28 Mar 2023
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)
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.
New neutrino detection method using water
Research published in the journal Physical Review Letters conducted by an international team of scientists including Joshua Klein, the Edmund J. and Louise W. Kahn Term Professor in the School of Arts & Sciences, has resulted in a significant breakthrough in detecting neutrinos.
The international collaborative experiment known as Sudbury Neutrino Observation (SNO+), located in a mine in Sudbury, Ontario, roughly 240 km (about 149.13 mi) from the nearest nuclear reactor, has detected subatomic particles, known as antineutrinos, using pure water. Klein notes that prior experiments have done this with a liquid scintillator, an oil-like medium that produces a lot of light when charged particles like electrons or protons pass through it.
"Given that the detector needs to be 240km, about half the length of New York state, away from the reactor, large amounts of scintillator are needed, which can be very expensive," Klein says. "So, our work shows that very large detectors could be built to do this with just water."
What neutrinos and antineutrinos are and why you should care
Klein explains that neutrinos and antineutrinos are tiny subatomic particles that are the most abundant particles in the universe and considered fundamental building blocks of matter, but scientists have had difficulty detecting them due to their sparse interactions with other matter and because they cannot be shielded, meaning they can pass through any and everything. But that doesn't mean they're harmful or radioactive: Nearly 100 trillion neutrinos pass through our bodies every second without notice.
These properties, however, also make these elusive particles useful for understanding a range of physical phenomena, such as the formation of the universe and the study of distant astronomical objects, and they "have practical applications as they can be used to monitor nuclear reactors and potentially detect the clandestine nuclear activities," Klein says.
Where they come from
While neutrinos are typically produced by high energy reactions like nuclear reactions in stars, such as the fusion of hydrogen into helium in the sun wherein protons and other particles collide and release neutrinos as a byproduct, antineutrinos, Klein says, are usually produced artificially, "for instance, nuclear reactors, which, to split atomic nuclei, produce antineutrinos as a result of radioactive beta decay from the reaction," he says. "As such, nuclear reactors produce large amounts of antineutrinos and make them an ideal source for studying them."
Why this latest finding is a breakthrough
"So, monitoring reactors by measuring their antineutrinos tells us whether they are on or off," Klein says, "and perhaps even what nuclear fuel they are burning."
Klein explains that a reactor in a foreign country could therefore be monitored to see if that country is switching from a power-generating reactor to one that is making weapons-grade material. Making the assessment with water alone means an array of large but inexpensive reactors could be built to ensure that a country is adhering to its commitments in a nuclear weapons treaty, for example; it is a handle on ensuring nuclear nonproliferation.
Why this hasn't been done before
"Reactor antineutrinos are very low in energy, and thus a detector must be very clean from even trace amounts of radioactivity," Klein says. "In addition, the detector must be able to 'trigger' at a low enough threshold that the events can be detected."
He says that, for a reactor as far away as 240km, it's particularly important that the reactor contain at least 1,000 tons of water. SNO+ satisfied all these criteria.
Leading the charge
Klein credits his former trainees Tanner Kaptanglu and Logan Lebanowski for spearheading this effort. While the idea for this measurement formed part of Kaptanglu's doctoral thesis, Lebanowski, a former postdoctoral researcher, oversaw the operation.
"With our instrumentation group here, we designed and built all the data acquisition electronics and developed the detector 'trigger' system, which is what allowed SNO+ to have an energy threshold low enough to detect the reactor antineutrinos."
More information: A. Allega et al, Evidence of Antineutrinos from Distant Reactors Using Pure Water at SNO+, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.091801
Journal information: Physical Review Letters
Provided by University of Pennsylvania New measurements suggest 'antineutrino anomaly' fueled by modeling error
Ground-breaking 'antineutrino' detector requires pure water only
A team of scientists has devised a new technique to detect antineutrinos from a distance using little more than pure water.
Christopher McFadden
Created: Mar 29, 2023/2023/03/29/image/jpeg/6YR4EX58gfINEXoA62DVly2rwnsS6glfTZoVOQLh.jpg)
A new Canadian detector can actually "see" antineutrinos.
A significant breakthrough in detecting subatomic particles known as antineutrinos has been achieved, according to recent research published in APS.
In the Sudbury Neutrino Observation (SNO+) experiment, an international group of scientists - including Joshua Klein, Edmund J., and Louise W. Kahn - working together in a mine in Sudbury, Ontario, found antineutrinos using pure water.
This is a groundbreaking achievement, as prior experiments have used a liquid scintillator, a costly medium due to the large amounts needed for detecting antineutrinos.
What are antineutrino and neutrinos?
Klein explains that neutrinos and antineutrinos are tiny subatomic particles and the most abundant particles in the universe. Yet, they have been challenging to detect due to their sparse interactions with other matter and because they cannot be shielded. But because of how they work, we can use them to learn about things like how the universe was made and how far away astronomical objects are.
Also, they can be used in the real world to watch over nuclear reactors and possibly learn about secret nuclear activities.
While neutrinos are produced by high-energy reactions like nuclear reactions in stars, antineutrinos are usually produced artificially by nuclear reactors. By measuring antineutrinos from reactors, scientists can tell if a reactor is on or off and maybe even what kind of nuclear fuel it is burning.
This method could help monitor a reactor in a foreign country and determine if the country is switching from a power-generating reactor to one making weapons-grade material.
However, reactor antineutrinos are low in energy, making it difficult to detect them. The detector must be clean from any trace amounts of radioactivity and have a low enough threshold to detect the events. Additionally, the reactor must contain at least 1,000 tons of water to monitor a reactor as far away as 149.13 miles (240 kilometers).
Klein says his former students Tanner Kaptanglu and Logan Lebanowski led the way. Kaptanglu's doctoral thesis was part of the idea for the measurement, and Lebanowski, who used to be a postdoctoral researcher, was in charge of the whole thing. The instrumentation group designed and built all the data acquisition electronics and developed the detector's trigger system, allowing SNO+ to have an energy threshold low enough to detect the reactor antineutrinos.
This breakthrough in detecting antineutrinos with just water could lead to large and inexpensive detectors, ensuring a country is adhering to its commitments in a nuclear weapons treaty and providing a handle on ensuring nuclear nonproliferation. This discovery also opens new ways to study and use these elusive particles in the real world.
You can read the study for yourself in the journal Physical Review Letters.
Study abstract:
"The SNO+ Collaboration reports the first evidence of reactor antineutrinos in a Cherenkov detector. The nearest nuclear reactors are located 240 km away in Ontario, Canada. This analysis uses events with energies lower than in any previous analysis with a large water Cherenkov detector. Two analytical methods are used to distinguish reactor antineutrinos from background events in 190 days of data and yield consistent evidence for antineutrinos with a combined significance of 3.5σ."
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