Neutrino detector measures active nuclear power plants hundreds of miles away – New Scientist

The Canadian particle experiment SNO+ has captured neutrinos generated in nuclear power plants hundreds of kilometers away. It is therefore possible to monitor the activity of nuclear reactors remotely.

A handful of flashes of light, that’s all they can handle after 190 days of measuring. Nevertheless, the physicists at the Sudbury Neutrino Observatory (SNO+) are cheering. After years of research, they present in the science magazine Physical Review Letters evidence that their detector has seen 14 neutrinos generated in nearby nuclear power plants. Such a remote measurement tells you what is happening in the reactor without you having to be in the power station.

According to the researchers, more than half of the captured neutrinos come from the Canadian nuclear power plants in Bruce, Darlington and Pickering in the state of Ontario, between 240 and 350 kilometers from the detector. The rest could come from another fifteen reactor cores in Canada and about a hundred in the US. SNO+ cannot trace the neutrinos to individual nuclear reactors, because information about the movement of the neutrino is lost during the measurement process.

Ultra-pure

Neutrinos are created in all kinds of nuclear reactions, such as nuclear fusion in the center of the sun and nuclear fission in nuclear power plants. These ‘ghost particles’ are the most common form of matter in the universe, but you will never see them. Neutrinos slip through the thickest shield without stopping. For example, they fly effortlessly through the meters of reinforced concrete around a nuclear reactor, and even right through the earth. Very rarely does a neutrino react with normal matter. That’s what neutrino detectors use.

SNO+ uses a twelve meter wide tank of ultrapure water to detect a tiny fraction of all passing neutrinos. The detector is located in the Canadian Creighton nickel mine, 2 kilometers underground. The thick rock roof above the detector ensures that the sensitive installation is hardly affected by other particles from the universe, such as cosmic rays.

The right type of neutrino

‘SNO+ was originally designed to capture high-energy neutrinos from the distant universe,’ says particle physicist Elizabeth Falcon at the University of Sussex, one of the scientists behind the research. ‘Such neutrinos are interesting as a source of information about supernovae, for example, but also because they may have played a role in the disappearance of all antimatter in the early days of the universe.’

Like the much larger neutrino detectors KM3NeT and IceCube, SNO+ searches for the weak light signal that is created when a neutrino hits an atomic nucleus of hydrogen in water. The detectors mainly find neutrinos that originate in the sun or in supernovas, because they have more energy. Reactor neutrinos have also been measured before with specialized detectors such as COMBLAND.

According to expert in neutrino observations Ernst Jan Buis of Delft University of Technology, which itself is not affiliated with SNO+, the measurement of reactor neutrinos by the experiment is an achievement in itself. ‘Water Cherenkov detectors such as SNO+ are usually not sensitive enough to measure neutrinos of this low energy.’

Interfering signals

The measurement therefore did not go without a hitch. The researchers had to put in a lot of effort to find their 14 reactor neutrinos. They were hidden in the noise of other measurement signals, such as the natural radioactive decay of thorium and uranium in the earth’s crust.

The biggest source of interference in the hunt for reactor neutrinos turned out to be the glass of the light detectors in SNO+, says Falk. They contain a whiff of radioactive material that emits tiny flashes of light, much like the flash of light that betrays a reactor neutrino. ‘We saw 10,000 times more of those interfering signals than reactor neutrinos. Fortunately, you mainly measure this interference signal in the one light detector where it originates, while the flash of a reactor neutrino is visible throughout the tank. That way we could tell them apart.’

supervision

What exactly is the use of a device that can see from a considerable distance how fast a nuclear power plant is running? According to Elisabeth Falk, you could use that information to find out if someone is pushing back fissile material, or even using a reactor to grow weapons-grade plutonium. ‘If you can carry out inspections on site, that is of course better. But that’s not always an option, like in North Korea or Iran.’

Reactor expert Bryan van der Ende of the Canadian nuclear research laboratory, which itself is not involved in the research, does question the possible application. According to him, remote monitoring requires much more detailed measurements than SNO+ is currently presenting. ‘If you want to know whether a reactor operator adheres to the rules, you must be able to see how many and what kind of nuclear reactions take place over time. Measuring only fourteen neutrinos in 190 days, which also come from different reactors, is not enough.’

Ernst-Jan Buis is also still skeptical about nuclear surveillance with neutrino detectors. ‘The problem remains that neutrinos are so difficult to measure that you need large installations.’ According to Buis, a ‘small’ set-up with only a few tons of detection material only has a measuring range of a few kilometres. ‘Ideally, you want to place your detector somewhere where there are few sources of interference. But for a monitoring system you don’t have a choice.’

Nevertheless, the performance of SNO+ has potential, thinks Van der Ende. ‘A neutrino detector with a range of hundreds of kilometers means that you can detect secret reactors in an area. Those are exactly the kind of installations where someone would try to illegally grow plutonium.’ .

Ghost particle
Every second, trillions of neutrinos rush through your body, originating from nuclear fusion in the sun. But in a lifetime, on average, only one collides with an atom in your body, so is the chance that these ghost particles react to other matter.

Wolfgang Pauli predicted the elusive particles in 1930 to plug holes in the theory of radioactive decay. It was not until 1956 that experiments actually observed the neutrino.