A Historic Journey into Neutrino Detection
Seventy years ago, physicists Clyde Cowan and Frederick Reines embarked on a groundbreaking experiment, constructing a custom-made 10-ton detector encased in thick lead walls and damp sandbags. They positioned it near a powerful nuclear reactor at the Savannah River Plant in South Carolina, dubbing the project “Poltergeist” in a nod to its aim of capturing a ghostly particle.
More than twenty-five years prior, scientists were perplexed by the apparent loss of energy during a radioactive process known as beta decay. In 1930, Austrian physicist Wolfgang Pauli proposed a radical hypothesis: an almost undetectable particle was silently carrying away the missing energy. Pauli confessed to a colleague, “I have done something terrible. I have postulated a particle that cannot be detected.” This elusive particle, later named the neutrino, is characterised by its negligible mass and charge, allowing it to traverse the Earth and our bodies with minimal obstruction.
The ambitious device deployed by Cowan and Reines in early 1956 aimed to uncover what Pauli deemed impossible. In June of that year, the two physicists from Los Alamos National Laboratory sent a telegram to Pauli, exclaiming, “We are pleased to inform you that we have detected neutrinos.”
Unlocking the Secrets of the Universe
This discovery shifted the focus to a broader question: if nuclear reactions produce neutrinos, could these particles be used to observe the nuclear fireworks within stars, including our Sun? This posed a significant challenge: how could one detect particles from distant stars when they can pass through nearly all matter undetected? It was suspected that capturing a particle which rarely interacts with matter would require an enormous volume of material for it to collide with, all while being shielded from other forms of radiation. Consequently, scientists sought to construct some of the largest, deepest, and most sophisticated experimental traps in scientific history—then simply wait.
In the 1960s, Raymond Davis Jr. and his colleagues at Brookhaven National Laboratory placed a tank 1.5 kilometres underground in the Homestake mine in South Dakota, filling it with nearly 400,000 litres of a chlorine-based cleaning fluid called perchloroethylene. On the rare occasions when a passing neutrino struck a chlorine nucleus, it would transform into a radioactive form of argon that could be detected and quantified. Over a 25-year span, the experiment recorded only about one-third of the predicted number of solar neutrinos, giving rise to what became known as the solar neutrino problem.
Advancements in Neutrino Detection Technology
It took decades to resolve this issue, aided by even more complex experiments. At the depths of the Kamioka mine in Japan, Masatoshi Koshiba constructed a different detector called Kamiokande, which utilised 3 million litres of ultra-pure water. Here, neutrinos occasionally interacted with the atomic nuclei of the water, resulting in the generation of an electron that moved so quickly it produced a flash of light known as Cherenkov radiation, which was then captured by detectors.
Kamiokande and Koshiba confirmed Davis’s deficiency, and subsequent detectors, including the larger Super-Kamiokande and the Sudbury Neutrino Observatory in Canada, elucidated the discrepancy. Neutrinos exist in three different “flavours” (electron, muon, and tau) and can oscillate, or change, between these states. This phenomenon necessitates that neutrinos possess mass, a concept that prior physical laws had not predicted. These flavours refer to types or varieties of the same particle.
Exploring New Frontiers in Particle Physics
The latest neutrino detectors continue this tradition of ambitious goals and remarkable results. The IceCube Neutrino Observatory, located beneath the Amundsen-Scott Station at the South Pole, employs Antarctic ice instead of water to detect these particles. Through its observations, scientists have constructed a map of the Milky Way composed entirely of neutrinos and traced some of these high-energy cosmic particles back to active galaxies powered by supermassive black holes. Meanwhile, KM3NeT, the Cubic Kilometre Neutrino Telescope located at the bottom of the Mediterranean Sea, has detected the highest-energy cosmic neutrino recorded to date, though its origin remains unknown.
