Raymond Davis Jr.

Raymond Davis Jr. (14 October 1914 – 31 May 2006) was an American chemist. He was the first to detect neutrinos from the Sun — and for three decades he held fast to a result that nobody could explain.

In 2002 he received the Nobel Prize in Physics for it, at the age of 88.

A chemist among physicists

Davis studied chemistry and took his doctorate in physical chemistry at Yale in 1942. After war service he came to Brookhaven National Laboratory in 1948 and looked for a task there.

That he was not a physicist is not incidental to this story. The task he set himself was regarded among physicists as hopeless. For a chemist it was a difficult separation problem — and separation problems are there to be solved.

The number with which it all began

John Bahcall calculated how many neutrinos the Sun delivers. From that it followed how many argon atoms would arise in a tank of perchloroethylene: about one every two days.

One atom every two days, in 615 tonnes of liquid, among about 10³⁰ other atoms. Most specialists considered it impossible to extract so few atoms and on top of that to be sure that none had been lost.

The set-up is described in detail on the page on the Homestake experiment.

The care that made the difference

What distinguished Davis was not the idea but the checking.

He put known, tiny quantities of argon into the tank and got them out again — the yield was over 90 per cent. This demonstrated that his procedure really does find almost every atom.

He checked whether his tank responded to anything other than solar neutrinos. Among other things he placed a set-up next to a nuclear reactor and there found, as expected, nothing — because reactors emit antineutrinos, and these cannot trigger the nuclear reaction that was used.

And he repeated his measurement over decades, run after run, each lasting two months.

The result that nobody wanted

From 1968 the figures were available: Davis found about one third of the rate calculated by Bahcall.

The obvious explanations were that Bahcall was calculating wrongly or Davis was measuring wrongly. Both checked for decades, and neither found an error.

These were not easy years. An experiment whose result everyone takes for a mistake is a hard situation to bear — especially when it takes decades before that changes.

The late confirmation

In 2002 the Sudbury Neutrino Observatory showed that the missing neutrinos had not disappeared but had merely been converted — into species to which Davis's chlorine tank could not respond in principle.

His measurement had been right from the start. It had simply measured something other than everyone assumed, and precisely therein lay the discovery.

In the same year he received the Nobel Prize, jointly with Masatoshi Koshiba.

After-effect

Davis died in 2006. His experiment ran from 1968 to 1994 — twenty-six years, in which he counted about two thousand argon atoms in total.

In the same tunnels of the Homestake mine the far detector of DUNE is being built today, with 68,000 tonnes of liquid argon. The mountain in which a single man counted atoms houses, a generation later, one of the largest undertakings in particle physics.

Davis's real legacy, however, is an attitude: not to explain away a result that does not fit the picture — but to keep testing it until it turns out that it was not the measurement that was wrong, but the picture.

Homestake experiment — his facility
John Bahcall — the prediction
Solar neutrino problem — the finding
Masatoshi Koshiba — his co-laureate

Sources

R. Davis Jr., D. S. Harmer, K. C. Hoffman: Search for Neutrinos from the Sun, Physical Review Letters 20, 1205 (1968).
Nobel Foundation: Nobel Prize in Physics 2002, citation and Nobel lecture.