Ray Davis counts solar neutrinos
October 29, 2008
On Sept. 11, 2000, Homestake Mining Co. announced it would close its gold mine in Lead, S.D., the following year. Within two weeks, neutrino physicists were proposing to convert the giant gold mine into an underground deep underground laboratory. They knew about Homestake because of Ray Davis, an unassuming nuclear chemist from Brookhaven National Laboratory. His breakthrough experiment at Homestake already had changed physics forever.
In the early 1960s, Davis conceived a way to peer into the innermost workings of the sun. His experiment would count subatomic particles called neutrinos, which are produced in fusion reactions all stars, including in the sun. The heart of his experiment would be a 100,000-gallon tank full of chlorine atoms, in the form of perchloroethyline (more commonly known as dry cleaning fluid.) Davis predicted that when the occasional neutrinos from our own sun collided with a chlorine atoms in the tank, they would change chlorine atoms into argon37. Using helium and charcoal, Davis devised a way to count the handful of argon37 atoms produced each month. Davis knew, however, that if he placed his experiment on the surface of the earth, it would have been overwhelmed by a shower of particles caused by cosmic radiation. Deep underground, his detector would be protected from this cosmic noise.
Neutrinos are unaffected by depth, for a couple of reasons. First, neutrinos are unimaginably small—about a million times smaller than electrons. Second, neutrinos have no charge, so they don't interact with charged particles. As a result, most neutrinos pass right through the earth.
Homestake Mining Co. agreed to excavate a cavern for Dr. Davis, at a location on the 4,850-foot level. The Davis experiment was far removed from the Homestake ore body and from the mine's infrastructure of tunnels and shafts, but it was close enough to the Yates Shaft to provide relatively easy access for researchers. The experiment itself was installed in 1965, and soon began taking data—that is, counting solar neutrinos.
There was, however, a problem. Ray Davis detected only about a third of the number of neutrinos predicted by theorists. At first, the scientific community thought the experiment must be wrong, but Davis persisted. So did his results. The discrepancy became known as "the solar neutrino problem." Finally, in the 1990s, underground experiments in Japan and Canada vindicated Davis. Neutrinos come in three types, or "flavors"—electron, muon and tau—and Davis was measuring only the electron neutrinos produced in the sun. Later experiments, however, suggested that the wily neutrino can change its flavor, or oscillate, as it flies through space a near the speed of light.
The solar neutrino detector at Homestake had turned out to be remarkably accurate, and in 2002 Ray Davis was awarded a share of the Nobel Prize for Physics for his work.
Today, the Large Underground Xenon (LUX) dark matter detector is being installed in the same cavern excavated for Davis in the 1960s. Another cavern has been excavated nearby for the Majorana Demonstrator experiment, which will search for neutrinoless double-beta decay. A third cavern has been carved out for infrastructure, such as chillers and an electrical substation. The entire—30,000 square feet, including 10,000 square feet of finished lab space—has been named the Davis Campus in honor of a nuclear chemist who had a vision about how to pierce the inner workings of the sun.