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Ray Davis heralded as neutrino pioneer

Ray Davis, winner of the Nobel Prize in Physics in 2002, takes a swim in the Homestake mine, circa 1971. (Photo courtesy of Brookhaven National Laboratory)

A tank filled with 100,000 gallons of dry-cleaning fluid and tucked into a corner of Lead's Homestake gold mine for two-and-a-half decades was the forerunner to the science now blossoming at the underground research facility.

The experiment designed by Ray Davis Jr. detected neutrinos from the sun, although only at a third the rate scientists had predicted.

"Upon collision with a neutrino, a chlorine atom transforms into a radioactive isotope of argon, which can then be extracted and counted," reads Wikipedia entries about Davis and the experiment. "A big target deep underground was needed to account for the very small probability of a successful neutrino capture, and to prevent interference from other forms of solar radiation.

"Davis investigated the detection of neutrinos by inverse beta decay, the process by which a neutrino brings enough energy to a nucleus to make certain stable isotopes into radioactive ones."

In describing Davis' work, The New York Times offered this explanation of neutrinos: "The sun, as a result of the fusion reactions that generate light and energy, continually throws out bountiful numbers of neutrinos; billions of solar neutrinos a second are flying through this computer screen right now. But neutrinos are notoriously taciturn particles, only rarely colliding with other particles."

Dubbed the Homestake Experiment, Davis' project had first been tried in a more shallow mine in Ohio but detected no neutrinos. That location did not provide the shield against cosmic rays offered by the hard rock at Homestake.

"When he started doing this, people thought he was almost crazy," said John Wilkerson, now the lead scientist on one of the experiments operating at the Sanford Underground Research Facility at Homestake. "It took 20 years of experimentation to show he was right."

When Davis and partner John Bahcall did detect neutrinos at Homestake but counted only a third the number that had been predicted, the scientific community assumed they had done something wrong. Fairly recently, other scientists have proved "neutrino oscillation" and shown there are three "flavors" of neutrinos. Davis' Homestake Experiment was sensitive to only one of those flavors.

Davis was awarded the Nobel Prize in Physics in 2002. The Davis Campus, underground in the old mine, is home to research facilities now situated where that huge tank of dry-cleaning fluid captured neutrinos. Scientists working there call it "hallowed ground" but say Davis would no longer recognize the once rocky cavern after extensive renovations.

The Homestake Experiment ran continuously from 1970 through 1994. Davis died at age 91 in June 2006.