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Ray Davis spent about 30 years counting single atoms of argon in a tank of cleaning fluid nearly a mile underground. He found fewer neutrinos from the Sun than theory predicted, and for decades many physicists suspected his experiment was at fault. It was not. This post follows his own account, the chemistry that made the detector work, and a short calculation on radioactive build-up.
Who was Ray Davis?
The Nobel Prize facts page records that Raymond Davis Jr. was born on 14 October 1914 in Washington, D.C., and died on 31 May 2006 in Blue Point, New York. He received a quarter of the 2002 Nobel Prize in Physics "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos".
He was a chemist. His Nobel autobiography says he earned a PhD in physical chemistry at Yale in 1942. When he joined Brookhaven National Laboratory in 1948, the chemistry chairman told him to go to the library and choose any project he liked. "Thus began a long career of doing just what I wanted to do and getting paid for it." He picked neutrinos, a field where little was known.
How can a chemist catch a neutrino?
Neutrinos barely interact with matter. The 2002 Nobel press release notes that thousands of billions pass through each of us every second unnoticed.
Davis used a method suggested by Bruno Pontecorvo in 1946. Very rarely, a neutrino hitting a chlorine-37 nucleus turns it into argon-37:
ν + ³⁷Cl → ³⁷Ar + e⁻
Argon is a noble gas, so it can be flushed out of a huge volume of chlorine-rich liquid. Argon-37 is radioactive with a half-life of 35 days, so each atom can be counted when it decays. Our post on Frederick Reines and Clyde Cowan covers the first detection of neutrinos from a reactor. Davis's early reactor tests saw nothing, partly because reactors emit antineutrinos, which this reaction does not detect.
What did the Homestake experiment find?
Davis's autobiography describes the final detector: a 100,000-gallon tank of perchloroethylene, a dry-cleaning solvent, at the 4,850-foot level of the Homestake Gold Mine in Lead, South Dakota. The press release gives its mass as 600 tonnes and says that over 30 years he captured about 2,000 neutrinos from the Sun. The first results appeared in 1968 as Search for Neutrinos from the Sun, alongside a theoretical prediction by John Bahcall and colleagues.
The signal came out at roughly one-third of the flux predicted by Bahcall's standard solar model. This was the solar neutrino problem. Davis wrote: "My opinion in the early years was that something was wrong with the standard solar model; many physicists thought there was something wrong with my experiment." His team's full analysis was published in The Astrophysical Journal in 1998.
Why does the 35-day half-life matter?
Argon-37 is made at a steady rate and decays at the same time, so it does not pile up forever. If atoms form at rate R and decay with constant λ, the number present after time t is
N = (R/λ)(1 − e^(−λt))
Worked example:
Decay constant. λ = ln 2 / 35 days ≈ 0.0198 per day.
Saturation. As t grows, N approaches R/λ ≈ 50R. Waiting forever never gives more than about 50 days' worth of production.
After 70 days (two half-lives): 1 − e^(−1.386) = 1 − 0.25 = 0.75 of the maximum.
After 105 days (three half-lives): 1 − 0.125 = 0.875.
So there is little to gain by waiting much beyond two or three half-lives before extracting. Common mistake: treating the build-up like simple decay and writing N = N₀e^(−λt). Here the source keeps adding atoms. Our post on radioactive decay and half-life covers the decay side.
How was the puzzle solved?
Davis's autobiography traces the answer. Kamiokande in Japan confirmed the shortfall for high-energy neutrinos in the late 1980s, and gallium experiments did so for low-energy ones in the 1990s. Then the Sudbury Neutrino Observatory in Canada measured all neutrino types together and found the total matched the solar model. About two-thirds had changed into other flavours on the way to Earth. Davis's chlorine method sees only electron neutrinos, so it had been right all along.
What can a student take from how Davis worked?
Use the tools you know. He brought radiochemistry to a physics problem.
Test your own method hard. Years of tests showed his procedures were sound, so he stood by the result.
Stay with a long question. He kept measuring after retiring from Brookhaven in 1984.
If build-up and decay equations still feel tangled, Fermi's AI tutor can work out what you already know and build a plan for the session.
Next step: use the formula above to find what fraction of the maximum argon-37 is present after 35 days.
