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American physicist and co-leader, with Frederick Reines, of the 1956 experiment at the Savannah River reactor that first directly detected the neutrino. Cowan's death in 1974 preceded the 1995 Nobel recognition of the discovery.
Contributions
Co-detection of the neutrino (1956)
Cowan was co-principal investigator with Reines on Project Poltergeist at Los Alamos and later at Hanford and Savannah River. He led much of the detector design and the analysis of the delayed-coincidence signature that established the detection unambiguously.
Liquid-scintillator detector technology
Cowan and Reines pioneered the use of cadmium-doped water targets surrounded by organic liquid scintillator for antineutrino detection — a technique still used by every large reactor-neutrino experiment today, from KamLAND to JUNO.
Cosmic-ray astrophysics
After leaving Los Alamos, Cowan moved to the Catholic University of America, where he continued experimental work on cosmic rays and atmospheric radioactivity until his death.
Legacy
Cowan's role in the 1956 neutrino detection was equal to Reines's, but his early death made him ineligible for the 1995 Nobel Prize. Reines publicly and repeatedly credited Cowan for the shared achievement. The delayed-coincidence technique that Cowan helped develop remains the standard for reactor neutrino detection seven decades later.
Background
Clyde Lorrain Cowan Jr. was born in Detroit in 1919. He served as an officer in the United States Army Air Forces during the Second World War. After the war he earned a PhD from Washington University in St. Louis in 1949 and joined Los Alamos Scientific Laboratory, where he began a collaboration with Frederick Reines on detector technology.
Project Poltergeist and Savannah River
Cowan and Reines began discussing neutrino detection around 1951. Their first concept was to detect neutrinos from a nuclear explosion; after reconsidering they moved to a reactor source. Beginning at the Hanford reactor in 1953 and moving to Savannah River in 1955, they deployed a detector of cadmium-loaded water sandwiched between two tanks of liquid scintillator.
Cowan was responsible for much of the detector assembly and for the careful accounting of backgrounds that allowed the team to claim an unambiguous detection. The signature — a prompt positron signal followed by a delayed neutron-capture signal a few microseconds later, with timing and energy consistent with inverse beta decay — emerged from months of running and cross-checks.
After Los Alamos
Cowan left Los Alamos in 1957, moving to the Catholic University of America in Washington, DC, where he continued research on cosmic rays and atmospheric radioactivity. He remained involved in neutrino discussions and played an advisory role in later detector projects.
He died on 24 May 1974 at age 54, twenty-one years before the Nobel committee recognized the 1956 achievement.