mass greater than approximately 20 MeV, surpassing those from the previous XENON10 and XENON1T experiments.

Measurement of the sodium and iodine scintillation quenching factors across multiple NaI(Tl) detectors to identify systematics

The amount of light produced by nuclear recoils in scintillating targets is strongly quenched compared to that produced by electrons. A precise understanding of the quenching factor is particularly interesting for weakly interacting massive particles (WIMP) searches and coherent elastic neutrino-nucleus scattering (CEνNS) measurements since both rely on nuclear recoils, whereas energy calibrations are more readily accessible from electron recoils. There is a wide variation among the current measurements of the quenching factor in sodium iodide (NaI) crystals, especially below 10 keV, the energy region of interest for dark matter and CEνNS studies. A better understanding of the quenching factor in NaI(Tl) is of particular interest for resolving the decades-old puzzle in the field of dark matter between the null results of most WIMP searches and the claim for dark matter detection by the DAMA/LIBRA collaboration. In this work, we measured sodium and iodine quenching factors for five small NaI(Tl) crystals grown with similar thallium concentrations and growth procedures. Unlike previous experiments, multiple crystals were tested, with measurements made in the same experimental setup to control systematic effects. The quenching factors agree in all crystals we investigated, and both sodium and iodine quenching factors are smaller than those reported by DAMA/LIBRA. The dominant systematic effect was due to the electron equivalent energy calibration originating from the nonproportional behavior of the NaI(Tl) light yield at lower energies, potentially the cause for the discrepancies among the previous measurements. Published by the American Physical Society 2024

Measurements of low-energy nuclear recoil quenching factors for Na and I recoils in the NaI(Tl) scintillator

Elastic scattering off nuclei in target detectors, involving interactions with dark matter and coherent elastic neutrino nuclear recoil (CEνNS), results in the deposition of low energy within the nuclei, dissipating rapidly through a combination of heat and ionization. The primary energy loss mechanism for nuclear recoil is heat, leading to consistently smaller measurable scintillation signals compared to electron recoils of the same energy. The nuclear recoil quenching factor (QF), representing the ratio of scintillation light yield produced by nuclear recoil to that of electron recoil at the same energy, is a critical parameter for understanding dark matter and neutrino interactions with nuclei. The low energy QF of NaI(Tl) crystals, commonly employed in dark matter searches and CEνNS measurements, is of substantial importance. Previous low energy QF measurements were constrained by contamination from photomultiplier tube (PMT)-induced noise, resulting in an observed light yield of approximately 15 photoelectrons per keVee (kilo-electron-volt electron-equivalent energy) and nuclear recoil energy above 5 keVnr (kilo-electron-volt nuclear recoil energy). Through enhanced crystal encapsulation, an increased light yield of around 26 photoelectrons per keVee is achieved. This improvement enables the measurement of the nuclear recoil QF for sodium nuclei at an energy of 3.8±0.6keVnr with a QF of 11.2±1.7%. Furthermore, a re-evaluation of previously reported QF results is conducted, incorporating enhancements in low energy events based on waveform simulation. The outcomes are generally consistent with various recent QF measurements for sodium and iodine. Published by the American Physical Society 2024

Constraints on cosmic-ray boosted dark matter with realistic cross section

Sub-MeV cold dark-matter particles are unable to produce electronic recoil in conventional dark-matter direct detection experiments such as XENONnT and LUX-ZEPLIN above the detector threshold. The mechanism of boosted dark matter comes into picture to constrain the parameter space of such low mass dark matter from direct detection experiments. We consider the effect of the leading components of cosmic rays to boost the cold dark matter, which results in significant improvements on the exclusion limits compared to the existing ones. To present concrete study results, we choose to work on models consisting of a dark-matter particle χ with an additional U(1)' gauge symmetry including the secluded dark photon, U(1)B-L, and U(1)L e-L μ . We find that the energy dependence of the scattering cross section plays a crucial role in improving the constraints. In addition, we systematically estimate the Earth shielding effect on boosted dark matter in losing energy while traveling to the underground detector through the Earth.

