Homestake experiment

The Homestake experiment was an underground solar-neutrino detector in South Dakota that used chlorine atoms to measure electron neutrinos. Its unexpectedly low event rate helped establish the solar neutrino problem.

Chlorine-37: A stable isotope of chlorine whose nucleus can capture an electron neutrino and become argon-37. Its neutrino-capture reaction was the experiment’s target process.
Solar neutrino problem: The historical discrepancy between observed solar neutrino rates and predictions from solar models. Homestake supplied the first persistent evidence of the deficit.
Raymond Davis Jr.: An American chemist who pioneered radiochemical neutrino detection and led the Homestake experiment. He designed and operated the detector over several decades.
Kamiokande: A Japanese water-Cherenkov detector that observed solar neutrinos and later measured neutrinos from Supernova 1987A. Its independent solar-neutrino measurements confirmed a deficit relative to expectations.
Neutrino: An electrically neutral elementary particle that interacts through the weak force and gravity. Homestake was built to detect this elusive particle from the Sun.
Argon-37: A radioactive argon isotope produced when chlorine-37 captures an electron neutrino. Homestake inferred neutrino captures by extracting and counting this product.
Solar neutrino flux: The number of solar neutrinos passing through a unit area per unit time. Homestake’s low capture rate raised questions about the predicted flux reaching Earth.
John N. Bahcall: An American astrophysicist who calculated solar-neutrino fluxes and helped develop the experiment’s theoretical basis. His predictions made the observed rate discrepancy quantifiable.
GALLEX: A gallium-based radiochemical experiment that measured low-energy solar neutrinos in Italy. It tested solar-neutrino predictions in an energy range inaccessible to Homestake.
Electron neutrino: The neutrino flavor associated with the electron and produced in several solar nuclear reactions. The chlorine reaction in Homestake was sensitive to this flavor.

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