Antimatter neutrinos detected from a nuclear reactor 240 km away

A water-based detector has been used to spot antineutrinos from nuclear reactions hundreds of kilometres away. It could be used to monitor distant nuclear activities.

Antimatter neutrinos created by a nuclear reactor were picked up by a detector located 240 kilometres away. The approach could be used to monitor nuclear activity from afar.

Antineutrinos colliding with protons generates a blue light

The universe is full of neutrinos and their antimatter counterparts called antineutrinos. They are commonly produced inside the sun and in nuclear reactors, but detecting them is difficult because they typically move through matter without affecting it. Despite this, Josh Klein at the University of Pennsylvania and his colleagues managed to record antineutrinos from a nuclear reactor hundreds of kilometres away.

Their detector, operated by a group of researchers called the SNO+ collaboration, consists of a large, hollow acrylic sphere filled with 905 tonnes of extremely pure water surrounded by nearly a thousand very sensitive detectors for electromagnetic radiation. It is located underground, under 2 kilometres of rock that most particles can’t penetrate, at a facility inside a mine in Sudbury, Canada.

Antineutrinos interact with matter only under very special conditions and simply pass through it otherwise, so they could make it through the rocks untouched. However, the pure water in the sphere has the right conditions for antineutrinos to interact – namely, a particularly high number of protons to collide with.

The detector is a large acrylic sphere that gets filled with very pure water and is surrounded by very sensitive light detectors
SNO+ Collaboration

Each such collision creates a positron, which is the antimatter equivalent of an electron, and a neutron. The positrons move very fast, creating a wake made of blue light. The neutron, on the other hand, combines with the nucleus of a nearby atom and undergoes a process that involves emitting a gamma ray. From past experiments with antineutrinos, the researchers knew that the blue light precedes the gamma ray by about 200 microseconds, so when they saw this precise pattern they knew there was an antineutrino in the water.

The team collected data for 190 days and determined that an antineutrino entered the detector 14 times. There is no direct way to establish where each antineutrino originated, but the data was a good match for what the three nearest Canadian reactors, the closest of which is 240 kilometres away, produce, says Klein. Specifically, the antineutrinos had energies expected only for particles created in a reactor and not for those originating inside the sun or the earth’s core.

Eventually, a similar approach could be used to detect processes involved in making nuclear weapons. However, the detectors would need to be much larger and more sensitive, says Federico Sanchez at the University of Geneva in Switzerland.

Physical Review Letters:  DOI: 10.1103/PhysRevLett.130.091801

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