Raymond Davis Jr.

American chemist and physicist whose Homestake solar neutrino experiment ran from 1968 to 1994 and first established the solar neutrino deficit. Shared the 2002 Nobel Prize in Physics with Masatoshi Koshiba and Riccardo Giacconi.

Contributions

The Homestake chlorine experiment

Davis deployed 615 tons of tetrachloroethylene — cleaning fluid — 1,478 m underground in the Homestake gold mine in South Dakota. The reaction νe + ³⁷Cl → ³⁷Ar + e⁻ produced argon atoms that Davis extracted chemically at a rate of about one argon atom every three days. The extraction, counting, and background-subtraction techniques he developed were chemically as demanding as any experiment in physics.

Establishing the solar neutrino problem

From 1968 onward Homestake consistently measured a νe flux about one third of the Standard Solar Model prediction. For thirty years this deficit resisted explanation. Davis's careful experimental work — repeatedly verified and cross-checked — ensured that the deficit could not be dismissed as an experimental artifact, forcing a theoretical resolution that ultimately came in the form of neutrino oscillations.

Radiochemical neutrino detection

Davis pioneered the radiochemical method — in which a neutrino-induced rare nuclear transmutation is detected by extracting the daughter isotope and counting its decays. The same principle was later used at SAGE and GALLEX with gallium targets sensitive to pp-chain neutrinos. These radiochemical experiments provided the essential low-energy data that supplemented and validated real-time Cherenkov and scintillator observations.

Legacy

Davis's thirty-year vigil at Homestake turned a chemical anomaly into one of the great unsolved problems of physics, and created the experimental framework that SNO and Super-K would eventually use to resolve it. The 2002 Nobel Prize citation honored him for 'pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos.'

Training and Brookhaven

Raymond Davis Jr. was born in Washington, DC in 1914. He earned his PhD in physical chemistry from Yale in 1942 and served in the US Army during the Second World War. In 1948 he joined Brookhaven National Laboratory, where he began work on radiochemical detection techniques for weak nuclear transitions. He remained affiliated with Brookhaven until the mid-1980s, and held an appointment at the University of Pennsylvania through his retirement and beyond.

The Homestake experiment

The chlorine-to-argon reaction $ν_{e} +^{37} Cl \to^{37} Ar + e^{-}$ had been proposed as a neutrino detector by Bruno Pontecorvo in 1946. Davis took it up in the late 1950s and ran a first small-scale effort at the Brookhaven reactor, setting an upper limit on reactor antineutrino capture — the wrong sign of the reaction, as it turned out, but formative for the technique.

In 1965 Davis began construction of the Homestake experiment: 100,000 gallons (615 tons) of tetrachloroethylene in a tank 4,850 feet below the Black Hills of South Dakota. The chemistry was straightforward in principle and extraordinarily demanding in practice. Each month he would:

Flush the tank with helium to collect the few dozen $^{37}$ Ar atoms produced
Trap the argon cryogenically
Load it into a proportional counter
Count $^{37}$ Ar decays (half-life 35 days) to extract the neutrino capture rate

The background — single atoms of cosmogenically produced argon — was controlled by depth, by materials choice, and by statistical subtraction. A single run took months.

The deficit

Homestake reported in 1968 a first measurement of about 3 solar neutrino units, where one SNU equals $1 0^{- 36}$ captures per target atom per second. The Standard Solar Model of John Bahcall predicted about 8 SNU. The deficit was already established at the first result and persisted through three decades of measurements.

The discrepancy, widely called the “solar neutrino problem”, generated an enormous theoretical literature. Proposed resolutions included incorrect nuclear cross-sections, a cooler solar core, energy loss through hypothetical particles, and — finally — neutrino oscillations. Davis’s own instinct was that the deficit was real and neutrino-physical in origin, but he resisted making strong claims beyond what his data directly supported.

Vindication

The SAGE and GALLEX gallium experiments confirmed a deficit at lower energies. Kamiokande and Super-Kamiokande confirmed the deficit at higher energies through elastic scattering. SNO, using heavy water, finally decomposed the total flux into flavor components and proved that the missing $ν_{e}$ had transformed into $ν_{μ}$ and $ν_{τ}$ . The deficit Davis had measured for thirty years was a quantitative match to the oscillation picture.

Davis received the 2002 Nobel Prize in Physics at the age of 88, sharing it with Masatoshi Koshiba (for Kamiokande’s observation of solar neutrinos and of SN 1987A) and Riccardo Giacconi (for X-ray astronomy). He died on 31 May 2006 at age 91.