The Nobel Prize in Physics 2002
Raymond Davis Jr., Masatoshi Koshiba, Riccardo Giacconi
Raymond Davis Jr.
Born: 14 October 1914, Washington, DC, USA
Died: 31 May 2006, Blue Point, NY, USA
Affiliation at the time of the award: University of Pennsylvania, Philadelphia, PA, USA
Prize motivation: "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos"
Field: Neutrino astrophysics
I was born in Washington, D.C. on October 14, 1914. My father was a photographer at the National Bureau of Standards. A self-educated man, he never finished high school, but, in his career at the National Bureau of Standards, he made many useful inventions, and eventually became chief of the Photographic Technology Section. His early influence led me in the direction of individual experimentation and designing my own apparatus. My mother, Ida Rogers Younger, was a native of the state of Virginia. She taught me to enjoy music, although she never succeeded in making me a performer. It was to please her that I spent several years as a choirboy, in spite of my inability to carry a tune. A bit later in life, I took pleasure in attending outdoor concerts at the Watergate, in the days before air traffic grew heavy enough to drown out the music.
My constant companion in childhood was my brother Warren, only 14 months my junior. Together we played street games on summer evening, paddled a canoe on the Potomac and, after my release from the choir, spent many weekends rifle-shooting with our father. In high school and college, I gathered a number of medals for marksmanship, but I have long since abandoned this activity, having concluded that the world would be a better place with fewer sharpshooters.
As my brother and I grew older, our interests diverged. He headed for a military career, while I became more interested in science. My father encouraged my interest, bringing me chemicals for my basement experiments, and helping me become a reasonably good photographer. My favorite reading matter was Smithsonian reports on many phases of science, obtained at my local branch library. Washington offered many educational opportunities for curious young minds.
I was educated in the Washington public schools, and attended the University of Maryland as a day student, graduating in 1938 with a degree in chemistry. After working for the Dow Chemical Company in Midland, Michigan for a year, I returned to the University of Maryland to take a Master's degree, before going on to Yale to pursue a doctorate. In 1942, I received my Ph.D. in physical chemistry, and immediately entered the Army as a reserve officer. Most of my war years were spent at Dugway Proving Ground in Utah, observing chemical weapons tests and, in my spare hours, exploring and photographing the surrounding territory, which included the Great Salt Lake and geologic evidence for its much larger predecessor, Lake Bonneville.
Upon my discharge from the Army in 1945, I went to work at the Monsanto Chemical Company's Mound Laboratory, in Miamisburg, Ohio, doing applied radiochemistry of interest to the Atomic Energy Commission. In the spring of 1948, I was able to join the newly created Brookhaven National Laboratory, which was dedicated to finding peaceful uses for atomic energy. In my first few months at Brookhaven, I lived at the Lindenmere, a summer hotel which had been leased by the laboratory to provide housing for new arrivals. It was there that I met my future wife, Anna Torrey, who was also employed at Brookhaven, in the Biology Department. Since this was a seaside community, I decided to build my own sailboat. This notion was viewed with scorn by most of my acquaintances but, with Anna's help, I built a 21-foot sloop, the Halcyon, which gave our family many years of pleasure. Now in the hands of her third owner, the Halcyon still sails the Great South Bay. Anna and I were married in late 1948 and, over the next fifteen years, five children were born to us: Andrew, a senior scientist at the University of Chicago who studies meteorites to learn about stars and the early history of the solar system, lives in River Forest, Illinois; Martha Kumler, a private tutor of high school students, lives in Honeoye Falls, New York; Nancy Klemm, a homemaker and restorer of windows in old houses, lives in Webster Groves, Missouri; Roger, a mechanical technician working on the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, lives in Center Moriches, New York; and Alan, an engineer with Boeing, lives in Seattle, Washington. Among them, they have given us eleven grandchildren. We have lived in the same house, in Blue Point, New York, for over fifty years.
My first act, on arriving at Brookhaven, was to report to the chairman of the Chemistry Department, Richard Dodson, and ask him what I was expected to do. To my surprise and delight, I was advised to go to the library, do some reading and choose a project of my own, whatever appealed to me. Thus began a long career of doing just what I wanted to do and getting paid for it. In the library, I read a 1948 review paper by H. R. Crane in Reviews of Modern Physics which led me to decide on an experiment in neutrino physics, a field in which little was known at the time, and which seemed well-suited to my background in physical chemistry.
In early experiments, I attempted to detect neutrinos from a reactor, using the chlorine-argon detection method suggested by Bruno Pontecorvo (in 1946). In this method, a 37Cl atom reacts with a neutrino to make an 37Ar atom. Argon is a noble gas and is easy to separate chemically from a large amount of chlorine-rich solvent. It is radioactive with a half-life of 35 days and can be counted with a gas-filled proportional counter. A first attempt, exposing a 1000-gallon tank of carbon tetrachloride at the Brookhaven Graphite Research Reactor, failed to detect any signal, as the neutrino flux at this reactor was too small to affect a target of this size. Furthermore, a reactor emits antineutrinos, and the Pontecorvo method only detects neutrinos. It was not known at that time that the two particles were not identical. Later, I built larger experiments, using one of the Savannah River reactors as the neutrino source. I eventually set a limit on the neutrino flux that was a factor of 20 below the antineutrino flux measured by Reines and Cowan in their elegant experiment that won Fred Reines his Nobel Prize.
