Ray Davis (1914-2006)

photo courtsey: Brookhaven National Laboratory, Long Island

Ray Davis, the physics Nobel laureate of 2002, passed away on Wednesday due to complications from Alzheimer's disease at his home in New York. He was 91.

Raymond Davis Jr. was born on Oct. 14, 1914 in Washington, D.C. He received bachelor's and master's degrees in chemistry from the University of Maryland and a doctorate in physical chemistry from Yale University in 1942. Davis spent most of his career (since 1948) at the Brookhaven National Laboratory on Long Island and was a pioneer of neutrino astrophysics.

Neutrinos are small particles that travel at nearly the speed of light, have little or no mass and no charge and interact only extremely rarely with other matter. Neutrinos took centerstage in astrophysics in 1920s. The first hints that the sun was nuclear-powered appeared at that time when experiments showed that a helium atom, which contains two protons and two neutrons, has less mass than four hydrogen atoms -- essentially four protons.

British astrophysicists concluded that the fusion of four hydrogen atoms into a helium atom in the interior of the sun could release substantial amounts of energy, plus two neutrinos. Researchers calculated that only one in trillion solar neutrinos that reached Earth would strike an atomic nucleus, the rest simply passing through unnoticed.

At that time many researchers believed that detection of neutrinos which hardly ever interacts with matter would be impossible. Davis was virtually the only one who thought otherwise. In an audacious experiment, Davis set out to detect these theoretically predicted solar neutrinos. He filled a giant tank with 600,000 litres of "cleaning fluid" (chemists usually call them perchloroethylene) -- 2,300 feet underground in the Barberton Limestone Mine in Ohio in 1961. His team was looking for the occurence of very rare events when a solar neutrino would interact with a chlorine atom to produce radioactive argon. But when he added these signals up, the team found that the Sun was producing only about a third of the neutrinos it should have been based on the best solar models available.

The main problems were coming from cosmic rays and other sources of radiation which were leaking through even the depth of 2300 feet and was causing error in his estimate.

In his second attempt in late 60s Davis installed a 100,000-gallon tank of perchloroethylene 4,850 feet below the ground in the Homestake Gold Mine in Lead, S.D. This time his team synthesized 100 atoms of radioactive argon, added them to the perchloroethylene, then successfully re-extracted them. After much tedious refinements in their observational techniques the detector began observing neutrinos. Over the 30 years of experimentation, about 2,000 neutrino events were observed, demonstrating conclusively the occurrence of nuclear fusion in the sun.

The story didn't end there. In fact a new story began. The number of neutrinos detected by Davis group was only about 1/3rd of the total expected by scientists. This is what came to be known as the "solar neutrino problem." The theoretical models predicting the numbers were developed principally by the late John Bahcall of the Institute for Advanced Study in Princeton, who died in August last year. During those decades strong suspicions were raised at Davis' experiment by several scientists that it was at fault.

Ultimately, other researchers concluded that both Davis and Bahcall were right: the neutrinos produced by the Sun, which are all "electron-type" neutrinos, were oscillating into muon- and tau-like neutrinos on their journey to Earth. These could not interact with chlorine. Those two other types of neutrinos ultimately were observed, but no one would have looked for them had Davis not went forward (despite criticism and scoffing of other researchers who doubted his experiments), fighted with all kinds of challenges offered by the nature and conclusively demonstrated that neutrinos could be detected in the first place.

From time to time in order to advance science to a new level and to open new doors of the universe, we need true leaders who can come forward and accept the challenge and fight against all kinds of adversities offered by nature or man-kind and finally establish the truth and expand the horizon. Raymond Davis Jr was one such leader and he'll be remembered as the physicist who could solve the mystery of the sun.