Measuring the antineutrino energy spectrum

Nuclear reactors, among the brightest terrestrial emitters of antineutrinos, have been central to neutrino physics. For more than a decade, physicists have puzzled over anomalous differences between measurements and predictions of the antineutrino emissions from nuclear reactors. These differences could reveal deficiencies in prediction methods and their underlying nuclear data, the existence of additional ‘sterile’ neutrinos beyond the three flavors predicted by the Standard Model of particle physics, or some combination of these two explanations. The Precision Oscillation and Spectrum Experiment (PROSPECT), is an LLNL-led neutrino physics experiment dedicated to exploring these two possibilities through precision measurements of the flux and energy spectrum of antineutrinos emitted from the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory.

PROSPECT has now published the world’s highest precision measurement of the antineutrino energy spectrum exclusively from U-235 fissions in Physical Review Letters, with this paper being highlighted as an ‘Editors Suggestion.’ This new analysis, led by NACS physicist Christian Roca-Catala, doubles the statistical precision of past PROSPECT measurements and reports for the first time a true antineutrino energy spectrum which removes the imprint of the PROSPECT detector response. NACS physicist Xianyi Zhang led the development of the antineutrino event reconstruction responsible for the significant increase in statistical power.

Examination of the new spectrum measurement further highlights differences with standard predictions and indicates that discrepancies exist for all fissile isotopes thereby narrowing the range of plausible explanations. This work provides a new high precision benchmark against which updated predictions can be tested and for possible applications of antineutrino detection to reactor safeguards. The new LLNL-led PROSPECT event selection with double the statistical power is now being used to expand the experiment’s sensitivity to the existence of sterile neutrinos, with world-leading results expected later this year.

[M. Andriamirado et al., (PROSPECT Collaboration), Final Measurement of the  235 U  Antineutrino Energy Spectrum with the PROSPECT-I Detector at HFIR, Phys. Rev. Lett. (2023), DOI: 10.1103/PhysRevLett.131.021802.]

–Physical and Life Sciences Communications Team