Buried under kilometers of rock in Ontario, Canada, a reservoir of the purest water shimmered as its particles barely collided.
It’s the first time water has been used to detect a particle known as an antineutrino, which originated from a nuclear reactor more than 240 kilometers (150 miles) away. This breakthrough promises neutrino experiments and observational technology that uses materials that are inexpensive, easy to obtain, and safe.
As some of the most abundant particles in the universe, neutrinos are exotic little things with a lot of potential for revealing deeper insights into the universe. Unfortunately, they are almost massless, carry no charge, and hardly interact with other particles at all. They flow mostly through space and rock alike, as if all matter were insubstantial. There’s a reason they’re called ghost particles.
Antineutrinos are the antiparticle counterpart of neutrinos. Normally, an antiparticle has the opposite charge of the particle equivalent; The antiparticle of a negatively charged electron, for example, is the positively charged positron. Because neutrinos carry no charge, only scientists can distinguish between the two Based on the fact An electron neutrino will appear alongside a positron, while an electron antineutrino will appear with an electron.
electron antineutrinos emitted During beta nuclear decay, a type of radioactive decay in which a neutron decays into a proton, an electron, and an antineutrino. One of these electrons antineutrinos can interact with a proton to produce a positron and a neutron, a reaction known as inverse beta decay.
Large liquid-filled tanks lined with photomultiplier tubes are used to detect this particular type of decay. It is designed to capture the faint glow of Cherenkov radiation Created by charged particles moving faster than light that can travel through a liquid, similar to a sonic boom caused by breaking the sound barrier. So they are very sensitive to very low light.
Antineutrinos are produced in huge quantities by nuclear reactors, but they are relatively low powered, which makes them hard to detect.
Enters SNO+. Buried under more than 2 kilometers (1.24 miles) of rock, it is the world’s deepest underground laboratory. This rocky shielding provides an effective barrier against cosmic ray interference, allowing scientists to obtain exceptionally well-resolved signals.
Today, the lab’s 780-ton spherical tank is filled with linear alkylbenzene, a flashing, light-amplifying liquid. Back in 2018, while the facility was undergoing calibration, it was filled with highly purified water.
By combing through 190 days of data collected during that calibration phase in 2018, the SNO+ collaboration found evidence of inverse beta decay. The neutron produced during this process is captured by a hydrogen nucleus in the water, which in turn produces a subtle bloom of light at a very specific energy level, 2.2 MeV.
Cherenkov water detectors generally struggle to detect signals below 3 MeV; But SNO+ filled with water was able to detect up to 1.4 MeV. This results in an efficiency of about 50 percent for detecting signals at 2.2 MeV, so the team thought it was worth their luck looking for signs of inverse beta decay.
An analysis of a candidate signal determined that it was likely caused by an antineutrino, with a confidence level of 3 sigma — a probability of 99.7 percent.
The result indicates that water detectors can be used to monitor the energy production of nuclear reactors.
Meanwhile, SNO+ is being used to help better understand neutrinos and antineutrinos. Because neutrinos It is impossible to measure directlyWe don’t know much about them. One of the biggest questions is whether neutrinos and antineutrinos are the exact same particles. A rare, never-before-seen dissolution would answer that question. SNO+ is currently searching for this decay.
“Surprisingly, pure water can be used to measure antineutrinos from reactors and over such large distances,” says physicist Logan Lipanovsky SNO+ collaboration and the University of California, Berkeley.
“We took great pains to extract just a few signals from the 190 days of data. The result is satisfactory.”
Research published in Physical review letters.
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