Systematic approach to measure the performance of microchannel-plate photomultipliers

In this paper, we present our approach to systematically measure numerous performance parameters of microchannel plate photomultiplier tubes. The experimental setups, the analyses and selected results are discussed. Although the techniques used may be different in other locations, the document is intended as a guide for comparable measurements with other types of microchannel plate photomultiplier tubes. Measurements are shown for the following performance parameters: spectral and spatial quantum efficiency, collection efficiency, gain as a function of voltage, position and magnetic field, time resolution, rate capability and lifetime. By using a dedicated 3-axis stepper and an FPGA-based data acquisition system, also inner PMT parameters are measured as a function of the active area, such as relative detection efficiency, dark count rate, time resolution, recoil electron and afterpulse distributions, as well as charge sharing and electronic crosstalk. In addition, some of the parameters are investigated inside a strong magnetic field. For many of these measurements, the change of most setup parameters and the subsequent analysis can be controlled semi-automatically by software scripts.

Measurement of solar pp neutrino flux using electron recoil data from PandaX-4T commissioning run* *This project is supported in part by the grants from the National Science Foundation of China (12090060, 12090063, 12105052, 12005131, 11905128, 11925502), the Office of Science and Technology, Shanghai Municipal Government (22JC1410100), and the National Science Foundation of Sichuan Province, China (2024NSFSC1371). We are grateful for

The proton-proton ( ) fusion chain dominates the neutrino production in the Sun. The uncertainty of the predicted neutrino flux is at the sub-percent level, whereas that of the best measurement is (10%). In this study, for the first time, we measure solar neutrinos in the electron recoil energy range from 24 to 144 keV using the PandaX-4T commissioning data with 0.63 tonne year exposure. The neutrino flux is determined as , which is consistent with the Standard Solar Model and existing measurements, corresponding to an upper flux limit of at 90% C.L..

Experimental Demonstration of a Tunable Energy-Selective Gamma-Ray Imaging System Based on Recoil Electrons

The domain of gamma-ray imaging necessitates technological advancements to surmount the challenge of energy-selective imaging. Conventional systems are constrained in their dynamic focus on specific energy ranges, a capability imperative for differentiating gamma-ray emissions from diverse sources. This investigation introduces an innovative imaging system predicated on the detection of recoil electrons, addressing the demand for adjustable energy selectivity. Our methodology encompasses the design of a gamma-ray imaging system that leverages recoil electron detection to execute energy-selective imaging. The system’s efficacy was investigated experimentally, with emphasis on the adaptability of the energy selection window. The experimental outcomes underscore the system’s adeptness at modulating the energy selection window, adeptly discriminating gamma rays across a stipulated energy spectrum. The results corroborate the system’s adaptability, with an adjustable energy resolution that coincides with theoretical projections and satisfies the established criteria. This study affirms the viability and merits of utilizing recoil electrons for tunable energy-selective gamma-ray imaging. The system’s conceptualization and empirical validation represent a notable progress in gamma-ray imaging technology, with prospective applications extending from medical imaging to astrophysics. This research sets a solid foundation for subsequent inquiries and advancements in this domain.

Phonon-mediated quantum efficiency measurement in semiconductors

Accurate quantum efficiency measurement not only provides crucial information for the photovoltaic cell industry but also supports experiments aimed at directly detecting dark matter and elastic neutrino interactions. The dark matter direct searches paradigm has recently expanded to include particles with masses below 1,MeV/c2, where the expected signal in an electron–recoil interaction is approximately in the eV range, just above the energy gap for silicon and germanium. A robust calibration method for ionization signals in this lower energy region is essential. This paper presents a method for measuring quantum efficiency and yield (q/E) in semiconductors using phonon-mediated calorimetry. The Neganov–Trofimov–Luke phonon amplification method in low-temperature semiconductor crystals has been employed to indirectly measure ionization down to single-electron accuracy. Specifically, at zero bias, the phonon readout directly quantifies the total energy deposited within the detector, independent of the ionization yield. This eliminates a significant source of systematic uncertainty in quantum efficiency estimates associated with total energy uncertainty. The paper includes results from an updated ionization efficiency measurement in a germanium detector.

