A tiny neutrino detector scored big at a nuclear reactor

A tiny neutrino detector has found its footing in a fresh setting — at a nuclear reactor. 

Conventional detectors of the subatomic particles require metric tons of material. But the new detector has a mass of less than 3 kilograms. Think chihuahua. And it successfully detected antineutrinos, the antimatter counterparts of neutrinos, streaming from a nuclear power plant in Leibstadt, Switzerland, researchers report in a paper submitted January 9 to arXiv.org.

“This is actually huge,” says neutrino physicist Kate Scholberg of Duke University, who was not involved with the research. “People have been trying to do this for many decades and now have finally succeeded.”

Similar scaled-down neutrino detectors have glimpsed neutrinos and antineutrinos created by laboratory sources of the particles. Nuclear reactors spit out relatively low-energy antineutrinos. By measuring those low-energy particles, such detectors could help test physics theories or reveal the inner workings of atomic nuclei. Some scientists have proposed that the compact devices could used to monitor nuclear reactors for activity that signals development of nuclear weapons.

Neutrinos are dastardly difficult to spot. For the most part, they interact with matter so rarely that detectors need to be enormous to provide more opportunities for neutrinos to interact within. 

But one type of interaction is more common, in which a neutrino or antineutrino bounces off an atomic nucleus rather than a proton or neutron. Detectors of neutrino-nucleus interactions can be built quite small — with a catch. They must be extremely sensitive. Observing the nucleus recoiling is like sensing the motion of a bowling ball hit by a ping-pong ball. The effect was first observed in 2017, using a laboratory source of the particles.

In the new study, a detector made of germanium crystals snagged about 400 antineutrinos from the Leibstadt reactor over 119 days. The number agrees with predictions from the established theory of particle physics, the standard model.

In a neutrino-nucleus interaction, the complexity of the nucleus, with its constituent protons and neutrons, is washed out. It’s truly like knocking into a bowling ball. And the lower the energy of the particles hitting that nucleus, the more like a bowling ball it is. With reactor antineutrinos, “it’s just such a gentle bump that it’s very clean,” Scholberg says. “Any sort of weird mushy stuff going on in the nucleus doesn’t matter.” 

The lack of mushiness makes the measurements more sensitive to potential new effects, such as undiscovered types of particles or unexpected magnetism in neutrinos. “This opens up a new channel in neutrino physics,” says physicist Christian Buck of the Max Planck Institute for Nuclear Physics in Heidelberg, a coauthor of the study. “There might be new physics in that channel that we do not know about now.” Other teams of scientists are already using the data to check for such effects, as reported in two papers submitted to arXiv.org on January 17 and January 21.

Scientists have previously found evidence for nucleus-thumping by neutrinos from the sun, using large detectors designed to spot dark matter, an invisible source of mass present in the cosmos.

This is not the first claimed observation of reactor antineutrinos bouncing off nuclei. Another team of scientists purportedly saw the effect in 2022, but the result didn’t fully jibe with accepted theories, making it controversial. The new study rules out the possibility that the 2022 claim was correct, Buck says.

Neutrino detectors of this type might be useful for monitoring nuclear reactors for clandestine activity, some physicists have suggested. The antineutrinos that spew out of nuclear reactors provide a signature of what’s going on within. Antineutrino energies, for example, can reveal the reactor’s quantity of plutonium, a material relevant for weapons production. But the energies of antineutrinos may be challenging to determine using the technique. The new experiment was also snuggled up close to the reactor, while some types of real-world monitoring would need to take place farther from the source. 

And while the detector’s small size is a plus for practical applications, it had to be thickly clad in lead and other material to shield it from particles that could mimic antineutrinos, making it less portable.

“This is still a very, very difficult way to do physics,” says neutrino physicist Jonathan Link of Virginia Tech in Blacksburg, who was not involved with the work. “But you always start with the first baby step.”