Inside the Experimental Traps Scientists Set for Ghostly Neutrinos

Seventy years ago, the physicists Clyde Cowan and Frederick Reines took a custom-built 10-ton detector, surrounded it with heavy pb walls and bedewed sandbags, and placed it adjacent a almighty atomic reactor astatine the Savannah River Plant successful South Carolina. They called the experimentation Project Poltergeist, designed arsenic it was to drawback a ghost.

More than a 4th of a period before, physicists had been puzzling implicit wherefore vigor appeared to beryllium mislaid during a radioactive process called beta decay. Something was missing, and determination was nary known physics to explicate it. Then successful 1930, the Austrian physicist Wolfgang Pauli projected a extremist solution: A virtually undetectable particle was silently carrying the missing vigor away. “I person done a unspeakable thing,” Pauli told a friend. “I person postulated a particle that cannot beryllium detected.” It would travel to beryllium known arsenic the neutrino. Having astir nary wide and nary charge, these particles tin walk done Earth and everything connected it, including our bodies, virtually unimpeded.

The monolithic instrumentality that Cowan and Reines deployed successful aboriginal 1956 was meant to find what Pauli thought was impossible. That June, the brace of physicists from the Los Alamos National Laboratory sent Pauli a telegram: “We are blessed to pass you that we person decidedly detected neutrinos.”

Attention past shifted to a broader question. If atomic reactions nutrient neutrinos, could we usage them to adjacent astatine the atomic fireworks wrong stars, including the sun? This presented a immense challenge: How tin you perchance drawback particles shooting from distant stars if these particles tin walk done astir thing undetected? The suspicion was that detecting a particle that seldom collides with substance requires a immense magnitude of substance for it to collide with. Moreover, the substance would person to beryllium shielded from the sound of different forms of radiation. So the reply scientists came up with was to physique immoderate of the biggest, deepest, and astir exotic experimental traps successful technological past … and past wait.

In the 1960s, Raymond Davis Jr. and colleagues astatine Brookhaven National Laboratory placed a vessel 1.5 kilometers underground successful the Homestake excavation successful South Dakota and filled it with astir 400,000 liters of a chlorine-based cleaning fluid called perchloroethylene. On the uncommon juncture that a passing neutrino struck a chlorine nucleus, it would beryllium transformed into a radioactive signifier of argon that could beryllium detected and counted. The experiment, which would tally for 25 years, recovered conscionable one-third the fig of neutrinos coming from the prima that had been predicted successful theoretical models. This became known arsenic the star neutrino problem.

Decades passed earlier it was solved—by yet much monolithic experiments. Deep successful the Kamioka excavation successful Japan, Masatoshi Koshiba built a antithetic benignant of detector called Kamiokande, which utilized 3 cardinal liters of ultrapure water. In this setup, neutrinos occasionally interact with atomic nuclei successful the water. The enactment creates an electron that moves truthful fast, it generates a flash of what’s called Cherenkov light. This airy gets picked up by detectors.

Kamiokande and Koshiba confirmed Davis’ shortfall, and a second, adjacent larger detector, Super-Kamiokande, arsenic good arsenic Canada’s Sudbury Neutrino Observatory, explained the discrepancy. Neutrinos travel successful 3 antithetic “flavors” (electron, muon, and tau) and tin oscillate, oregon switch, betwixt them. To bash so, neutrinos indispensable person mass, which the laws of physics failed (and inactive fail) to predict.

Newer neutrino detectors proceed the contented of expansive ambitions and astonishing results. The IceCube Neutrino Observatory beneath the Amundsen-Scott South Pole Station uses Antarctic crystal alternatively of water. It has developed a representation of the Milky Way made up lone of neutrinos and traced these high-energy cosmic particles backmost to progressive galaxies powered by supermassive achromatic holes. On the level of the Mediterranean Sea, the Cubic Kilometer Neutrino Telescope (KM3NET) has detected the highest-energy cosmic neutrino connected record. Its root remains unknown.