Transition Edge Sensors with Sub-eV Resolution And Cryogenic Targets (TESSERACT) at the underground laboratory of Modane (LSM)

The future TESSERACT experiment will search for individual galactic DM particles below the proton mass through interactions with advanced, ultra-sensitive detectors. Currently TESSERACT is in a design phase aiming to produce fully defined detector technologies that will explore DM masses down to 10 MeV. It is designed to be sensitive to DM candidates interacting with the detector target material in producing both nuclear recoil DM (NRDM) and electron recoil (ERDM). To do so, multiple target materials will be used with varying detection strategies to ensure the capability to both actively reject the so-called low-energy excess and discriminate nuclear recoils against electron recoils. In addition to maximizing sensitivity to a variety of DM interactions, this provides an independent handle on instrumental backgrounds. Nowadays, the TESSERACT project encompasses two US-based technologies, namely HeRALD using superfluid helium as a target material, and SPICE using polar crystals (Al2O3 and SiO2) and scintillating crystals such as GaAs. In these proceedings, we discuss the recent proposal to host the future TESSERACT experiment at the Modane Underground Laboratory (LSM) and add a third French-based cryogenic semiconducting (Ge, Si) detector technology to the TESSERACT payload.

First Measurement of Discrimination between Helium and Electron Recoils in Liquid Xenon for Low-Mass Dark Matter Searches.

We report the first measurement of discrimination between low-energy helium recoils and electron recoils in liquid xenon. This result is relevant to proposed low-mass dark matter searches which seek to dissolve light target nuclei in the active volume of liquid-xenon time projection chambers. Low-energy helium recoils were produced by degrading α particles from ^{210}Po with a gold foil situated on the cathode of a liquid xenon time-projection chamber. The resulting population of helium recoil events is well separated from electron recoils and is also offset from the expected position of xenon nuclear recoil events.

Detection of astrophysical neutrinos at prospective locations of dark matter detectors

We study the prospects for detection of solar, atmospheric neutrino, and diffuse supernova neutrino background (DSNB) fluxes at future large-scale dark matter detectors through both electron and nuclear recoils. We specifically examine how the detection prospects change for several prospective detector locations [Sanford Underground Research Facility (SURF), SNOlab, Gran Sasso, China Jinping Underground Laboratory (CJPL), and Kamioka] and improve upon the statistical methodologies used in previous studies. Because of its ability to measure lower neutrino energies than other locations, we find that the best prospects for the atmospheric neutrino flux are at the SURF location, while the prospects are weakest at CJPL because it is restricted to higher neutrino energies. On the contrary, the prospects for the DSNB are best at CJPL, due largely to the reduced atmospheric neutrino background at this location. Including full detector resolution and efficiency models, the CNO component of the solar flux is detectable via the electron recoil channel with exposures of ∼103 ton-yr for all locations. These results highlight the benefits for employing two detector locations, one at high and one at low latitude. Published by the American Physical Society 2024

Evaluating the capability of detecting recoil-electron tracks using an electron-tracking Compton camera with a silicon-on-insulator pixel sensor

Sub-MeV gamma-ray line observations are crucial for the understanding of high-energy astrophysical processes. In this energy range, the presence of a significant amount of background poses a serious problem. Therefore, it is essential to reduce this background to achieve high-sensitivity observations. An electron-tracking Compton camera can efficiently mitigate the impact of background on scientific results thanks to its ability to fully resolve the Compton process. In addition if we utilize a semiconductor detector as the sensor, the electron-tracking Compton camera can achieve high-energy resolution for the detection of gamma-ray lines. To operate the electron-tracking Compton camera effectively, the sensor needs to coincidence events for each detection signal and resolve the complex structure of recoiled-electron tracks of a few hundred μm. We focused on the silicon-on-insulator pixel sensor XRPIX2b developed for X-ray observations, which satisfies requirements. We developed a prototype electron-tracking Compton camera using XRPIX2b and carried out a quantitative evaluation of its detection capability for estimating the recoil directions of electrons at various angles. As a result, we successfully detected short recoil-electron tracks with a length of 100μm corresponded to scattering angles of 30°-40° for 511 keV gamma rays entering the sensor. In term of angular resolution measure and scatter plane deviation of a reconstructed image, the evaluations showed results of 15° and 40° (full width at half maximum), respectively, in cases where 511 keV gamma rays were scattered at 90° and recoil electrons tracked the surface of the sensor detection plane. We confirmed that these results were consistent with simulation results, within approximately a 15% margin.

