High Flux Isotope Reactor

This Letter reports the first scientific results from the observation of antineutrinos emitted by fission products of ^{235}U at the High Flux Isotope Reactor. PROSPECT, the Precision Reactor Oscillation and Spectrum Experiment, consists of a segmented 4 ton ^{6}Li-doped liquid scintillator detector covering a baseline range of 7-9 m from the reactor and operating under less than 1 m water equivalent overburden. Data collected during 33 live days of reactor operation at a nominal power of 85 MW yield a detection of 25 461±283 (stat) inverse beta decays. Observation of reactor antineutrinos can be achieved in PROSPECT at 5σ statistical significance within 2 h of on-surface reactor-on data taking. A reactor model independent analysis of the inverse beta decay prompt energy spectrum as a function of baseline constrains significant portions of the previously allowed sterile neutrino oscillation parameter space at 95% confidence level and disfavors the best fit of the reactor antineutrino anomaly at 2.2σ confidence level.

The PROSPECT Collaboration presents their improved results with new limits on the oscillation of electron antineutrinos to light sterile neutrinos and energy spectrum measurements with several short baselines.

Surface structures and a surface structure phase transition are identified that are distinct from the known bulk ferroelectric-paraelectric phase transition of crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%). The temperature-dependent changes in the surface structure are accompanied by the physical rotation of the polar group ${(\mathrm{C}\mathrm{H}}_{2}{\ensuremath{-}\mathrm{C}\mathrm{F}}_{2}).$ These changes in the surface structure are compared to the bulk phase transition. We show that the bulk structural transition, while distinct from the surface, is qualitatively similar in both thick and thin Langmuir-Blodgett-grown films.

© 2016 IOP Publishing Ltd. The precision reactor oscillation and spectrum experiment, PROSPECT, is designed to make a precise measurement of the antineutrino spectrum from a highly-enriched uranium reactor and probe eV-scale sterile neutrinos by searching for neutrino oscillations over a distance of several meters. PROSPECT is conceived as a 2-phase experiment utilizing segmented 6Li-doped liquid scintillator detectors for both efficient detection of reactor antineutrinos through the inverse beta decay reaction and excellent background discrimination. PROSPECT Phase I consists of a movable 3 ton antineutrino detector at distances of 7-12 m from the reactor core. It will probe the best-fit point of the disappearance experiments at 4σ in 1 year and the favored region of the sterile neutrino parameter space at in 3 years. With a second antineutrino detector at 15-19 m from the reactor, Phase II of PROSPECT can probe the entire allowed parameter space below 10 eV2 at 5σ in 3 additional years. The measurement of the reactor antineutrino spectrum and the search for short-baseline oscillations with PROSPECT will test the origin of the spectral deviations observed in recent experiments, search for sterile neutrinos, and conclusively address the hypothesis of sterile neutrinos as an explanation of the reactor anomaly.

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 $\ensuremath{\sim}14.6$ solar days of data from 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.

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

We address the reactivity of lithiated graphite–anode material for Li-ion batteries with standard organic solvents used in batteries (ethylene carbonate and dimethyl carbonate) by following changes in neutron scattering signals. The reaction produces a nanosized layer, the solid-electrolyte interphase (SEI), on the graphite particles. We probe the structure and chemistry of the SEI using small-angle neutron scattering (SANS) and inelastic neutron scattering. The SANS results show that the SEI fills 20–30 nm sized pores, and inelastic scattering experiments with H/D substitution show that this “chemical” SEI is primarily organic in nature; that is, it contains a large amount of hydrogen. The graphite–SEI particles show surface fractal scattering characteristic of a rough particle–void interface and are interconnected. The observed changes in the SEI structure and composition provide new insight into SEI formation. The chemically formed SEI is complementary and simpler in composition to the electrochemically formed SEI, which involves a number of different reactions and products that are difficult to deconvolute.

