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

There is an increasing necessity for low background active materials as ton-scale, rare-event and cryogenic detectors are developed. Poly(ethylene-2,6-naphthalate) (PEN) has been considered for these applications because of its robust structural characteristics, and its scintillation light in the blue wavelength region. Radioluminescent properties of PEN have been measured to aid in the evaluation of this material. In this article we present a measurement of PEN's quenching factor using three different neutron sources; neutrons emitted from spontaneous fission in Cf-252 , neutrons generated from a DD generator, and neutrons emitted from the 13 C(alpha,n) 16 O and the 7 Li(p,n) 7 Be nuclear reactions. The fission source used time-of-flight to determine the neutron energy, and the neutron energy from the nuclear reactions was defined using thin targets and reaction kinematics. The Birks' factor and scintillation efficiency were found to be kB = 0.12 +/- 0.01 mm MeV-1 -1 and S = 1.31 +/- 0.09 MeVeeMeV-1 ee MeV -1 from a simultaneous analysis of the data obtained from the three different sources. With these parameters, it is possible to evaluate PEN as a viable material for large-scale, low background physics experiments.

The cross section of the ^{13}C(α,n)^{16}O reaction is needed for nuclear astrophysics and applications to a precision of 10% or better, yet inconsistencies among 50 years of experimental studies currently lead to an uncertainty of ≈15%. Using a state-of-the-art neutron detection array, we have performed a high resolution differential cross section study covering a broad energy range. These measurements result in a dramatic improvement in the extrapolation of the cross section to stellar energies potentially reducing the uncertainty to ≈5% and resolving long standing discrepancies in higher energy data.

We present a novel cryogenic VUV spectrofluorometer designed to characterize wavelength shifters (WLS) crucial for experiments based on liquid argon (LAr) scintillation light detection. Wavelength shifters like 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) or polyethylene naphthalate (PEN) are used in these experiments to shift the VUV scintillation light to the visible region. Precise knowledge of the optical properties of the WLS at liquid argon's temperature (87 K) and LAr scintillation wavelength (128 nm) is necessary to model and understand the detector response. The cryogenic VUV spectrofluorometer was commissioned to measure the emission spectra and relative wavelength shifting efficiency (WLSE) of samples between 300 K to 87 K for VUV (120 nm to 190 nm) and UV (310 nm) excitation. New mitigation techniques for surface effects on cold WLS were established. As part of this work, the TPB-based wavelength shifting reflector (WLSR) featured in the neutrinoless double-beta decay experiment LEGEND-200 was characterized. The WLSE was observed to increase by (54 ± 5) % from room temperature (RT) to 87 K. PEN installed in LEGEND-200 was also characterized, and a first measurement of the relative WLSE and emission spectrum at RT and 87 K is presented. The WLSE of amorphous PEN was found to be enhanced by at least (37 ± 4) % for excitation with 128 nm and by (52 ± 3) % for UV excitation at 87 K compared to RT.

The cross section of the 13C(alpha, n)16O reaction is needed for nuclear astrophysics and applications to a precision of 10% or better, yet inconsistencies among 50 years of experimental studies currently lead to an uncertainty of approximate to 15%. Using a state-of-the-art neutron detection array, we have performed a high resolution differential cross section study covering a broad energy range. These measurements result in a dramatic improvement in the extrapolation of the cross section to stellar energies potentially reducing the uncertainty to approximate to 5% and resolving long standing discrepancies in higher energy data.

Polyethylene Naphthalate (PEN) plastic scintillator has been identified as potential self-vetoing structural material in low-background physics experiments. Radio-pure scintillating components have been produced from PEN using injection compression molding technology. These low-background PEN components will be used as optically active holders to mount the Germanium detectors in the Legend-200 neutrinoless double beta decay experiment. In this paper, we present the measurement of the optical properties of these PEN components. The scintillation light emission spectrum, time constant, attenuation and bulk absorption length as well as light output and light yield are reported. In addition, the surface of these PEN components has been characterized and an estimation of the surface roughness is presented. The light output of the final Legend-200 detector holders has been measured and is reported. These measurements were used to estimate the self-vetoing efficiency of these holders.

Poly Ethylene Naphthalate (PEN) is an industrial polymer plastic which is investigated as a low background, transparent, scintillating and wavelength shifting structural material. PEN scintillates in the blue region and has excellent mechanical properties both at room and cryogenic temperatures. Thus, it is an ideal candidate for active structural components in experiments for the search of rare events like neutrinoless double-beta decay or dark matter recoils. Such optically active structures improve the identification and rejection efficiency of backgrounds events, like this improving the sensitivity of experiments. This paper reports on the production of radiopure and transparent PEN plates These structures can be used to mount germanium detectors operating in cryogenic liquids (LAr, LN). Thus, as first application PEN holders will be used to mount the Ge detectors in the LEGEND-200 experiment. The whole process from cleaning the raw material to testing the PEN active components under final operational conditions is reported.

