The Homestake Experiment (1968): Catching Neutrinos From the Sun

The Homestake experiment, led by Raymond Davis Jr., first tried in 1968 to catch neutrinos from fusion in the Sun’s core. It found at most a third of the predicted number. That shortfall, the solar neutrino problem, lasted 30 years and revealed that neutrinos have mass.

The question

Hans Bethe’s 1939 theory of stellar fusion said the Sun shines by turning hydrogen into helium. Those reactions should also release neutrinos, tiny particles that barely interact with matter and fly straight out of the Sun’s core. Light from the core takes a long, roundabout path to the surface, but neutrinos arrive at Earth minutes later, carrying news of the fusion as it happens. Could anyone detect them, and would the number match the theory?

What the paper found

• The experiment set an upper limit on the flux of solar neutrinos of about one-third of the theoretical prediction.
• Either the models of the Sun were wrong, or something was happening to the neutrinos on their way to Earth.
• That mismatch became known as the solar neutrino problem.

How they did it

Detecting neutrinos means waiting for an extremely rare collision. Davis’s detector was a tank holding about 100,000 gallons of perchloroethylene, a dry-cleaning fluid rich in chlorine, placed deep in the Homestake gold mine in South Dakota. The rock overhead shielded it from cosmic rays, which would otherwise swamp the signal.

Very occasionally a solar neutrino hits a chlorine atom and turns it into a radioactive argon atom. The team periodically flushed the argon out of the tank and counted it, a few atoms at a time, by watching for their radioactive decays. From the number of argon atoms they inferred how many neutrinos had passed through.

Why it mattered

This was the first attempt to look directly inside a star, and it opened neutrino astronomy. It also created one of the great puzzles of 20th-century physics. Astrophysicists checked and rechecked their solar models; physicists checked the experiment. Neither seemed wrong. Raymond Davis Jr. shared the 2002 Nobel Prize in Physics for pioneering contributions to astrophysics, in particular the detection of cosmic neutrinos.

What we know now

The puzzle was solved in 2002, when the Sudbury Neutrino Observatory showed that solar neutrinos change flavor on their way to Earth. The Sun makes the predicted number of neutrinos, but many arrive as types the chlorine tank could not see. Neutrinos that change flavor must have mass, a discovery beyond the Standard Model of particle physics. Neutrino astronomy has since grown to include IceCube’s high-energy neutrinos from beyond the solar system, detected by the IceCube Neutrino Observatory at the South Pole. For how fusion shapes a star’s life, see our guide to stars.

Who wrote it

Raymond Davis Jr. led the experiment with Don S. Harmer and Kenneth C. Hoffman. The physicist John Bahcall later told the whole story in “Solving the Mystery of the Missing Neutrinos”, describing a puzzle that scientists struggled with for four decades before it was solved between 2001 and 2003.

Read the original

Davis, R., Jr., Harmer, D. S. and Hoffman, K. C. (1968). “Search for Neutrinos from the Sun.” Physical Review Letters 20, 1205-1209. doi:10.1103/PhysRevLett.20.1205

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