PROSPECTing new perspective on reactor-produced neutrinos

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Lawrence Livermore is a founding member of PROSPECT, the Precision Oscillation and Spectrum Experiment. The sophisticated neutrino–antineutrino detection system—the first aboveground detector of its kind—is sited at Oak Ridge National Laboratory’s High Flux Isotope Reactor (HFIR)

While neutrinos are one of the most common particles in the universe, their elusive nature makes it challenging to understand their behavior.

In new research from the Precision Reactor Oscillation and Spectrum (PROSPECT) Experiment, scientists have produced final results from measurements of neutrinos emitted by a nuclear reactor at the Oak Ridge National Laboratory. Their work, recently published in Physical Review Letters and highlighted as an Editor’s Suggestion, provides new insight into a puzzling and persistent difference that scientists have encountered between predictions and measurements of neutrinos emitted by nuclear reactors.

Neutrinos are nearly weightless, have no electric charge and come in three types, or “flavors.” The “electron-flavored” neutrino produced in abundance by nuclear reactors is the lighter cousin of its namesake, but due to its lack of charge, it does not stick to or bounce off of atoms the way that electrons do. Instead, it passes through matter effortlessly, leaving almost no trace of its existence. Despite the feebleness of their interactions, neutrinos play an essential role in processes as small as radioactive decay and as large as the clumping of matter in the universe.

While particle physicists think of nuclear reactors as ultra-bright neutrino factories, most people know them for the carbon-free electricity that they generate. Heat is generated inside a reactor core as uranium and plutonium atoms in its fuel split in two. The resulting fragments, consisting of dozens of elements of varying weights, are unstable and release neutrinos as they decay. In contrast to the way heat generated in a reactor is readily captured to generate electric power that runs our lights, stoves, and electric cars, the billions-upon-billions of ghostly neutrinos made each second easily escape.

In 2018, at the High Flux Isotope Reactor (HFIR) in Oak Ridge, Tennessee, a tiny fraction of those neutrinos (around a thousand per day) were detected by PROSPECT, a specialized ton-scale particle detector resting only eight meters from HFIR’s center – one concrete wall away from the water pool surrounding the core. Every so often, a single hydrogen nucleus in the detector’s four-ton liquid center was transformed into a neutron by an interacting neutrino, creating unique pairs of light flashes that were captured by surrounding sensors.

“To access the neutrino physics potential of HFIR, PROSPECT built a detector that uses unique features of the neutrino signal to find ‘the needle in the haystack’ and overcome a challenging background environment.” said Nathaniel Bowden, PROSPECT co-spokesperson and group leader for rare event detection at the Lawrence Livermore National Laboratory. ”This was the first time an experiment has been able to do physics with reactor antineutrinos on the Earth’s surface, without being underground.”

In their new results, PROSPECT used their entire 2018 neutrino dataset, carefully cleaned of backgrounds, to map the energies of neutrinos emitted by the unstable nuclear fragments in the HFIR core, which burns only highly-enriched uranium fuel. They then compared their map to a model formed from large nuclear databases containing decades’ worth of measurements of the properties of these rare elements, many of which only exist on Earth inside burning nuclear fuel.

This comparison revealed a significant discrepancy also seen by previous neutrino efforts: higher-energy reactor neutrinos are more common than these models predict. By noting that a similar difference also is seen by neutrino experiments at commercial reactor cores burning a mix of uranium and plutonium, PROSPECT collaborators were able to prove that models are likely wrong for fragments from both of these fuel components.

Bowden said this conclusion is relevant to nuclear physicists and reactor experts, who rely on these same nuclear databases, among other things, to model and validate other aspects of nuclear reactor behavior. While the jury is still out on the true underlying cause of this anomaly, there is broad agreement that further study by both neutrino physicists and nuclear theorists and experimenters is required.

The PROSPECT detector was designed, built and operated by dozens of scientists from 12 universities and four U.S. national laboratories. It was funded in part by the Heising-Simons Foundation and in part by a Department of Energy program dedicated to jump-starting small neutrino physics efforts in order to maintain a vibrant mix of experiments alongside forthcoming large-scale international neutrino projects.

“The PROSPECT detector was constructed over a period of about a year with major student involvement,” said PROSPECT co-spokesperson Pieter Mumm of the National Institute of Standards and Technology. “It nicely demonstrated how small, nimble experiments can quickly respond to emerging scientific questions while providing unique opportunities for young scientists to play a central role.”

PROSPECT has used its year of reactor neutrino data as the basis for more than a dozen technical and physics publications, including new searches for dark matter and even ghostlier neutrino types called sterile neutrinos.

PROSPECT is working to execute a second phase of its experiment that fields a more mobile detector that can be deployed at multiple reactors. Such a detector would allow this neutrino energy anomaly to be observed at both high-enriched and commercial reactor cores through a common lens. Bowden said a paired measurement set would greatly improve PROSPECT’s precision, giving nuclear data experts and theorists an even better magnifying glass.