For the first time, pure water has been used to detect low-energy antineutrinos produced by nuclear reactors. The work was done by the international SNO+ collaboration and could lead to safe and affordable new ways to monitor nuclear reactors from a distance.
Situated 2 km underground near an active mine in Sudbury, Canada, the SNO+ detector is the successor to the earlier Sudbury Neutrino Observatory (SNO). In 2015, SNO’s director Art McDonald shared the Nobel Prize for Physics for the experiment’s discovery of neutrino oscillation – which suggests that neutrinos have tiny masses.
Neutrinos are difficult to detect because they rarely interact with matter. This is why neutrino detectors tend to be very large and are located underground – where background radiation is lower.
At the heart of SNO was a large sphere of ultra-pure heavy water in which energetic neutrinos from the Sun would very occasionally interact with the water. This produces a flash of radiation that can be detected.
Careful measurements
SNO is currently being upgraded as SNO+, and as part of the process ultra-pure normal water was temporarily used as the detection medium. This was replaced by a liquid scintillator in 2018, but not before the team was able to made a series of careful measurements. And these threw up a surprising result.
“We found our detector was performing beautifully, and that it might be possible to detect antineutrinos from distant nuclear reactors using pure water,” explains Mark Chen. He is the SNO+ director and is based at Queen’s University in Kingston, Canada. “Reactor antineutrinos have been detected using liquid scintillators in heavy water in the past, but using just pure water to detect them, especially from distant reactors, would be a first.”
It had been difficult to detect reactor antineutrinos in pure water because the particles have lower energies than solar neutrinos. This means that the detection signals are much fainter – and therefore are easily overwhelmed by background noise.
Lower background
As part of SNO+’s upgrades, the detector was fitted with a nitrogen cover gas system, which significantly lowered these background rates. This allowed the SNO+ collaboration to explore an alternative approach to detecting reactor antineutrinos.
The detection process involves a neutrino interacting with a proton, resulting in the creation of a positron and a neutron. The positron creates an immediate signal whereas the neutron can be absorbed sometime later by a hydrogen nucleus to create a delayed signal.
“What enabled SNO+ to accomplish this detection are very low backgrounds and excellent light collection, enabling a low energy detection threshold with good efficiency,” Chen explains. “It’s the latter – a consequence of the first two features – that enabled the observation of antineutrinos interacting in pure water.”
“Dozen or so event”
“As a result, we were able to identify a dozen or so events that could be attributed to interactions from antineutrinos in pure water,” says Chen. “It’s an interesting result because the reactors that produced those antineutrinos were hundreds of kilometres away.” The statistical significance of the antineutrino detection was 3.5σ, which is below the threshold of a discovery in particle physics (which is 5σ).
Reactor antineutrinos detected in pure water in an experimental first
The result could have implications for the development of techniques used to monitor nuclear reactors. Recent proposals have suggested that antineutrino detection thresholds could be lowered by doping pure water with elements like chlorine or gadolinium – but now, the results from SNO+ show that these costly, potentially dangerous materials may not be necessary to achieve the same quality of results.
Although SNO+ can no longer make this type of measurement, the team hopes that other groups could soon develop new ways to monitor nuclear reactors using safe, inexpensive, and easily attainable materials, at distances that will no disrupt reactor operation.
The research is described in Physical Review Letters.