Nonfuel antineutrino contributions in the ORNL High Flux Isotope Reactor (HFIR)

Abstract

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 νe is important when making theoretical predictions. One source of νe that is often neglected arises from the irradiation of the nonfuel materials in reactors. The νe 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 νe 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 νe source for the the Precision Reactor Oscillation and Spectrum Measurement (PROSPECT) experiment. Overall, we observe that the nonfuel νe 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 νe contribution.

PROSPECT-II physics opportunities

“…PROSPECT has observed the HFIR core, which burns HEU fuel and generates a neutrino flux ∼99% from 235 U fissions. Antineutrino emission from non-fuel isotopes [59] and nearby spent fuel [60,61] may also contribute to the signal observed in reactor-based experiments. In PROSPECT, nonfuel isotopes contribution about 1% of the antineutrino signal, and spent fuel contributions are negligible.…”

“…The broader impact of these measurements was discussed in detail in the previous section. Beyond these flagship measurements, PROSPECT also provided new estimates of non-fuel ν e production by research reactors [59] and set new limits on low-mass WIMP dark matter [117].…”

Nuclear structure opportunities with GeV radioactive beams at FAIR

“…With that, a possible accuracy of 2% both experimentally and theoretically could be reached, allowing to constrain the slope parameter by ±10 to ±20 MeV, depending on the accuracy of the reaction model. The first set of measurements was taken with beams of [124][125][126][127][128][129][130][131][132][133][134] Sn ions and carbon target at an energy range of 0.4-1 GeV/ nucleon [213]. The data from the experiment are currently under analysis.…”

White Paper on Light Sterile Neutrino Searches and Related Phenomenology

“…Reactor and site-specific sources of νe are another such source that has to be included for each individual experiment separately. These may include νe originating from the neutrons interacting with non-fuel reactor elements [61] and spent nuclear fuel [62,63] placed in close proximity to the detectors.…”