Measurement of the Antineutrino Spectrum from U235 Fission at HFIR with PROSPECT

Abstract

This Letter reports the first measurement of the 235 U νe 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.) νe-induced inverse beta decays (IBD), 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 νe energy region of 5-7 MeV.Reactor ν e experiments have been central to the understanding of neutrinos, including the first observation of ν e [1], the discovery of ν e oscillations [2], observation of ν e produced within the Earth [3], and the measurement of the neutrino mixing angle θ 13 [4][5][6]. Most of these experiments were located at low-enriched uranium (LEU) nuclear power reactors where more than 99 % of emitted ν e come from the beta decay of fission products of four isotopes ( 235 U, 238 U, 239 Pu, and 241 Pu). At power reactors, the emitted ν e flux and spectrum evolve over time as the isotopic composition changes in the fuel cycle. Comparisons between theoretical predictions and experimental results reveal a ∼6 % global flux deficit [7-10] and disagreement of the energy spectrum [11][12][13][14] and flux-evolution [15,16]. Explanations for these possibly independent phenomena may lie in the complex nuclear physics of reactors [17][18][19][20][21][22][23][24], physics beyond the Standard Model such as eV-scale sterile neutrinos [8], or both [25-27]. New experiments at compact-core, highly enriched uranium (HEU) research reactors enable short baseline searches for sterile neutrino oscillations and the measurement of the nearly time-independent emission of ν e from 235 U fission [28][29][30]. PROSPECT has recently reported a search for sterile neutrinos at the High Flux Isotope Reactor (HFIR) [31]. This Letter reports the first measurement of the ν e energy spectrum from HFIR by the PROSPECT experiment and the higheststatistics 235 U spectral measurement since the ILL experiment observed ∼5000 ν e candidates in 1981 [32].Located at Oak Ridge National Laboratory, HFIR is an 85 megawatt thermal (MW th ) HEU research reactor. The cylindrical reactor core (diameter: 0.435 m, height: 0.508 m) contains 93 % 235 U enriched fuel, leading to a ∼99 % 235 U fission fraction. Each 24-day operating cycle uses fresh fuel, minimizing 239 Pu and 241 Pu production. The PROSPECT detector is deployed in a ground-level room at a center-to-center distance of (7.9 ± 0.1) m from the reactor core. The core center is located 40°below the horizontal and the surrounding building provides less than one meter-water-equivalent of concrete overburden.

Extraction of the U235 and Pu239Adey1, An2, Balantekin3 et al. 2019Phys. Rev. Lett.Self Cite6561100View full textAdd to dashboardCite

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“…The use of 4 tons of PSD capable 6 Li-doped LS (LiLS) provides fast neutron and neutron capture identification, while a 2D segmented geometry (14.5 cm pitch) provides event localization and topology. An emphasis on efficient, uniform light collection results in very good energy resolution for an organic scintillator detector (Ashenfelter et al, 2018b), which has been utilized in a measurement of the 235 U reactor antineutrino energy spectrum (Ashenfelter et al, 2019). Initial background predictions for PROSPECT (Ashenfelter et al, 2016) are in good agreement with the data reported in (Ashenfelter et al, 2018a), including observation of spectral features due to multiple neutron and neutron inelastic processes.…”

“…4 Beta decays following neutron capture on materials in a reactor also contribute to the neutrino flux. The effect is small for typical power reactors (Huber and Jaffke, 2016), but can be significant for certain research reactor configurations (Ashenfelter et al, 2019). 5 Following common usage, this review uses "neutrino" as a general term for both neutrinos and antineutrinos.…”

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