Nuclear Reactor Mystery Solved, With No Need For New Particles

sciencehabit shares a report from Science Magazine: A physics mystery has come to an end, with a resolution about as shocking as "the butler did it." For a decade, physicists have pondered why nuclear reactors pump out fewer particles called neutrinos than predicted. Some suggested the elusive bits of matter might be morphing into weirder, undetectable "sterile" neutrinos. Instead, new results pin down what other experiments had suggested: that theorists overestimated how many neutrinos a reactor should produce. [...] In a reactor's core, uranium and plutonium nuclei split in a chain reaction, and the antineutrinos come from the radioactive "beta decay" of the lighter nuclei left behind. In such decay, a neutron in a nucleus changes into a proton while emitting an electron and an electron antineutrino. To predict the total flux of antineutrinos, physicists had to account for the amounts and decays of myriad different nuclei. That accounting pointed to a shortfall, but in 2017, physicists from the Daya Bay Reactor Neutrino Experiment in China called it into question. They studied antineutrinos from six commercial reactors, burning fuel with 4% uranium-235 atoms, which can sustain a chain reaction, and 96% uranium-238 atoms, which can't. As the uranium-235 is consumed, neutrons from its fission convert uranium-238 into plutonium-239, which also sustains a chain reaction. Daya Bay physicists found the antineutrino deficit shrank as the amount of uranium-235 fell, suggesting theorists had overestimated the flux of antineutrinos originating from uranium-235. Now, physicists working at a small research reactor in France have confirmed that suspicion. The reactor at the Laue-Langevin Institute (ILL) produces copious neutrons for studies of materials. It also uses fuel containing 93% uranium-235. So, by studying the antineutrinos from it, researchers working with a neutrino detector called STEREO could measure the flux of antineutrinos from uranium-235 alone. The detector consists of six identical oil-filled segments lined up like teeth and spanning a distance of 9 to 11 meters from the reactor's core. Rarely, a proton in the oil will absorb an electron antineutrino to turn into a neutron while ejecting a positron -- sort of the reverse of beta decay. As the positron streaks through the oil, it produces light in proportion to the energy of the original neutrino. STEREO researchers showed the spectrum of energies of electron antineutrinos remained the same as distance from the core increased. That observation clashes with the idea that some are morphing into sterile neutrinos, because lower energy neutrinos should morph faster than higher energy ones, changing the spectrum as the neutrinos advance. STEREO researchers also showed the total flux of antineutrinos from uranium-235 was lower than the one used in theorists' models, as they report today in Nature. Taken together, the observations put an end to the reactor antineutrino deficit as evidence for a 1-eV sterile neutrino, says David Lhuillier, a neutrino physicist at France's Atomic Energy Commission and spokesperson for the 26-member STEREO team. "Can it be explained by a sterile neutrino of mass around 1 eV? The answer is no." Other experiments -- such as one called PROSPECT at Oak Ridge National Laboratory -- had reached similar conclusions, Lhuillier notes.

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