Raymond Davis died last week at age 91. He was an experimental physicist par excellence; in 2002 he received a Nobel Prize for his work on detecting solar neutrinos. Experimentalists are a funny breed: they have to understand theories well enough to figure out how to attack them at their most vulnerable points, and they have to understand technology well enough to figure out how to design apparatus to measure things that have never been seen before.

Ray Davis realized that everybody thought they understood how stars work — nuclear fusion — but nobody had actually taken a look into the center of a star and observed this happening. Merging hydrogen atoms into heavier elements creates neutrinos, subatomic particles that scarcely interact at all with ordinary matter. By putting tons of chlorine-rich cleaning fluid deep in an underground chamber to shield it from cosmic rays, Davis calculated that he could, just barely, sense the neutrinos flooding out from the core of the Sun and thereby get data about how the Sun really worked. Every month a few dozen chlorine atoms in the cleaning fluid should get changed into argon atoms, if theory was correct. Kenneth Chang writes in a New York Times obituary published on 2 June 2006:

"Ray was the most optimistic person you could ever encounter," said Kenneth Lande, a professor of physics at the University of Pennsylvania and a collaborator on the Homestake experiment. "For Ray, this was a challenge. The greater the challenge, the more fun it was to attack it."

Dr. Davis was meticulous, Dr. Lande said, willing to tackle almost every challenge from other physicists. Dr. Lande said that William A. Fowler, an astrophysicist at the California Institute of Technology, asked Dr. Davis if he could inject 100 argon atoms into the detector and then pull them out, to demonstrate that his detection method worked. Argon, a gas, was flushed out by a stream of helium bubbles and then trapped in ultracold charcoal. The number of atoms were counted by observing the radioactive decay. "Most people would have said, 'Come on,' " Dr. Lande said. "Ray went off and he in fact made 100 atoms, put them in and took them out."

Ray Davis's equipment worked well, but it only detected a third as many solar neutrinos as theory said should be there. Figuring out what happened to the missing two-thirds took decades more ...

(cf. LatePhysicists (24 Sep 2000), Nobel Neutrinos (13 Oct 2002), ...)

TopicScience - TopicProfiles - 2006-06-03

(correlates: Nobel Neutrinos, PullPush, GlassDarkly, ...)