Frederick Reines

American physicist who, with Clyde Cowan, made the first direct detection of the neutrino at the Savannah River reactor in 1956. Awarded the 1995 Nobel Prize in Physics. Later a pioneer of high-energy neutrino detection and astrophysical neutrinos.

Contributions

Legacy

Reines moved neutrino physics from theory to experimental reality. He founded the Department of Physics at UC Irvine in 1966 and led it for decades. The 1995 Nobel Prize was awarded to him alone — Cowan having died in 1974 — 'for the detection of the neutrino.' The citation honored work that had been completed four decades earlier and still required an eyewitness among the laureates.

Early career and Los Alamos

Frederick Reines was born in Paterson, New Jersey in 1918. He earned his PhD from NYU in 1944 and joined the Manhattan Project at Los Alamos under J. Robert Oppenheimer. After the war he remained at Los Alamos for nearly two decades, where he first met Clyde Cowan in the early 1950s.

Project Poltergeist

The initial concept for detecting the neutrino — proposed by Reines and Cowan around 1951 — was to use a nuclear explosion as a brief, intense neutrino source and drop a detector by parachute near ground zero. Los Alamos director Norris Bradbury approved the idea, but the two physicists themselves reconsidered, realizing that a sustained source such as a nuclear reactor would give them integration time and repeatability.

The project, which they nicknamed Project Poltergeist, was moved first to the Hanford production reactor in Washington State in 1953, and then — after excessive cosmic-ray background at Hanford — to the Savannah River Plant in South Carolina in 1955. The detector consisted of 200 liters of cadmium-doped water as the target sandwiched between two tanks of liquid scintillator each instrumented with photomultiplier tubes. The inverse-beta-decay signature of prompt positron annihilation followed by delayed neutron capture on cadmium was unambiguous.

The 1956 detection

On 14 June 1956, Reines and Cowan cabled Pauli at ETH Zurich:

We are happy to inform you that we have definitely detected neutrinos from fission fragments by observing inverse beta decay of protons. Observed cross section agrees well with expected six times ten to the minus forty-four square centimeters.

Pauli replied by telegram the following day: “Everything comes to him who knows how to wait.”

The paper describing the result appeared in Science in 1956 under the title “Detection of the free neutrino: a confirmation.”

Later work

Reines left Los Alamos for Case Western Reserve in 1959 and then for UC Irvine in 1966, where he founded the Department of Physics. His research turned successively to:

• Atmospheric neutrinos at deep-underground sites in India (Kolar Gold Fields, 1965) and South Africa (East Rand, 1967) — the first observations of neutrinos produced by cosmic rays
• Solar neutrinos, through chlorine, gallium, and Cherenkov programs
• Proton decay searches at the Irvine-Michigan-Brookhaven (IMB) detector in Ohio
• Supernova neutrinos, through IMB’s detection of SN 1987A

The IMB collaboration, with Reines as a founding member, detected 8 of the 24 neutrinos observed worldwide from SN 1987A — confirming the thermal core-collapse signal and opening the field of extragalactic neutrino astronomy.

The 1995 Nobel Prize

Reines shared the 1995 Nobel Prize in Physics with Martin Perl (for the discovery of the tau lepton), “for pioneering experimental contributions to lepton physics.” The citation specifically honored Reines’s detection of the neutrino nearly four decades earlier. Clyde Cowan, who had died in 1974, was not eligible. Reines accepted the prize alone, crediting Cowan explicitly in his Nobel lecture.

Reines died in Orange, California, on 26 August 1998, aged 80.

External references

• en.wikipedia.org
• nobelprize.org