3. Ghostlike Neutrinos

    • A neutrino, or little neutral one, has no electrical charge, almost no mass, and travels at nearly the velocity of light.

    • The neutrino was proposed in 1933 by Wolfgang Pauli to explain radioactive beta decay, and first observed in 1956 by Clyde Cowan and Frederick Reines in Project Poltergeist involving a nuclear reactor on Earth. Reines received the 1995 Nobel Prize in Physics for this accomplishment.

    • The fusion reactions that generate the Sunís energy also create great quantities of electron neutrinos that pass effortlessly through both the Sun and the Earth; billions of solar neutrinos are streaking right through you every second.

    • The amount and energies of the neutrinos produced by the Sun depend on the nuclear fusion reactions in its core.

    • Computer models are used to determine the flux of solar neutrinos expected at the Earth.

    • Massive subterranean neutrino detectors have captured a small number of the electron neutrinos generated by nuclear reactions in the Sun. These instruments detected only one-third to one-half the expected amount of electron neutrinos, a discrepancy known as the solar neutrino problem.

    • Solar neutrinos were first detected in 1967, in an experiment designed by Raymond Davis, Jr. It consisted of a huge vat of cleaning fluid placed deep underground in the Homestake Gold Mine near Lead, South Dakota. Solar neutrinos occasionally convert chlorine in the cleaning fluid into argon, but the amount of argon produced, and therefore the number of solar neutrinos, is less than that expected. Davis received the 2002 Nobel Prize in Physics for his pioneering detection of solar neutrinos.

    • A second solar neutrino detector, located in the Kamioka zinc mine in Japan, uses pure water to detect the neutrinos and to show that they come from the Sun. The 2002 Nobel Prize in Physics was shared by Masatoshi Koshiba for this experiment, which also detected a very few neutrinos from the supernova SN 1987A.

    • By measuring the internal velocity of sound waves, scientists have taken the temperature of the Sunís energy-generating core, showing that it agrees with model predictions, at 15.6 million kelvin, and apparently ruling out any astrophysical solution to the solar neutrino problem.

    • Observations with the Super Kamiokande neutrino detector indicate that atmospheric neutrinos, generated by cosmic rays, change type and oscillate between flavors, therefore possessing a small mass.

    • The solar neutrino problem has now been resolved by observations at the Sudbury Neutrino Observatory, which uses heavy water. These observations indicate that solar neutrinos switch between types, or flavors, on their way from the center of the Sun to the Earth. Some of the Sunís electron neutrinos transform into other types of neutrinos, the muon or tau neutrinos, which were undetectable by the pioneering neutrino experiments.

    • Beams of neutrinos generated by nuclear reactors on Earth have been sent through the planet and detected on its other side, confirming that the solar electron neutrino deficit is due to the transformation of some electron neutrinos into other types of neutrinos.

Copyright 2010, Professor Kenneth R. Lang, Tufts University