Other early interests included the development, with Oliver Schaeffer, of a method of geological dating using 36Cl in surface rocks. With the later advent of accelerator mass spectrometry, this has become a useful tool in geochemistry, but our counting techniques were not sensitive enough to make the method work. We turned to measuring 36Cl in meteorites. Measuring the 36Cl radioactivity and the total accumulated decay product, 36Ar, in a meteorite allowed us to determine how long the meteorite had been exposed in space. Our interest in meteorite exposure ages continued for many years. We also worked on measuring cosmic-ray production of 37Ar and 39Ar in a variety of freshly fallen meteorites. Our greatest success in this work was with the Lost City meteorite. The track of this meteorite was photographed as it fell, allowing its orbit to be determined. Our measurement of radioactive argon isotopes allowed us to deduce the cosmic ray intensity gradient in the inner solar system. During the era of the moon landings, I was involved in measuring 37Ar, 39Ar, tritium and 222Rn in lunar rocks and in the lunar atmosphere (trapped in the rock boxes brought back by the astronauts). During processing of the Apollo 12 samples, one of the glove boxes in Houston leaked and I had the interesting experience of being quarantined with the astronauts and a few other unlucky scientists for two weeks until it was clear that we were not infected with any lunar diseases.
Following the Savannah River experiments, I began thinking about detecting neutrinos from the Sun. The first step was a pilot experiment located 2,300 feet underground in the Barberton Limestone Mine, near Akron, Ohio. Observing neutrinos from the Sun had the potential of testing the theory that the hydrogen-helium fusion reactions are the source of the Sun's energy. In the 1950s, however, the proton-proton chain of reactions was believed to be the principal neutrino source, but this chain only emitted low-energy neutrinos, below the threshold of the chlorine-argon reaction.
A new measurement of the nuclear reaction 3He+4Heg7Be+g by Holmgren and Johnston in 1958, suggested that one of the terminal reactions in the proton-proton chain would produce energetic neutrinos which could be measured by the chlorine-argon radiochemical method. Encouraged by these developments, and with the support of the Brookhaven National Laboratory and the U.S. Atomic Energy Commission, I built a much larger experiment in the Homestake Gold Mine in Lead, South Dakota. The detector itself consisted of a 100,000-gallon tank filled with perchloroethylene, a solvent most commonly used for dry cleaning of clothing. The experiment was located nearly a mile underground, at the 4850 foot level of the mine. Initially, we observed no solar neutrino signal and expressed our results only as upper limits. Subsequent refinements in technique and, particularly, in counting methods, continued over the years, producing a solar neutrino signal approximately one-third of the expected flux from the standard solar model calculated by John Bahcall. This was the genesis of the so-called "solar neutrino problem".
The solar neutrino problem caused great consternation among physicists and astrophysicists. My opinion in the early years was that something was wrong with the standard solar model; many physicists thought there was something wrong with my experiment. Years of measurements produced consistent answers and many tests showed that there were no problems with experimental procedures. Many distinguished physicists suggested explanations for the low solar neutrino flux that now seem fanciful. Trevor Pinch, a sociologist, made a study of how scientists responded to the solar neutrino problem. The disagreement between the measured solar neutrino flux and that predicted by the standard solar model was confirmed for energetic 8B neutrinos by the Kamiokande II experiment in the late 1980s and for the lower energy pp neutrinos by the gallium experiments GALLEX and SAGE in the middle 1990s. Only recently, observations at the Sudbury Neutrino Observatory (SNO) in the Inco Nickel Mine in Sudbury, Ontario, Canada, have indicated that, indeed, the total number of solar neutrinos emitted agrees with the standard solar model prediction, but that two-thirds of the neutrinos change in the course of their journey to the Earth into other flavors (m and t neutrinos), a phenomenon known as neutrino oscillation. Only electron neutrinos can be detected with the Cl-Ar radiochemical method.
I retired from Brookhaven in 1984, but wasn't ready to give up measuring solar neutrinos, because I thought it important that the Homestake experiment measure the solar neutrino flux at the same time as new solar neutrino experiments. I transferred administration of the Homestake experiment to the University of Pennsylvania and have been a Research Professor there since that time. The experiment continued to measure the solar neutrino flux until the late 1990s, when the Homestake Mine ceased operating.
Meanwhile, to my surprise, a whole new field of neutrino physics has developed in directions I never imagined in the Homestake days.
|Boris Pregel Prize, New York Academy of Sciences, 1957|
|Cyrus B. Comstock Prize, U. S. National Academy of Sciences, 1978|
|American Chemical Society Award for Nuclear Applications in Chemistry, 1979|
|Tom W. Bonner Prize, American Physical Society, 1988|
|Honorary Doctor of Science, University of Pennsylvania, 1990|
|W. K. H. Panofsky Prize, American Physical Society, 1992|
|Beatrice M. Tinsley Prize, American Astronomical Society, 1995|
|George Ellery Hale Prize, American Astronomical Society, 1996|
|Honorary Doctor of Science, Laurentian University, 1997|
|Bruno Pontecorvo Prize, Joint Institute for Nuclear Research, Dubna Russia, 1999|
|Wolf Prize, 2000|
|Honorary Doctor of Science, University of Chicago, 2000|
|U. S. National Medal of Science, 2001|
|Nobel Prize in Physics, 2002|
From Les Prix Nobel. The Nobel Prizes 2002, Editor Tore Frängsmyr, [Nobel Foundation], Stockholm, 2003
This autobiography/biography was written at the time of the award and later published in the book series Les Prix Nobel/Nobel Lectures. The information is sometimes updated with an addendum submitted by the Laureate.
Raymond Davis Jr. died on May 31, 2006.
Copyright © The Nobel Foundation 2002
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