50 years of neutrinos
In 1965, Ray Davis began building an experiment deep in the Homestake mine with the hope of counting neutrinos, subatomic particles produced in fusion reactions inside stars. Using a 100,000-gallon tank full of perchloroethylene, or dry cleaning fluid, Davis predicted that when neutrinos interacted with the chlorine atoms, they would change into argon atoms, which he and his team would detect.
Davis?s fascination with neutrinos began at Brookhaven in 1948 when he read a review paper by H.R. Crane. ?Thus began a long career of doing just what I wanted to do and getting paid for it,? he wrote in an autobiographical sketch for the Nobel Prize committee.
Davis?s research took him to Brookhaven?s Graphite Research Reactor in New York and the Savannah River Plant in South Carolina. Then, in the late 1950s, he built a pilot experiment 2,300 feet underground in a limestone mine near Akron, Ohio. In 1965, he went deeper still?to Homestake?s 4850 Level.
His experiment worked. In the mid-70s, Davis saw the telltale signs of neutrinos in the tank. But there was a problem?despite more than two decades of research, Davis found only one-third of the neutrinos predicted by his collaborator, astrophysicist John Bahcall, leading to the ?solar neutrino problem.?
?The solar neutrino problem caused great consternation among physicists and astrophysicists,? Davis wrote. ?My opinion in the early years was that something was wrong with the standard solar model; many physicists thought there was something wrong with my experiment.?
The ?problem? lead scientists at underground laboratories from around the world to seek the answer to this riddle. Eventually, the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, solved the mystery. As they travel through space neutrinos oscillate, or change flavors, between electron, muon and tau neutrinos. The SNO detector is sensitive to all flavors, but Davis?s experiment detected only the electron type. In 2002, he won a share of the Nobel Prize in Physics for his work.
A lot has changed since Davis moved to Homestake 50 years ago. ?To my surprise, a whole new field of neutrino physics has developed in directions I never imagined in the Homestake days,? Davis wrote in 2002. One has only to look at the neutrino research at Sanford Lab now and future experiments to grasp just how far science has come in its search to understand this most elusive particle.