PEN is an industrial polyester plastic which has become interesting for the physics community as a new type of plastic scintillator. PEN scintillates in the blue regime, which is ideal for most photosensor devices. In addition, PEN has excellent mechanical properties and very good radiopurity has been achieved. Thus, it is an ideal candidate for active structural components in low-background experiments. One possible application are holders for germanium detectors operating in cryogenic liquids (LAr, LN2). Such structures can help to reject surface and external backgrounds, boosting the sensitivity of experiments. In this contribution, the R\&D on PEN is outlined and an evaluation of the first production of PEN structures for the LEGEND-200 experiment is reported.

Liquid argon is commonly used as a detector medium for neutrino physics and dark matter experiments in part due to its copious scintillation light production in response to its excitation and ionization by charged particle interactions. As argon scintillation appears in the vacuum ultraviolet (VUV) regime and is difficult to detect, wavelength-shifting materials are typically used to convert VUV light to visible wavelengths more easily detectable by conventional means. In this work, we examine the wavelength-shifting and optical properties of poly(ethylene naphthalate) (PEN), a recently proposed alternative to tetraphenyl butadiene (TPB), the most widely-used wavelength-shifter in argon-based experiments. In a custom cryostat system with well-demonstrated geometric and response stability, we use 128~nm argon scintillation light to examine various PEN-including reflective samples' light-producing capabilities, and study the stability of PEN when immersed in liquid argon. The best-performing PEN-including test reflector was found to produce 34% as much visible light as a TPB-including reference sample, with widely varying levels of light production between different PEN-including test reflectors. Plausible origins for these variations, including differences in optical properties and molecular orientation, are then identified using additional measurements. Unlike TPB-coated samples, PEN-coated samples did not produce long-timescale light collection increases associated with solvation or suspension of wavelength-shifting material in bulk liquid argon.

We present a detailed report on sterile neutrino oscillation and U-235 (nu) over bar (e) 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 (nu) over bar (e) produced in beta decays of U-235 fission products. New limits on the oscillation of (nu) over bar (e) 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 (nu) over bar (e) to sterile neutrino oscillation and disfavor the reactor antineutrino anomaly best-fit point at the 2.5 sigma confidence level. The reported U-235 (nu) over bar (e) energy spectrum measurement shows excellent agreement with energy spectrum models generated via conversion of the measured U-235 beta spectrum, with a chi(2)/d.o.f . of 31/31. PROSPECT is able to disfavor at 2.4 sigma confidence level the hypothesis that U-235 (nu) over bar (e) 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.

M. Andriamirado,6 A. B. Balantekin,16 H. R. Band,17 C. D. Bass,8 D. E. Bergeron,10 D. Berish,13 N. S. Bowden,7 J. P. Brodsky,7 C. D. Bryan,11 R. Carr,9 T. Classen,7 A. J. Conant,4 G. Deichert,11 M. V. Diwan,2 M. J. Dolinski,3 A. Erickson,4 B. T. Foust,17 J. K. Gaison,17 A. Galindo-Uribarri,12, 14 C. E. Gilbert,12, 14 C. Grant,1 B. T. Hackett,12, 14 S. Hans,2 A. B. Hansell,13 K. M. Heeger,17 D. E. Jaffe,2 X. Ji,2 D. C. Jones,13 O. Kyzylova,3 C. E. Lane,3 T. J. Langford,17 J. LaRosa,10 B. R. Littlejohn,6 X. Lu,12, 14 J. Maricic,5 M. P. Mendenhall,7 A. M. Meyer,5 R. Milincic,5 I. Mitchell,5 P. E. Mueller,12 H. P. Mumm,10 J. Napolitano,13 C. Nave,3 R. Neilson,3 J. A. Nikkel,17 D. Norcini,17 S. Nour,10 J. L. Palomino,6 D. A. Pushin,15 X. Qian,2 E. Romero-Romero,12, 14 R. Rosero,2 P. T. Surukuchi,17 M. A. Tyra,10 R. L. Varner,12 D. Venegas-Vargas,12, 14 P. B. Weatherly,3 C. White,6 J. Wilhelmi,17 A. Woolverton,15 M. Yeh,2 A. Zhang,2 C. Zhang,2 and X. Zhang7

Poly(ethylene naphthalate), PEN, is an industrial polyester which has been shown to scintillate in the blue wavelength region. Combined with measurements of a high intrinsic radiopurity, this has sparked interest in the material for use in low-background experiments.

