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Nuclear physics has long played a central role in our efforts to better understand the natural world. Several experiments are currently well positioned to improve limits in searches for physics Beyond the Standard Model (BSM). Many experiments in nuclear physics have traditionally used semiconductor or scintillation detectors for particle detection, yet these face fundamental performance limitations that greatly restrict the sensitivity achievable. A new detector paradigm for charged particle detection could potentially open up orders of magnitude improvement in sensitivity in searches for BSM physics. We are working to achieve this by adapting Thermal Kinetic Inductance Detectors (TKIDs) for external charged particle detection. These cryogenic detectors are used in X-ray and gamma spectroscopy as well as dark matter searches and have been shown to have photon energy resolutions on the order of tens of eV. They can be multiplexed to create large area detectors. Thus far, however, TKIDs have not yet been developed for external (non-embedded) charged particle detection. Creating a TKID with a sensitivity of 100s of eV or better suitable for external charged particle detection would significantly impact the next generation of nuclear experiments, allowing orders of magnitude improvements in sensitivity.

This Letter reports one of the most precise measurements to date of the antineutrino spectrum from a purely U235-fueled reactor, made with the final dataset from the PROSPECT-I detector at the High Flux Isotope Reactor. By extracting information from previously unused detector segments, this analysis effectively doubles the statistics of the previous PROSPECT measurement. The reconstructed energy spectrum is unfolded into antineutrino energy and compared with both the Huber-Mueller model and a spectrum from a commercial reactor burning multiple fuel isotopes. A local excess over the model is observed in the 5MeV to 7MeV energy region. Comparison of the PROSPECT results with those from commercial reactors provides new constraints on the origin of this excess, disfavoring at 2.2 and 3.2 standard deviations the hypotheses that antineutrinos from U235 are solely responsible and non-contributors to the excess observed at commercial reactors respectively.

This paper presents an energy calibration scheme for an upgraded reactor antineutrino detector for the Precision Reactor Oscillation and Spectrum Experiment (PROSPECT). The PROSPECT collaboration is preparing an upgraded detector, PROSPECT-II (P-II), to advance capabilities for the investigation of fundamental neutrino physics, fission processes and associated reactor neutrino flux, and nuclear security applications. P-II will expand the statistical power of the original PROSPECT (P-I) dataset by at least an order of magnitude. The new design builds upon previous P-I design and focuses on improving the detector robustness and long-term stability to enable multi-year operation at one or more sites. The new design optimizes the fiducial volume by elimination of dead space previously occupied by internal calibration channels, which in turn necessitates the external deployment. In this paper, we describe a calibration strategy for P-II. The expected performance of externally deployed calibration sources is evaluated using P-I data and a well-benchmarked simulation package by varying detector segmentation configurations in the analysis. The proposed external calibration scheme delivers a compatible energy scale model and achieves comparable performance with the inclusion of an additional AmBe neutron source, in comparison to the previous internal arrangement. Most importantly, the estimated uncertainty contribution from the external energy scale calibration model meets the precision requirements of the P-II experiment.

The Precision Reactor Oscillation and Spectrum Experiment, PROSPECT, has made world-leading measurements of reactor antineutrinos at short baselines. In its first phase, conducted at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory, PROSPECT produced some of the strongest limits on eV-scale sterile neutrinos, made a precision measurement of the reactor antineutrino spectrum from \\(^{235}\\)U, and demonstrated the observation of reactor antineutrinos in an aboveground detector with good energy resolution and well-controlled backgrounds. The PROSPECT collaboration is now preparing an upgraded detector, PROSPECT-II, to probe yet unexplored parameter space for sterile neutrinos and contribute to a full resolution of the Reactor Antineutrino Anomaly, a longstanding puzzle in neutrino physics. By pressing forward on the world's most precise measurement of the \\(^{235}\\)U antineutrino spectrum and measuring the absolute flux of antineutrinos from \\(^{235}\\)U, PROSPECT-II will sharpen a tool with potential value for basic neutrino science, nuclear data validation, and nuclear security applications. Following a two-year deployment at HFIR, an additional PROSPECT-II deployment at a low enriched uranium reactor could make complementary measurements of the neutrino yield from other fission isotopes. PROSPECT-II provides a unique opportunity to continue the study of reactor antineutrinos at short baselines, taking advantage of demonstrated elements of the original PROSPECT design and close access to a highly enriched uranium reactor core.

If dark matter has mass lower than around 1 GeV, it will not impart enough energy to cause detectable nuclear recoils in many direct-detection experiments. However, if dark matter is upscattered to high energy by collisions with cosmic rays, it may be detectable in both direct-detection experiments and neutrino experiments. We report the results of a dedicated search for boosted dark matter upscattered by cosmic rays using the PROSPECT reactor antineutrino experiment. We show that such a flux of upscattered dark matter would display characteristic diurnal sidereal modulation, and use this to set new experimental constraints on sub-GeV dark matter exhibiting large interaction cross-sections.

The neutron spin rotation (NSR) collaboration used parity-violating spin rotation of transversely polarized neutrons transmitted through a 0.5 m liquid helium target to constrain weak coupling constants between nucleons. While consistent with theoretical expectation, the upper limit set by this measurement on the rotation angle is limited by statistical uncertainties. The NSR collaboration is preparing a new measurement to improve this statistically-limited result by about an order of magnitude. In addition to using the new high-flux NG-C beam at the NIST Center for Neutron Research, the apparatus was upgraded to take advantage of the larger-area and more divergent NG-C beam. Significant improvements are also being made to the cryogenic design. Details of these improvements and readiness of the upgraded apparatus are presented. We also comment on how recent theoretical work combining effective field theory techniques with the 1/ N c expansion of QCD along with previous NN weak measurements can be used to make a prediction for d ϕ/ dz in 4 He. An experiment using the same apparatus with a room-temperature target was carried out at LANSCE to place limits on parity-conserving rotations from possible fifth-force interactions to complement previous studies. We sought this interaction using a slow neutron polarimeter that passed transversely polarized slow neutrons by unpolarized slabs of material arranged so that this interaction would tilt the plane of polarization and develop a component along the neutron momentum. The results of this measurement and its impact on the neutron-matter coupling g A 2 from such an interaction are presented. The NSR collaboration is also preparing a new measurement that uses an upgraded version of the room-temperature target to be run on the NG-C beamline; and it is expected to constrain g A 2 by at least two additional orders of magnitude for λ c between 1 cm and 1 μm.