The oscillation of neutrinos and the myriad mysteries they present have spurred a new generation of detectors. The Jiangmen Underground Neutrino Observatory (JUNO) in China commenced operations in 2025 and published unprecedented measurements of neutrino oscillation in June 2026. Additionally, the Hyper-Kamiokande in Japan and the Deep Underground Neutrino Experiment (DUNE) in the United States are slated to begin operations by the end of this decade.
Revealing the Secrets of the Elusive Neutrino
Thanks to these audacious experiments, the particle that Pauli believed could never be captured has gradually been revealing its secrets. The formula for discovery has remained unchanged for seven decades: think big, delve deep, and exercise patience.
The Super-Kamiokande facility in Japan rises twelve stories high and is filled with detectors capable of capturing the light flashes produced by neutrino interactions. Typically filled with ultra-pure water, this image was taken during maintenance activities. The various flavours of neutrinos that traverse the Super-Kamiokande detector create characteristic patterns on the detector walls, showcasing the trails of a muon neutrino on the left and an electron neutrino on the right—two fundamental particles within the Standard Model of particle physics.
Located 2.1 kilometres underground in the Creighton mine in Ontario, Canada, the Sudbury Neutrino Observatory (SNO) was filled with heavy water, which contains deuterium instead of regular hydrogen atoms. Its findings provided evidence that neutrinos can change, or “oscillate,” between different types. A panoramic view of the detector reveals a 12-metre diameter acrylic sphere containing 1,000 tonnes of ultra-pure heavy water at its core.
Innovative Approaches to Neutrino Research
SNO+ succeeded the Sudbury Neutrino Observatory in the Creighton mine, reusing some of its predecessor’s equipment. However, instead of heavy water, it was filled with linear alkylbenzene (LAB), which is commercially used to manufacture liquid soap. LAB produces signals that are 50 times brighter than those generated by heavy water. Hanging from the ceiling is the experiment’s director, Art McDonald.
The Borexino neutrino detector, buried near Abruzzo, Italy, at the Gran Sasso National Laboratory, provided the first direct evidence that the Sun fuses hydrogen into helium through multiple pathways. In the image above, a researcher examines a photomultiplier tube, which amplifies the light generated by neutrino interactions.
These digital optical modules (DOM), roughly the size of a basketball, serve as the main sensors for the IceCube Neutrino Observatory at the South Pole. Scientists strung over 5,000 DOMs onto cables like pearls on a necklace, then used jets of hot water to bury them up to 2.5 kilometres beneath the Antarctic ice. In this case, the ice performs the same function as liquids in other detectors: a neutrino collision creates a high-speed electron, and the resulting flash is detected by the DOMs.
Future Prospects in Neutrino Research
This data visualisation displays a neutrino propagating through the buried chains of DOMs in the IceCube Neutrino Observatory. The event, which occurred in 2010, corresponds to a high-energy neutrino generated by sources within the Milky Way. The size of each sphere corresponds to the brightness of the signal detected by each DOM, with colours indicating the arrival time: red for the earliest and blue for the latest.
Staff are deploying digital optical monitors for the underwater neutrino detector KM3NET, which is still under construction. Around 200,000 optical sensors will be anchored to the seabed of the Mediterranean Sea at a depth of approximately 3,500 metres off the coast of Sicily, with a smaller installation near Toulon, France. The sensors will detect the Cherenkov light generated by secondary particles resulting from neutrino interactions with water or surrounding rocks.
The Jiangmen Underground Neutrino Observatory (JUNO) in China, shown here under construction in 2023, is currently the largest neutrino detector in the world. It began collecting data in August 2025, with one of its primary objectives being to decipher the still unresolved mystery of the mass of each type of neutrino.
The Deep Underground Neutrino Experiment (DUNE) will generate trillions of neutrinos and send them 1,300 kilometres deep for study as they interact with two detectors: one located 60 metres underground at the Fermi National Accelerator Laboratory, and the other at a depth of 1.5 kilometres at the Sanford Underground Research Facility in Lead, South Dakota. Smaller-scale prototypes of the detectors, known as ProtoDUNE, are currently being tested at CERN, the particle physics laboratory straddling the border between Switzerland and France.