We comment on the claimed observation [arXiv:arXiv:2005.05301] of sterile neutrino oscillations by the Neutrino-4 collaboration. Such a claim, which requires the existence of a new fundamental particle, demands a level of rigor commensurate with its impact. The burden lies with the Neutrino-4 collaboration to provide the information necessary to prove the validity of their claim to the community. In this note, we describe aspects of both the data and analysis method that might lead to an oscillation signature arising from a null experiment and describe additional information needed from the Neutrino-4 collaboration to support the oscillation claim. Additionally, as opposed to the assertion made by the Neutrino-4 collaboration, we also show that the method of 'coherent summation' using the $L/E$ parameter produces similar results to the methods used by the PROSPECT and the STEREO collaborations.

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.

The DUNE IDR describes the proposed physics program and technical designs of the DUNE Far Detector modules in preparation for the full TDR to be published in 2019. It is intended as an intermediate milestone on the path to a full TDR, justifying the technical choices that flow down from the high-level physics goals through requirements at all levels of the Project. These design choices will enable the DUNE experiment to make the ground-breaking discoveries that will help to answer fundamental physics questions. Volume 1 contains an executive summary that describes the general aims of this document. The remainder of this first volume provides a more detailed description of the DUNE physics program that drives the choice of detector technologies. It also includes concise outlines of two overarching systems that have not yet evolved to consortium structures: computing and calibration. Volumes 2 and 3 of this IDR describe, for the single-phase and dual-phase technologies, respectively, each detector module's subsystems, the technical coordination required for its design, construction, installation, and integration, and its organizational structure.

This Letter reports the first measurement of the ^{235}U ν[over ¯]_{e} energy spectrum by PROSPECT, the Precision Reactor Oscillation and Spectrum experiment, operating 7.9 m from the 85  MW_{th} highly enriched uranium (HEU) High Flux Isotope Reactor. With a surface-based, segmented detector, PROSPECT has observed 31678±304(stat) ν[over ¯]_{e}-induced inverse beta decays, the largest sample from HEU fission to date, 99% of which are attributed to ^{235}U. Despite broad agreement, comparison of the Huber ^{235}U model to the measured spectrum produces a χ^{2}/ndf=51.4/31, driven primarily by deviations in two localized energy regions. The measured ^{235}U spectrum shape is consistent with a deviation relative to prediction equal in size to that observed at low-enriched uranium power reactors in the ν[over ¯]_{e} energy region of 5-7 MeV.

PROSPECT, the Precision Reactor Oscillation and SPECTrum experiment, is a short-baseline reactor antineutrino experiment designed to provide precision measurements of the U-235 product (nu) over bar (e) spectrum, utilizing an optically segmented 4-ton liquid scintillator detector. PROSPECT 's segmentation system, the optical grid, plays a central role in reconstructing the position and energy of (nu) over bar (e) interactions in the detector. This paper is the technical reference for this PROSPECT subsystem, describing its design, fabrication, quality assurance, transportation and assembly in detail. In addition, the dimensional, optical and mechanical characterizations of optical grid components and the assembled PROSPECT target are also presented. The technical information and characterizations detailed here will inform geometry-related inputs for PROSPECT physics analysis, and can guide a variety of future particle detection development efforts, such as those using optically reflecting materials or filament-based 3D printing.

This work reports the production and characterization of lithium-loaded liquid scintillator (LiLS) for the Precision Reactor Oscillation and Spectrum Experiment (PROSPECT) . Fifty-nine 90 liter batches of LiLS (6Li mass fraction 0.082%±0.001%) were produced and samples from all batches were characterized by measuring their optical absorbance relative to air, light yield relative to a pure liquid scintillator reference, and pulse shape discrimination capability. Fifty-seven batches passed the quality assurance criteria and were used for the PROSPECT experiment.

The Precision Reactor Oscillation and Spectrum (PROSPECT) Experiment is a reactor neutrino experiment designed to search for sterile neutrinos with a mass on the order of 1 eV/c2 and to measure the spectrum of electron antineutrinos from a highly-enriched 235U nuclear reactor. The PROSPECT detector consists of an 11 by 14 array of optical segments in 6Li-loaded liquid scintillator at the High Flux Isotope Reactor in Oak Ridge National Laboratory. Antineutrino events are identified via inverse beta decay and read out by photomultiplier tubes located at the ends of each segment. The detector response is characterized using a radioactive source calibration system. This paper describes the design, operation, and performance of the PROSPECT source calibration system.