We present a detailed report on sterile neutrino oscillation and U-235 antineutrino energy spectrum measurement results from the PROSPECT experiment at the highly enriched High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. In 96 calendar days of data taken at an average baseline distance of 7.9 m from the center of the 85 MW HFIR core, the PROSPECT detector has observed more than 50,000 interactions of antineutrinos produced in beta decays of U-235 fission products. New limits on the oscillation of antineutrinos to light sterile neutrinos have been set by comparing the detected energy spectra of ten reactor-detector baselines between 6.7 and 9.2 meters. Measured differences in energy spectra between baselines show no statistically significant indication of antineutrinos to sterile neutrino oscillation and disfavor the Reactor Antineutrino Anomaly best-fit point at the 2.5\\(\\sigma\\) confidence level. The reported U-235 antineutrino energy spectrum measurement shows excellent agreement with energy spectrum models generated via conversion of the measured U-235 beta spectrum, with a \\(\\chi^2\\)/DOF of 31/31. PROSPECT is able to disfavor at 2.4\\(\\sigma\\) confidence level the hypothesis that U-235 antineutrinos are solely responsible for spectrum discrepancies between model and data obtained at commercial reactor cores. A data-model deviation in PROSPECT similar to that observed by commercial core experiments is preferred with respect to no observed deviation, at a 2.2\\(\\sigma\\) confidence level.

The violation of Baryon Number, \\(B\\), is an essential ingredient for the preferential creation of matter over antimatter needed to account for the observed baryon asymmetry in the universe. However, such a process has yet to be experimentally observed. The HIBEAM/NNBAR %experiment program is a proposed two-stage experiment at the European Spallation Source (ESS) to search for baryon number violation. The program will include high-sensitivity searches for processes that violate baryon number by one or two units: free neutron-antineutron oscillation (\\(n n\\)) via mixing, neutron-antineutron oscillation via regeneration from a sterile neutron state (\\(n [n',n'] n\\)), and neutron disappearance (\\(n n'\\)); the effective \\( B=0\\) process of neutron regeneration (\\(n [n',n'] n\\)) is also possible. The program can be used to discover and characterise mixing in the neutron, antineutron, and sterile neutron sectors. The experiment addresses topical open questions such as the origins of baryogenesis, the nature of dark matter, and is sensitive to scales of new physics substantially in excess of those available at colliders. A goal of the program is to open a discovery window to neutron conversion probabilities (sensitivities) by up to three orders of magnitude compared with previous searches. The opportunity to make such a leap in sensitivity tests should not be squandered. The experiment pulls together a diverse international team of physicists from the particle (collider and low energy) and nuclear physics communities, while also including specialists in neutronics and magnetics.

Reactor neutrino experiments have seen major improvements in precision in recent years. With the experimental uncertainties becoming lower than those from theory, carefully considering all sources of \\(\\overline{\\nu}_{e}\\) is important when making theoretical predictions. One source of \\(\\overline{\\nu}_{e}\\) that is often neglected arises from the irradiation of the nonfuel materials in reactors. The \\(\\overline{\\nu}_{e}\\) rates and energies from these sources vary widely based on the reactor type, configuration, and sampling stage during the reactor cycle and have to be carefully considered for each experiment independently. In this article, we present a formalism for selecting the possible \\(\\overline{\\nu}_{e}\\) sources arising from the neutron captures on reactor and target materials. We apply this formalism to the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory, the \\(\\overline{\\nu}_{e}\\) source for the the Precision Reactor Oscillation and Spectrum Measurement (PROSPECT) experiment. Overall, we observe that the nonfuel \\(\\overline{\\nu}_{e}\\) contributions from HFIR to PROSPECT amount to 1\\% above the inverse beta decay threshold with a maximum contribution of 9\\% in the 1.8--2.0~MeV range. Nonfuel contributions can be particularly high for research reactors like HFIR because of the choice of structural and reflector material in addition to the intentional irradiation of target material for isotope production. We show that typical commercial pressurized water reactors fueled with low-enriched uranium will have significantly smaller nonfuel \\(\\overline{\\nu}_{e}\\) contribution.

Neutron spin rotation is expected from quark-quark weak interactions in the Standard Model, which induce weak interactions among nucleons that violate parity. We present the results from an experiment searching for the effect of parity violation via the spin rotation of polarized neutrons in a liquid \\(^{4}\\)He medium. The value for the neutron spin rotation angle per unit length in \\(^{4}\\)He, \\(d\\phi/dz =(+2.1 \\pm 8.3 (stat.) \\pm 2.9 (sys.))\\times10^{-7}\\) rad/m, is consistent with zero. The result agrees with the best current theoretical estimates of the size of nucleon-nucleon weak amplitudes from other experiments and with the expectations from recent theoretical approaches to weak nucleon-nucleon interactions. In this paper we review the theoretical status of parity violation in the \\(\\vec{n}+^{4}\\)He system and discuss details of the data analysis leading to the quoted result. Analysis tools are presented that quantify systematic uncertainties in this measurement and that are expected to be essential for future measurements.