We report the discovery of a compressibility phase transition at 160 K in crystalline copolymer films of vinylidene fluoride ( $70%$) with trifluoroethylene ( $30%$). This phase transition is distinct from the known bulk ferroelectric-paraelectric phase transition at 353 K and surface ferroelectric phase transition at 295 K. The new phase transition is characterized by an increase in the effective Debye temperature from 48 to 245 K along the $〈010〉$ direction as the temperature falls below 160 K. This phase transition is evident in neutron scattering, x-ray diffraction, angle-resolved photoemission, and in the dipole active phonon modes in electron energy-loss spectroscopy.

A concerted effort over the past few years has been focused on enhancing the core model for the High Flux Isotope Reactor (HFIR), as part of a comprehensive study for HFIR conversion from high-enriched uranium (HEU) to low-enriched uranium (LEU) fuel. At this time, the core model used to perform analyses in support of HFIR operation is an MCNP model for the beginning of Cycle 400, which was documented in detail in a 2005 technical report. A HFIR core depletion model that is based on current state-of-the-art methods and nuclear data was needed to serve as reference for the design of an LEU fuel for HFIR. The recent enhancements in modeling and simulations for HFIR that are discussed in the present report include: (1) revision of the 2005 MCNP model for the beginning of Cycle 400 to improve the modeling data and assumptions as necessary based on appropriate primary reference sources HFIR drawings and reports; (2) improvement of the fuel region model, including an explicit representation for the involute fuel plate geometry that is characteristic to HFIR fuel; and (3) revision of the Monte Carlo-based depletion model for HFIR in use since 2009 but never documented in detail, with the development of a new depletion model for the HFIR explicit fuel plate representation. The new HFIR models for Cycle 400 are used to determine various metrics of relevance to reactor performance and safety assessments. The calculated metrics are compared, where possible, with measurement data from preconstruction critical experiments at HFIR, data included in the current HFIR safety analysis report, and/or data from previous calculations performed with different methods or codes. The results of the analyses show that the models presented in this report provide a robust and reliable basis for HFIR analyses.

The Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes (TALSPEAK) process is a solvent extraction based method for separating trivalent lanthanides (Ln(3+)) from trivalent actinide cations in used nuclear fuel reprocessing. In conventional TALSPEAK, the extractant solution is di(2-ethylhexyl)phosphoric acid (HDEHP) in 1,4-diisopropylbenzene (DIPB). The aqueous medium is diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA) in a concentrated lactic acid (HL) buffer. Lanthanides are extracted by HDEHP/DIPB, while the actinides remain in the aqueous phase as DTPA complexes. Lactic acid is extracted both independently of the lanthanides and as Ln/HL/HDEHP mixed complex(es). Previous results indicate that lanthanides are extracted both as the mixed complex and as a binary Ln(DEHP·HDEHP)(3) species. Small-angle neutron scattering (SANS) has been applied to study the self-organization properties of solute molecules in xylene solutions containing HDEHP, HL, selected lanthanide ions, and water. The scattering results demonstrate that the dominant HDEHP species is the hydrogen bonded dimer, (HDEHP)(2). Absent lanthanides, lactic acid is extracted as the 1:3 complex (HL·(HDEHP)(3)). Scattering in samples containing up to 0.005 M lanthanides (prepared by extracting lanthanides from aqueous media containing 1.0 M buffered lactic acid) indicates that the dominant metal complex is Ln(DEHP·HDEHP)(3). At 0.013 M extracted lanthanide, the scattering results indicate lower Ln:DEHP stoichiometry and larger scattering particles. At higher metal concentrations, the SANS results indicate large aggregates, the largest aggregates achieving a size equivalent to 20 HDEHP monomers as the primary scattering entity. Analysis of particle shapes indicates best fits with a uniform oblate spheroid particle. These results are discussed in connection with the results of a number of complementary observations that have been made on this system.

