Raymond Davis jnr: Raymond Davis jnr, who won the Nobel Prize in physics in 2002 for capturing solar neutrinos in a tank deep underground in a South Dakota mine, providing the conclusive evidence that the sun is powered by nuclear fusion, has died at his home in Blue Point, New York, aged 91.
His achievement in capturing the wispy, extraordinarily elusive solar neutrinos was, according to the Swedish Royal Academy of Sciences, "considerably more difficult" than sifting through the entire Sahara desert to find a few key grains of sand.
When Davis received word of his prize, he said he was "embarrassed" to be singled out because the work had required the efforts of a large number of physicists, chemists and others, like virtually any other major physics project undertaken today.
Colleagues, however, noted that Davis provided the key inspiration for the project and was the driving force in keeping it alive and thriving for 30 years.
The first hints that the sun was nuclear-powered came in the 1920s 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 - small particles that travel at nearly the speed of light, have little or no mass and no charge and that interact only extremely rarely with other matter.
Researchers calculated that only one in a trillion solar neutrinos that reached Earth would strike an atomic nucleus, the rest simply passing through unnoticed. The vast majority of researchers thought detecting them would be impossible. Davis was virtually the only one who thought otherwise.
His first attempt at detecting the elusive particles involved placing a large tank of perchloroethylene - commonly used as dry-cleaning fluid - 2,300ft underground in the Barberton limestone mine in Ohio in 1961. Burying the detector deep beneath the Earth's surface was supposed to shield it from cosmic rays and other sources of radiation, but some cosmic rays leaked through at this depth, overwhelming any potential signal from neutrinos.
Later that decade, Davis installed a 100,000-gallon tank of perchloroethylene 4,850ft below the ground in the Homestake Gold Mine in Lead, South Dakota. There, too, his initial results were unrewarding and other researchers scoffed at his efforts, arguing that he could not even detect the small number of neutrinos expected.
To prove them wrong, Davis synthesised 100 atoms of radioactive argon, added them to the perchloroethylene, then successfully re-extracted them. Eventually, their observation techniques were refined and 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.
However their number was only about a third of the total expected by scientists, creating what came to be known as the "solar neutrino problem". Ultimately, other researchers concluded that about two-thirds of the solar neutrinos changed into a different type of neutrino during their passage to Earth - a type that could not interact with chlorine.
Those neutrinos ultimately were observed, but no one would have looked for them had Davis not demonstrated that neutrinos could be detected in the first place.
Raymond Davis jnr was born on October 14th, 1914, in Washington, DC. He often cited his father's influence in leading him to the design of his own experimental equipment.
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. Upon graduation, he entered the army and spent most of the war years observing chemical weapons tests at the Dugway Proving Grounds in Utah.
After the war, he worked for Monsanto until 1948, when he was invited to join the newly created Brookhaven National Laboratory on Long Island, where he spent the rest of his active career. Shortly after his arrival, he met Anna Torrey, who worked in the biology department. They married that year and lived in the same house on Long Island for more than 50 years.
He officially retired from Brookhaven in 1984, but wasn't ready to give up measuring solar neutrinos, so he transferred administration of the project to the University of Pennsylvania and became a research professor there. The work continued until the late 1990s, when the Homestake mine closed.
On the occasion of Davis's Nobel prize, the late John Bahcall said he was "not only an extraordinary scientific person, but also an extraordinary human being. Ray treats the janitor in the laboratory with the same friendliness and respect that he does the most senior scientist and for that, he is loved by his colleagues."
He is survived by his wife Anna, three sons and two daughters.
Raymond Davis: born October 14th, 1914, died May 31st, 2006