William A. Fowler (1911–1995)

Professor William A. ('Willy') Fowler died on 14 March 1995 at age 83 in Pasadena, California, where he had lived and worked for 62 years at the California Institute of Technology. Fowler shared the 1983 Nobel Prize in Physics with Subrahmanyan Chandrasekhar (who also died this year). He will long be remembered not only for his founding role in the science of nuclear astrophysics, for his many contributions to our understanding of nuclear and neutrino processes in stars, supernovae, and the Big Bang, for his measurement, calculation, and systematic tabulation of nuclear data for astrophysical application, but also for the simple joy and infectious enthusiasm he brought to his work.

Fowler was born in Pittsburgh, Pennsylvania, 9 August 1911, but spent his youth, after age 2, in Lima, Ohio. There, frequenting the yards of the Pennsylvania Railroad and the Lima Locomotive Works, he developed an early affection for things mechanical and electrical. After attending Lima Central High School, Willy graduated from Ohio State University in 1933 with a degree in Engineering Physics and, at age 22, during the height of the Depression, was admitted to graduate school at Caltech. He thus began work with Charles Lauritsen, partly for support, but soon for the research opportunities afforded in the new Kellogg Radiation Laboratory. At first, Fowler only operated the large high voltage x-ray tube that was used for experimental work in cancer therapy (as did Lauritsen's other graduate students for support), but when a second tube was adapted for charged particle acceleration, he began research into the induced radioactivity of light elements. The first work used the electroscopes developed by Charlie, known later as Lauritsen electroscopes, but Willy constructed a cloud chamber of his own and went on to study the properties of "mirror nuclei" such as¹¹ C,¹³ N,¹⁵ O ,¹⁷ F, finding some of the earliest evidence for the charge independence of the nuclear force. Fowler completed his PhD on this work in 1936 and, during the next three years, as a Research Fellow at Kellogg, explored with Lauritsen the cross sections for the interaction of carbon and fluorine with protons. This work was made possible by the construction by Fowler and Charlie and Tommy Lauritsen of a high pressure gas-insulated Van de Graaf accelerator, but it was also given added impetus in 1938 when the draft of Bethe's paper on the CNO-cycle in stars appeared.

However, World War II intervened and, from 1941 through 1945, Willy and the Lauritsens were chiefly occupied with the war effort. Just prior to this, in 1940 Willy married Ardianne Olmstead and embarked on a working honeymoon during which they toured Van de Graaf accelerators in the United States. During the first 8 months of 1941, he worked in Washington D.C. at the Carnegie Institution of Washington's Department of Terrestrial Magnetism on proximity fuses for anti-aircraft shells, both radio and photoelectric. But when it became apparent that problems with the rockets were paramount, both he and the Lauritsens returned to Caltech to work with others on ordnance. Out of this effort eventually arose the Naval Weapons Center at China Lake. In 1943 Fowler and the Lauritsens shifted the focus of their work from Pasadena to Los Alamos where they worked on the conventional explosives and fuse aspects of the Manhattan Project. All three were at the Alamagordo test and, in 1944, Willy went briefly to the South Pacific to observe the use of rockets in combat.

At the end of World War II, Fowler and the Lauritsens returned to nuclear physics, with Fowler opting to start precision, low-energy cross-section measurements on nuclear reactions known to be important in the stars, beginning again with¹² C(p,gamma)¹³ N. Equipment had to be constructed and the first results were not published until 1950, but by 1953, it was clear that the CN cycle was not responsible for most of the energy production in the sun and work began on the nuclear reactions of the p-p chain of hydrogen burning reactions.

Following visits by Ed Salpeter in 1951 and Fred Hoyle in 1952, nuclear studies at Kellogg expanded to include helium burning and the production of heavier elements, especially by the triple-alpha process. Alvin Tollestrup and Fowler and Charlie Lauritsen had measured the binding energy of⁸ Be and found it unstable, conclusively showing that there was not only an unstable mass gap at A = 5, but one at 8 as well. Armed with knowledge of the⁸ Be binding energy, Salpeter went on to formulate the well known reaction process responsible for helium burning in stars. But it was still too inefficient.

Hoyle met with the Kellogg group after a public lecture on cosmology. His work on red giant stars had led him to suspect the existence of a 0⁺ resonance in the¹² C nucleus near the threshold of⁸ Be* + alpha, necessary in order that the triple-alpha process proceed at a reasonable rate. There was some skepticism. The state had not been found in an earlier search carried out by Malm and Beuchner at MIT, but Ward Whaling and other "Kelloggites" searched for and soon located a state in¹² C at the predicted energy (the spin and parity were later determined to be 0⁺). Thus began a fruitful collaboration between Hoyle and Fowler which led Willy to spend the following year (1954-55) in Cambridge. There Willy also met Geoff and Margaret Burbidge who, at the time, were working on observations and theory related to the synthesis of the very heavy elements (above iron) in stars. Fowler invited both back to Pasadena and obtained temporary positions for them.

