A step toward resolving the reactor antineutrino anomaly

It’s no coincidence that some of the world’s leading neutrino-detection experiments are located in the vicinity of nuclear power plants. Hundreds of the fission products of nuclear-fuel isotopes such as uranium-235 and plutonium-239 undergo beta decay and release enormous amounts of both electrons and electron antineutrinos. More than a decade ago, researchers including Alain Letourneau of the French Alternative Energies and Atomic Energy Commission noticed that multiple experiments had reported fewer electron-antineutrino detections than expected from the nearby reactors.

A precise measurement in 2016 by the Daya Bay collaboration in China confirmed a roughly 6% overall shortfall in detections and introduced another puzzle: a surplus of antineutrinos in the 4–6 MeV energy range (see Physics Today, May 2016, page 16). Since then, particle physicists have pored over theoretical calculations and experimental data but have yet to explain the discrepancies. Now Letourneau and colleagues have proposed a solution. The reactor antineutrino anomaly, they say, could stem from flaws in the electron-spectrum data that underpin theoretical models.

An important clue toward resolving the anomaly came in 2017, when Daya Bay researchers reported that the antineutrino flux changed along with the relative amounts of ²³⁵U and ²³⁹Pu in the nearby reactors’ fuel supplies (see “As nuclear fuel ages, an antineutrino anomaly changes,” Physics Today online, 10 May 2017). The results disfavored one suggested mechanism, that the missing electron antineutrinos had morphed into a theorized class of undetectable neutrinos known as sterile neutrinos. In their new attempt to explain both the original anomaly and the dependence on fuel composition, Letourneau and colleagues focused on the fact that the leading model that is used to predict antineutrino fluxes is based on measurements of another beta-decay product, electrons, and then is applied to electron antineutrinos. So the researchers developed their own model that estimates the beta-decay rates of hundreds of fission fragments, including some for which little experimental data exist, and calculates the predicted spectra for both electrons and electron antineutrinos.

The team’s predictions for antineutrino flux largely mirror those of the leading model. But the spectra forecasted by the new model better match the experimental findings of Daya Bay and others, including the overall antineutrino flux and the bump at around 5 MeV. As for isotopes, the old and new models are in good agreement for species such as ²³⁹Pu but not for ²³⁵U. The researchers conclude that the neutrino-physics community should work to replace the 40-year-old spectral measurements of ²³⁵U and other isotopes that are currently used to inform theory with newer, more precise ones. Eliminating subtle systematic errors that were present in previous data, they say, could be the ticket to eliminating the anomaly. Working toward that goal, Letourneau and another team reported fresh measurements of the ²³⁵U antineutrino spectrum acquired at the Institut Laue–Langevin research reactor. (A. Letourneau et al., Phys. Rev. Lett. 130, 021801, 2023.)