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How A Pair of Old Mines Helped Win a Nobel Prize in Physics

How do you avoid cosmic background radiation from skewing your research? Do your research a kilometre or more underground.

by Matthew Braga
Oct 6 2015, 2:30pm

Image: Roy Kaltschmid, Berkeley Lab/Flickr, Berkeley Lab

On Tuesday it was announced that Arthur B. McDonald of Queen's University in Canada, and Takaaki Kajita of the University of Tokyo in Japan, won the Nobel prize for physics "for the discovery of neutrino oscillations, which shows that neutrinos have mass."

According to the Nobel prize committee, "the discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe."

And a pair of old mines on opposite sides of the world helped them do it.

The Nobel prize-winning work was done in two parts: half at the Sudbury Neutrino Observatory (SNO) in Canada, and half at the Super-Kamiokande detector in Japan. At the SNO, researchers studied neutrinos that came from the sun, while at the Super-Kamiokande detector, researchers studied neutrinos created by reactions between cosmic rays and the Earth's atmosphere.

Image: Johan Jarnestad/The Royal Swedish Academy of Sciences

The SNO site is unique, having been built 6,800 feet—just over 2 kilometers—underground in a defunct mine near Sudbury, Ontario (in INCO's old Creighton nickel mine, to be exact). Super-Kamiokande, meanwhile, was built in an old zinc mine a kilometer underground. The depth at both facilities is intended to shield the observatories' neutrino detectors from cosmic radiation at the earth's surface, which can cause false readings.

The neutrino detectors at both facilities consist of a tank of specially treated water surrounded by light detectors. When a neutrino collides with an atomic nucleus or an electron in the water, a faint flash of blue light results, which is known as Cherenkov light. "The shape and intensity of the Cherenkov light reveals what type of neutrino it is caused by, and from where it comes," a background paper on the discovery reads.

Image: Johan Jarnestad/The Royal Swedish Academy of Sciences

Over the course of McDonald and Kajita's research, it was found that the expected number of neutrinos did not match the actual number of neutrinos detected—the conclusion being that the expected neutrinos had undergone a transformation on their way to the detector. And the only way the neutrinos could have undergone a transformation was if they had mass.

The discovery is important because the "Standard Model of the innermost parts of matter had been immensely successful and for over twenty years it had resisted all experimental challenges," the backgrounder reads, referring to the theory in particle physics that governs nuclear interactions between particles. "But the model requires that neutrinos are massless. The experiments have thus revealed the first apparent crack in the Standard Model. It has become obvious that the Standard Model cannot be the complete theory of how the fundamental constituents of the universe function."