This paper describes the design and performance of a 50 liter, two-segment 6 Li-loaded liquid scintillator detector that was designed and operated as prototype for the PROSPECT (Precision Reactor Oscillation and Spectrum) Experiment. The two-segment detector was constructed according to the design specifications of the experiment. It features low-mass optical separators, an integrated source and optical calibration system, and materials that are compatible with the 6 Li-doped scintillator developed by PROSPECT. We demonstrate a high light collection of 850±20 PE/MeV, an energy resolution of σ = 4.0±0.2% at 1 MeV, and efficient pulse-shape discrimination of low dE / dx (electronic recoil) and high dE / dx (nuclear recoil) energy depositions. An effective scintillation attenuation length of 85±3 cm is measured in each segment. The 0.1% by mass concentration of 6 Li in the scintillator results in a measured neutron capture time of τ = 42.8±0.2 μs. The long-term stability of the scintillator is also discussed. The detector response meets the criteria necessary for achieving the PROSPECT physics goals and demonstrates features that may find application in fast neutron detection.

This paper describes the design and performance of a 50 liter, two-segment $^{6}$Li-loaded liquid scintillator detector that was designed and operated as prototype for the PROSPECT (Precision Reactor Oscillation and Spectrum) Experiment. The two-segment detector was constructed according to the design specifications of the experiment. It features low-mass optical separators, an integrated source and optical calibration system, and materials that are compatible with the $^{6}$Li-doped scintillator developed by PROSPECT. We demonstrate a high light collection of 850$\pm$20 PE/MeV, an energy resolution of $σ$ = 4.0$\pm$0.2% at 1 MeV, and efficient pulse-shape discrimination of low $dE/dx$ (electronic recoil) and high $dE/dx$ (nuclear recoil) energy depositions. An effective scintillation attenuation length of 85$\pm$3 cm is measured in each segment. The 0.1% by mass concentration of $^{6}$Li in the scintillator results in a measured neutron capture time of $τ$ = 42.8$\pm$0.2 $μs$. The long-term stability of the scintillator is also discussed. The detector response meets the criteria necessary for achieving the PROSPECT physics goals and demonstrates features that may find application in fast neutron detection.

The effective Debye temperature of poly(vinylidene fluoride-trifluoroethylene) copolymers was measured using photoemission and neutron diffraction techniques. An effective Debye temperature of 53±11 K is obtained from the photoemission data and 69±3.5 K from neutron diffraction measurements. This effective Debye temperature is a consequence of the temperature-dependent dynamic motions perpendicular to the surface of these crystalline polymer films.

A joint determination of the reactor antineutrino spectra resulting from the fission of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ has been carried out by the Daya Bay and PROSPECT Collaborations. This Letter reports the level of consistency of $^{235}\mathrm{U}$ spectrum measurements from the two experiments and presents new results from a joint analysis of both data sets. The measurements are found to be consistent. The combined analysis reduces the degeneracy between the dominant $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ isotopes and improves the uncertainty of the $^{235}\mathrm{U}$ spectral shape to about 3%. The $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ antineutrino energy spectra are unfolded from the jointly deconvolved reactor spectra using the Wiener-SVD unfolding method, providing a data-based reference for other reactor antineutrino experiments and other applications. This is the first measurement of the $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ spectra based on the combination of experiments at low- and highly enriched uranium reactors.

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 ( 6 Li 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.

This Letter reports one of the most precise measurements to date of the antineutrino spectrum from a purely ^{235}U-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 5-7 MeV energy region. Comparison of the PROSPECT results with those from commercial reactors provides new constraints on the origin of this excess, disfavoring at 2.0 and 3.7 standard deviations the hypotheses that antineutrinos from ^{235}U are solely responsible and noncontributors to the excess observed at commercial reactors, respectively.