Sub-GeV dark matter search at ILC beam dumps

Light dark matter particles may be produced in electron and positron beam dumps of the International Linear Collider (ILC). We propose an experimental setup to search for such events, the Beam-Dump eXperiment at the ILC (ILC-BDX). The setup consists of a muon shield placed behind the beam dump, followed by a multi-layer tracker and an electromagnetic calorimeter. The calorimeter can detect electron recoils due to elastic scattering of dark matter particles produced in the dump, while the tracker is sensitive to decays of excited dark-sector states into the dark matter particle. We study the production, decay and scattering of sub-GeV dark matter particles in this setup in several models with a dark photon mediator. Taking into account beam-related backgrounds due to neutrinos produced in the beam dump as well as the cosmic-ray background, we evaluate the sensitivity reach of the ILC-BDX experiment. We find that the ILC-BDX will be able to probe interesting regions of the model parameter space and, in many cases, reach well below the relic target.

Search for Boosted Dark Matter in COSINE-100.

We search for energetic electron recoil signals induced by boosted dark matter (BDM) from the galactic center using the COSINE-100 array of NaI(Tl) crystal detectors at the Yangyang Underground Laboratory. The signal would be an excess of events with energies above 4MeV over the well-understood background. Because no excess of events are observed in a 97.7 kg·yr exposure, we set limits on BDM interactions under a variety of hypotheses. Notably, we explored the dark photon parameter space, leading to competitive limits compared to direct dark photon search experiments, particularly for dark photon masses below 4MeV and considering the invisible decay mode. Furthermore, by comparing our results with a previous BDM search conducted by the Super-Kamionkande experiment, we found that the COSINE-100 detector has advantages in searching for low-mass dark matter. This analysis demonstrates the potential of the COSINE-100 detector to search for MeV electron recoil signals produced by the dark sector particle interactions.

Diurnal modulation of electron recoils from DM-nucleon scattering through the Migdal effect

Halo dark matter (DM) particles could lose energy due to the scattering off nuclei within the Earth before reaching the underground detectors of DM direct detection experiments. This Earth shielding effect can result in diurnal modulation of the DM-induced recoil event rates observed underground due to the self-rotation of the Earth. For electron recoil signals from DM-electron scatterings, the current experimental constraints are very stringent such that the diurnal modulation cannot be observed for halo DM. We propose a novel type of diurnal modulation effect: diurnal modulation in electron recoil signals induced by DM-nucleon scattering via the Migdal effect. We set so far the most stringent constraints on DM-nucleon scattering cross section via the Migdal effect for sub-GeV DM using the S2-only data of PandaX-II and PandaX-4T with improved simulations of the Earth shielding effect. Based on the updated constraints, we show that the Migdal effect induced diurnal modulation of electron events can still be significant in the low energy region, and can be probed by experiments such as PandaX-4T in the near future.

Characterization of a 220Rn source for low-energy electronic recoil calibration of the XENONnT detector

Low-background liquid xenon detectors are utilized in the investigation of rare events, including dark matter and neutrinoless double beta decay. For their calibration, gaseous 220Rn can be used. After being introduced into the xenon, its progeny isotope 212Pb induces homogeneously distributed, low-energy (<30 keV) electronic recoil interactions. We report on the characterization of such a source for use in the XENONnT experiment. It consists of four commercially available 228Th sources with an activity of 55 kBq. These sources provide a high 220Rn emanation rate of about 8 kBq. We find no indication for the release of the long-lived 228Th above 1.7 mBq. Though an unexpected 222Rn emanation rate of about 3.6 mBq is observed, this source is still in line with the requirements for the XENONnT experiment.