A tiny neutrino detector makes a big impact at a nuclear reactor

A compact method of detecting subatomic particles offers new insights into physics theories.

A tiny neutrino detector has successfully operated at a nuclear reactor, demonstrating a new way to observe these elusive particles.

Traditional neutrino detectors require massive structures weighing metric tons, but this new device, weighing less than 3 kilograms, has proven its effectiveness. Comparable in size to a chihuahua, it successfully detected antineutrinos—the antimatter counterparts of neutrinos—at a nuclear power plant in Leibstadt, Switzerland. Researchers reported their findings in a paper submitted to arXiv.org on January 9.

“This is a major achievement,” says neutrino physicist Kate Scholberg of Duke University, who was not involved in the research. “Scientists have been attempting this for decades, and now, they’ve finally succeeded.”

Neutrino interactions with atomic nuclei are unique in that they bypass the complex structure of protons and neutrons, resembling the impact of a bowling ball. At the low-energy levels associated with reactor antineutrinos, this effect becomes even cleaner, eliminating interference from nuclear complexities.

This clarity enhances sensitivity to potential new physics, such as undiscovered particles or unexpected neutrino magnetism. “This opens up a new avenue in neutrino physics,” says physicist Christian Buck of the Max Planck Institute for Nuclear Physics in Heidelberg, a coauthor of the study. “We might discover unknown phenomena in this domain.” Other researchers are already analyzing the data to investigate such possibilities, as detailed in two papers submitted to arXiv.org on January 17 and January 21.

Scientists have previously observed neutrinos interacting with nuclei in large detectors originally designed to study dark matter. However, this is the first widely accepted observation of reactor antineutrinos bouncing off nuclei. A previous claim from 2022 suggested a similar detection, but its inconsistencies with accepted theories made it controversial. The new study refutes that earlier claim, Buck says.

These compact neutrino detectors could potentially monitor nuclear reactors for clandestine activities. Since nuclear reactors emit a distinctive antineutrino signature, detecting their energies could reveal plutonium levels, a key factor in weapons production. However, challenges remain in precisely determining these energies. Additionally, the new experiment was conducted very close to the reactor, whereas real-world monitoring may require greater distances.

Smaller neutrino detectors have previously detected neutrinos and antineutrinos from laboratory sources. Since nuclear reactors emit relatively low-energy antineutrinos, measuring these particles could refine physics theories and enhance our understanding of atomic nuclei. Some researchers suggest these devices could be employed for nuclear reactor monitoring to detect illicit nuclear weapons development.

Neutrinos are notoriously difficult to detect, as they rarely interact with matter. Conventional detectors rely on their vast size to increase the chances of interaction. However, one particular type of neutrino interaction—where a neutrino or antineutrino bounces off an atomic nucleus—is more frequent. This allows for much smaller detectors, but these must be exceptionally sensitive. Observing such a recoil is akin to detecting a bowling ball’s motion after being hit by a ping-pong ball. The effect was first observed in 2017 using a laboratory particle source.

In this latest study, a detector composed of germanium crystals captured approximately 400 antineutrinos from the Leibstadt reactor over 119 days. The findings align with predictions from the Standard Model of particle physics. Despite its small size, the detector required heavy shielding with lead and other materials to block background signals that could mimic antineutrino interactions, limiting its portability.

“This remains an incredibly challenging way to conduct physics,” says neutrino physicist Jonathan Link of Virginia Tech, who was not involved in the research. “But every major breakthrough begins with a first step.”