A miniature neutrino detector promises to test the laws of physics

Neutrinos from a nuclear reactor are captured by a method known as coherent scattering.

Physicists caught a neutrino from a nuclear reactor using a device weighing only a few kilograms, an order of magnitude less massive than standard neutrino detectors. This technique opens up new ways of stress testing the known laws of physics and detecting abundant neutrinos formed in the hearts of collapsing stars.

"They finally did it," says Kate Scholberg, a physicist at Duke University in Durham, North Carolina. "And they have a very beautiful result." An experiment called CONUS+ is described on July 30.

Difficult quarry

Neutrinos are elementary particles that have no electric charge and usually do not interact with other substances, which makes them extremely difficult to detect. Most neutrino experiments capture these elusive particles by observing the flashes of light that are generated when a neutrino collides with an electron, proton or neutron. These collisions occur extremely rarely, so such detectors usually have a mass of tons or thousands of tons to provide enough target material to collect neutrinos in appropriate quantities.

Scholberg and her colleagues first demonstrated the mini-detector technique in 2017, using it to catch neutrinos produced by an accelerator at the Oak Ridge National Laboratory in Tennessee. Oak Ridge particles have slightly higher energy than those produced in reactors. As a result, the discovery of reactor neutrinos was even more difficult, she says. But low-energy neutrinos also allow you to more accurately test the standard model of physics.

Scholberg's COHERENT detector was the first to use a phenomenon called coherent scattering, in which a neutrino "disperses" the entire atomic nucleus, not the constituent particles of the atom.

Coherent scattering uses the fact that matter particles can act as waves - and the lower the energy of particles, the longer their wavelength, says Christian Buck, leader of CONUS cooperation. If the wavelength of a neutrino is similar to the diameter of the nucleus, "then the neutrino sees the nucleus as one. He doesn't see the internal structure," says Buck, who is a physicist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. The neutrino does not interact with any subatomic particles, but causes the return of the nucleus, depositing a tiny amount of energy in the detector.

View of the core

Coherent scattering occurs more than 100 times more often than the interactions used in other detectors, where the neutrino "sees" the nucleus as aggregates of smaller particles with an empty space between them. This higher efficiency means that the detectors can be smaller and still notice the same number of particles in the same period of time. "Now you can afford to build detectors on a kilogram scale," Buck says.

The disadvantage is that neutrinos deposit much less energy in the nucleus. According to Buck, the recoil caused by the neutrino core is comparable to the recoil produced on the ship by a ping-pong ball, and until recent years it has been extremely difficult to measure.

The CONUS detector consists of four pure germanium modules, each weighing 1 kilogram. He worked at a nuclear reactor in Germany from 2018 until the reactor was closed in 2022. Then the team moved the detector, Upgraded to CONUS+, to the Leibstadt nuclear power plant in Switzerland. From the new location, the team now reports that during 119 days of work it has observed about 395 collisions - in accordance with the forecasts of the standard particle physics model.

After the landmark result of COHERENT 2017, which was obtained with the help of detectors made of cesium iodide, Scholberg's team repeated the feat with the help of detectors made of argon and germanium. Separately, last year, two experiments originally developed for dark matter hunting reported hints of low-energy coherent scattering of neutrinos produced by the Sun. Scholberg says that the standard model makes very clean predictions of the rate of coherent scattering and how it changes with different types of atomic nuclei, which makes it extremely important to compare results from as many detecting materials as possible. And if the sensitivity of technology improves even more, coherent scattering can help promote the state of art of solar science.

Researchers say that coherent scattering is unlikely to completely replace any existing neutrino detection technologies. But it can detect all three known types of neutrinos (and their corresponding antiparticles) to low energies, while some other methods can capture only one type. This ability means that it can complement massive detectors that are aimed at capturing neutrinos at higher energies, such as the Hyper-Kamiokande Observatory, which is currently being built in Japan.