2002 Nobel Prize in Physics

Raymond Davis Jr. (University of Pennsylvania) and Masatoshi Koshiba (Univ. of Tokyo) shared half of this years Nobel prize in physics. Their citation reads "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos".

Until we started to understand the nature of the nuclear forces, the origin of solar energy was a mystery. Because of the fundamental work in Nuclear Physics done by Hans Bethe, George Gamow, Willy Fowler, Ray Davis, M. Koshiba their many collaborators, we now know that Sun's energy originates from a series of nuclear reactions going on in its core. These nuclear reactions emit neutrinos which travel through the material in the Sun very easily. Solar models, developed by John Bahcall and others, predict the solar neutrino flux which reaches the Earth.

In the 1960's Davis and collaborators started detecting these neutrinos by capturing them on chlorine nuclei in a big vat of cleaning fluid. To reduce the background coming from other cosmic rays this experiment was placed in the Homestake Gold Mine in Lead, South Dakota. Davis's experiment and several other similar experiments performed later using gallium nuclei were sensitive to only one flavor of neutrinos, the electron neutrino. In the 1980's water-based detectors were developed which are sensitive to all neutrino flavors. Masatoshi Koshiba, who shared the 2002 Nobel prize in physics with Davis, was instrumental in building the Kamioka light-water detector in Japan. This detector not only first detected other neutrino flavors, but also observed for the first time neutrinos coming from the collapse of a supernova (Supernova 1987A) along with the IMB detector which was operating in the Morton Salt Mine in Ohio and the BAKSAN detector in Russia.

Prof. Masatoshi Koshiba

The crucial confirmation of Davis' results came last year from the Sudbury Neutrino Observatory (SNO) operating in INCO's Creighton Mine in Sudbury, Ontario. SNO uses heavy-water as well as light-water to detect the total solar neutrino flux. The SNO results are completely consistent with the Homestake measurements of Davis and are in agreement with the solar model predictions. Nuclear physicists continue to play a leadership role in the SNO experiment. These accomplishments of the SNO scientists were recently recognized by the APS: SNO Institute Director Art McDonald was awarded the 2003 Bonner Prize. One of the gradute students in the experiment, Karsten Heeger (whose thesis supervisor was Hamish Robertson, a U.S. co-spokesperson of SNO), was awarded the 2003 American Physical Society Division of Nuclear Physics Dissertation Award.

Knowledge of nuclear physics was essential in building chlorine, gallium, and SNO detectors and in providing a low-background environment to operate them. Currently efforts are under way to convert the Homestake mine or one of the several other possible sites into a National Underground Science Laboratory. Neutrino physics and nuclear astrophysics are major research areas of nuclear physics. Following the path of Bethe, Fowler, and Davis, nuclear physicists will continue exploring our universe.

Related links:

A biography of Ray Davis Jr.
Sudbury Solar Neutrino Detector
Super-Kamiokande Official Home Page
National Underground Science Laboratory at Homestake

The other half of the prize was won by Riccardo Giacconi (Associated Universities Inc.) "for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources"

Artist's conception of a binary sytem. (NASA/HEASARC)

The Nobel Prize for Prof. Giacconi recognizes his enablement of the birth of X-ray astronnomy. This led to the discovery of neutron stars that accerete matter from a binary companion, releasing large amounts of gravitational energy. These objects periodically emit large flashes of X-rays as a result of nuclear reactions taking place in the built-up material. The exact details of what takes place awaits further illucidation of the nature of neutrons stars and the underlying nuclear physics. New nuclear physics facilities such as RIA will make a major contribution to this understanding.

The importance of X-ray sources was also recently recognized by the APS when Michael C.F. Wiescher, from the University of Notre Dame was awarded the 2003 Bethe Prize. The award was given for his contributions to the experimental foundation of nuclear astrophysics, especially the delineation of the processes involved in explosive hydrogen burning in novae and x-ray bursters; and for providing an intellectual bridge between experimental nuclear astrophysicists and their theoretical colleagues.

Related links:

Chandra X-Ray Observatory
The Rare Isotope Accelerator, RIA

For more information:
Link to the Nobel Prize Committee web site
APS Media information for the 2002 Nobel Prize in Physics