In a groundbreaking achievement, a subterranean tank of exceptionally pure water, encased beneath several kilometers of rock in Ontario, Canada, registered an ephemeral flash as a scarcely detectable particle traversed its molecular structure. This marked the inaugural occasion on which water was employed to discern an antineutrino, emanating from a nuclear reactor located over 240 kilometers (150 miles) away.

The remarkable findings, elucidated in a 2023 study, herald a new era for neutrino investigations and the development of monitoring technologies utilizing cost-effective, readily available, and inherently safe materials.

Neutrinos, among the universe’s most prolific particles, exhibit intriguing properties that offer profound insights into the cosmos. However, their almost negligible mass, lack of charge, and minimal interactions with other particles render them elusive—consequently, these particles have earned the moniker “ghost particles.”

Antineutrinos serve as the antiparticle counterparts to neutrinos. Unlike other particle-antiparticle pairs, which possess opposite charges—consider the negatively charged electron and its positively charged counterpart, the positron—the distinction between neutrinos and antineutrinos is evidenced solely by their interactions during decay processes.

Electron antineutrinos are products of nuclear beta decay—an inherently radioactive process wherein a neutron transmutes into a proton, electron, and antineutrino. In this interaction, an antineutrino can subsequently engage with a proton, resulting in the generation of a positron and another neutron, a phenomenon referred to as inverse beta decay.

Detection of this decay typically necessitates large tanks filled with liquid scintillators, outfitted with photomultiplier tubes designed to capture the faint illumination produced by Cherenkov radiation—analogous to a sonic boom but in the realm of light. Despite their prolific generation in nuclear reactors, antineutrinos are low in energy, complicating the detection process.

The SNO+ laboratory, situated over 2 kilometers (1.24 miles) underground, offers an environment remarkably shielded from cosmic interference, enabling the acquisition of exceptionally high-resolution signals. In the calibration phase of 2018, the lab’s 780-tonne tank, originally filled with ultrapure water, identified evidence of inverse beta decay after analyzing 190 days of collected data.

This discovery indicates that water-based detectors could potentially serve as instruments for monitoring nuclear reactor power outputs and enhances our comprehension of neutrinos and antineutrinos—two entities that, due to their illusive nature, remain poorly understood.

As physicist Logan Lebanowski from the SNO+ collaboration has remarked, “The ability to utilize pure water for the detection of antineutrinos from reactors at such substantial distances is profoundly intriguing.” This research was subsequently published in Physical Review Letters.

An earlier version of this article was published in April 2023.