Prize in Physics: Raymond Davis, Jr. for Contributions to Neutrino
Research and Our Understanding of the Sun
Raymond Davis, Jr., research professor of physics, is a winner of
the 2002 Nobel Prize in physics, the Nobel Foundation announced
last Tuesday in Stockholm.
Davis shares the honor with Masatoshi Koshiba of the University
of Tokyo and Riccardo Giacconi of Associated Universities Inc. in
Nobel Prize to Dr. Davis and Dr. Koshiba was awarded in recognition
of their ground-breaking research into the emission of neutrinos
produced by nuclear fusion reactions in the center of the sun. The
observation of these neutrinos demonstrated conclusively that the
sun is powered by the fusion of hydrogen nuclei into helium nuclei.
awarding of the Nobel Prize to Professor Raymond Davis is a great
moment for this extraordinary researcher, for the University and
for the world of science," said President Judith Rodin. "His
pathbreaking work has given rise to the discipline of neutrino astrophysics,
a field that has already told us much about our own sun and other
astronomical objects and may yield equally stunning insights into
the nature of matter itself.
Davis is a truly outstanding scientist and an inspiration to his
peers worldwide. We offer him and his colleagues our deepest and
most heartfelt congratulations."
source of the sun's energy has challenged scientists for centuries.
In the 19th century it was assumed that the sun's energy resulted
from its gravitational collapse. But with the advent of radioactive
dating in the beginning of the 20th century, the age of the Earth
was determined to be roughly 4 billion years. Only nuclear reactions
within the sun could supply energy for such a long time; gravitational
collapse could provide solar power only for a few tens of millions
of years, insufficient to have fostered the biological evolution
of species on Earth.
the interior of the sun is opaque to all modes of observation other
than neutrinos, directly observing these nuclear reactions proved
enormously challenging," said Dr. Kenneth Lande, a Penn professor
of physics who has collaborated with Dr. Davis since the 1970s.
"Ray Davis conceived, built and ran the first experiment to
detect neutrinos from the core of the sun."
in 1967, Dr. Davis detected solar neutrinos by observing the neutrino-induced
conversion of chlorine atoms into argon atoms. The observed rate
was one argon atom produced every two days in a 615-ton neutrino
detector Dr. Davis constructed a mile underground in the Homestake
Gold Mine in Lead, S.D. The subterranean location served to screen
out cosmic radiation that would otherwise produce too many distracting
signals. Since neutrinos rarely interact with matter, they passed
easily through the Earth to reach the detector, essentially a 100,000-gallon
tank filled with perchloroethylene, a common, chlorine-rich dry
cleaning fluid that could be manufactured cheaply in large quantities.
number of neutrinos Dr. Davis detected reaching the Earth was only
one-third that predicted by detailed models of nuclear reactions
within the sun. One of the explanations for this discrepancy was
that some electron neutrinos produced in solar fusion reactions
convert into other neutrino species--specifically, muon and tau
neutrinos--during the eight-minute flight from the solar core to
experiments at the Kamiokande and Superkamiokande detectors in Japan
and the Sudbury Neutrino Observatory in Sudbury, Ontario--research
in which Penn scientists have played a pivotal role--have confirmed
phenomenon of neutrino flavor conversion is one of our first views
of a new, previously unknown class of particle interactions that
may help in understanding the evolution of the universe," said
Dr. Lande, who has been responsible for the operation of the South
Dakota detector since 1990.
conversions require that neutrinos have mass and that the masses
of the various neutrino species are different. By combining the
results of the various neutrino observations, we have established
that the masses of these neutrinos are amazingly small, of the order
of a billionth the mass of the electron, but not zero as previously
thought. However, because of the enormous number of neutrinos in
the universe, the total mass of neutrinos is comparable to the total
mass of all the visible matter of the universe."
Davis was an adjunct professor of astronomy at Penn from 1973 to
1983 and at Dr. Lande's behest, Dr. Davis returned to Penn in 1985
as a research professor. He had been a research collaborator in
chemistry at the Brookhaven Lab in Upton, N.Y. for 37 years, from
1948-1985. His research was supported by the Department of Energy
from 1965 to 1984; since 1985 his work at Penn and the operation
of the Homestake neutrino detector have been supported by the National
Davis, 87, a resident of Blue Point, N.Y., received a B.S. in 1937
and an M.S. in 1939, both from the University of Maryland. In 1942
he earned a Ph.D. from Yale University. He served in the U.S. Army
during World War II and worked at Monsanto Chemical Company for
two years before joining Brookhaven Lab in 1948.
this year Dr. Davis received the 2001 National Medal of Science
from President George W. Bush (Almanac
May 21, 2002). He is also a member of the National Academy of
Sciences and the American Academy of Arts and Sciences.
researchers who have contributed significantly to Dr. Davis' experiment
include Kenneth Hoff-man, Daniel Harmer, John Evans, John Galvin,
Keith Rowley, R.W.Stoenner and Bruce Cleveland at Brookhaven Lab
and Dr. Kenneth Lande, Paul Wildenhain, James Distel, C.K. Lee,
Alicia Weinberger and Timothy Daily at Penn. In addition, Jack Ullman
from Lehman College of CUNY and Edward Fireman of the Harvard-Smithsonian
Observatory participated in these experiments.
the comments made at the Press Conference, see www.upenn.edu/almanac/v49/n08/pressconf.html.
Additional information about Dr. Davis is available at www.sas.upenn.edu/home/news/davis.html.
Almanac, Vol. 49, No. 8, October 15, 2002