The PROSPECT and STEREO collaborations present a combined measurement of the pure $^{235}\mathrm{U}$ antineutrino spectrum, without site specific corrections or detector-dependent effects. The spectral measurements of the two highest precision experiments at research reactors are found to be compatible with ${\ensuremath{\chi}}^{2}/\mathrm{ndf}=24.1/21$, allowing a joint unfolding of the prompt energy measurements into antineutrino energy. This ${\overline{\ensuremath{\nu}}}_{e}$ energy spectrum is provided to the community, and an excess of events relative to the Huber model is found in the 5--6 MeV region. When a Gaussian bump is fitted to the excess, the data-model ${\ensuremath{\chi}}^{2}$ value is improved, corresponding to a $2.4\ensuremath{\sigma}$ significance.

Light collection and pulse-shape discrimination in elongated scintillator cells for the PROSPECT reactor antineutrino experiment, Ashenfelter, J., Balantekin, B., Band, H.R., Barclay, G., Bass, C.D., Berish, D., Bowden, N.S., Bowes, A., Brodsky, J.P., Bryan, C.D., Cherwinka, J.J., Chu, R., Classen, T., Commeford, K., Davee, D., Dean, D., Deichert, G., Diwan, M.V., Dolinski, M.J., Dolph, J., Dwyer, D.A., Gaison, J.K., Galindo-Uribarri, A., Gilje, K., Glenn, A., Goddard, B.W., Green, M., Han, K., Hans, S., Heeger, K.M., Heffron, B., Jaffe, D.E., Langford, T.J., Littlejohn, B.R., Caicedo, D.A. Martinez, McKeown, R.D., Mendenhall, M.P., Mueller, P., Mumm, H.P., Napolitano, J., Neilson, R., Norcini, D., Pushin, D., Qian, X., Romero, E., Rosero, R., Saldana, L., Seilhan, B.S., Sharma, R., Sheets, S., Stemen, N.T., Surukuchi, P.T., Varner, R.L., Viren, B., Wang, W., White, B., White, C., Wilhelmi, J., Williams, C., Wise, T., Yao, H., Yeh, M., Yen, Y.R., Zangakis, G., Zhang, C., Zhang, X.

A meter-long, 23-liter EJ-309 liquid scintillator detector has been constructed to study the light collection and pulse-shape discrimination performance of elongated scintillator cells for the PROSPECT reactor antineutrino experiment. The magnitude and uniformity of light collection and neutron-gamma discrimination power in the energy range of antineutrino inverse beta decay products have been studied using gamma and spontaneous fission calibration sources deployed along the cell axis. We also study neutron-gamma discrimination and light collection abilities for differing PMT and reflector configurations. Key design features for optimizing MeV-scale response and background rejection capabilities are identified.

A low mass optical grid for the PROSPECT reactor antineutrino detector, Ashenfelter, J., Balantekin, A.B., Band, H.R., Bass, C.D., Bergeron, D.E., Berish, D., Bowden, N.S., Brodsky, J.P., Bryan, C.D., Cherwinka, J.J., Classen, T., Conant, A.J., Davee, D., Dean, D., Deichert, G., Detweiler, A.E., Diwan, M.V., Dolinski, M.J., Erickson, A., Febbraro, M., Foust, B.T., Gaison, J.K., Galindo-Uribarri, A., Gebre, Y., Gilbert, C.E., Gilje, K.E., Gustafson, I.F., Hackett, B.T., Hans, S., Hansell, A.B., Heeger, K.M., Hermanek, K.H., Insler, J., Jaffe, D.E., Jones, D.C., Kyzylova, O., Lane, C.E., Langford, T.J., LaRosa, J., Littlejohn, B.R., Lu, X., Caicedo, D.A. Martinez, Matta, J.T., McKeown, R.D., Mendenhall, M.P., Minock, J.M., Mueller, P.E., Mumm, H.P., Napolitano, J., Neilson, R., Nikkel, J.A., Norcini, D., Nour, S., Pushin, D.A., Qian, X., Romero-Romero, E., Rosero, R., Sarenac, D., Surukuchi, P.T., Tyra, M.A., Varner, R.L., Viren, B., White, C., Wilhelmi, J., Wise, T., Yeh, M., Yen, Y.-R., Zhang, A., Zhang, C., Zhang, X.