All four authors were thus in place for the famous Reviews of Modem Physics (1957) paper that would become known simply and universally as B² FH. Besides the confluence of key characters, other necessary catalytic events were taking place including, all within a few years, the publication by Suess and Urey of abundance systematics, the discovery by Merrill of technetium in S-stars, and the release of low energy neutron capture cross sections measured during the war. Fowler would always remember the publication of B² FH as the high point in his career. It was not the first time that the possibility of making heavy elements in stars had been suggested. Eddington, Hoyle, and others had published speculations along these lines. Nor was it the only delineation of nucleosynthetic processes published that year. Al Cameron in Canada discussed similar work in his Chalk River Report. But it was the first time a coherent theory encompassing the origin of all the elements was put forward with sufficient observational, calculational, and nuclear physical detail to be considered a full theory of stellar nucleosynthesis. B² FH catalogued eight nuclear processes as responsible for the synthesis of the elements with A greater than 4 — hydrogen and helium burning, the alpha-process, the e-process, the r, s, and p-processes and the x-process. Nowadays the alpha-process has been supplanted by carbon, neon, and oxygen burning; and the e-process is better understood as a combination of hydrostatic and explosive silicon burning (with iron made as⁵¹ Ni, not⁵⁶ Fe), the x-process is now attributed to cosmic ray spallation, but much remains the same.

Based upon their understanding of the origin of the heavy transuranic isotopes, Hoyle and Fowler, in 1959, applied the technique of radioactive dating, or "nuclear cosmochronology," to obtain a value around 10 billion years for the age of the Galaxy. Fowler continued to refine this estimate with new data and calculations the remainder of his life, though the answer never strayed far from 10-12 billion years.

During 1960-1963, Hoyle and Fowler worked on two projects of great consequence. One was the nature of the energy sources for radio galaxies for which they proposed, in advance of the discovery of the cosmological nature of quasars, the existence of point sources of energy of 10⁵ to 10⁸ solar masses. They had in mind supermassive stars and most of the rotation and appreciated (following conversations with Feynman) the critical role of general relativity in these objects. But they missed the important possibility that the object might be a rotating supermassive black hole. In this period they also worked on the evolution of massive stars and the nature of supernovae, laying the groundwork for our modem view of these events. Type I supernovae were identified as the product of the explosion of white dwarf stars; Type II were the collapse of the unstable iron core in massive stars. While their models for Type II were crude by modem standards, their identification of the critical role of neutrino losses was described with some accuracy.

It was about this time (1963) that Bahcall, Fowler, Iben and Sears published the first detailed solar model prediction of the neutrino flux from the sun following measurements of the key cross sections for³ He(alpha,gamma)⁷ Be and⁷ Be(p,gamma)⁸ B. Fowler (and contemporaneously Al Cameron) had previously written to Davis immediately following the Holmgren and Johnson (1958) measurement of a much larger than expected cross section for³ He(alpha,gamma)⁷ Be, pointing out the implications of a much stronger solar neutrino signal from⁸ B, but it was not until the early 60s decade that Davis made plans for a much larger 100,000 gallon chlorine experiment, constructed eventually in the Homestake Gold Mine. Fowler offered encouragement and assisted in obtaining funding (1964) for this experiment.

In 1967, Fowler and Hoyle, with Willy's new research fellow Bob Wagoner, published their famous study of nucleosynthesis in the Big Bang. Jim Peebles had already calculated the cosmological synthesis of helium in 1966, but in this new work, the abundances of all isotopes lighter than carbon were considered. It was shown that for the conditions appropriate to our early universe, the isotopes¹ H,² H,³ He,⁴ He and⁷ Li could (and presumably all were) produced in amounts consistent with their cosmic abundances.

Throughout the 1970s and early 1980s, Fowler and several colleagues, notably Georgeanne Caughlan of Montana State University, worked on one of Fowler's more enduring legacies: the systematization and tabulation of all the thermonuclear reaction rates below A = 24 that are important for energy generation and nucleosynthesis in the stars. The first of these compilations had been published in the Astrophysical Journal in 1962. Continual revisions and expansions occurred, the last tabulation appearing in Atomic Data and Nuclear Data Tables in 1988. With other colleagues, Fowler also carried out theoretical calculations of reaction rates for strong, weak, and electromagnetic reactions on heavier nuclei up to mass 60 that are still the current standard in nucleosynthesis theory. He particularly emphasized the role of a thermal population of excited nuclear states in determining the correct value for all these rates, an effect that had usually been omitted in previous calculations.

During the 1970s Fowler was also especially active in public service, serving on the National Science Board (1968-74); the Space Science Board (1970-1973; 1977-1980); the Council of the National Academy of Sciences (1974-1977); and as President of the American Physical Society (1976). He received many honors and prizes in addition to the 1983 Nobel Prize in Physics, among them the Presidential Medal of Merit (1948); Guggenheim Fellowships to visit Cambridge, England (1954-1955; 1961-1962); the Liege Medal (1955), the Bonner Prize of the APS (1970); the National Medal of Science (1974); the Eddington Medal of the RAS (1978); and the Bruce Gold Medal of the ASP (1979).

Throughout these latter years, Fowler remained especially interested in the synthesis of the elements in the early universe, particularly in inhomogeneous cosmologies, continuing to revise and update the appropriate nuclear reaction rates. He actively pursued these interests, engaging colleagues in conversation on this work until a few hours before his death.

In 1988, Fowler's wife of 48 years passed away and he married Mary Dutcher late in 1989. In addition to his wife Mary and two daughters by his first wife, Mary Fowler Galowin and Martha Fowler Schoenemann, he leaves behind 50 graduate students, and hundreds of former research fellows, colleagues, and friends, all of whom will sorely miss him.

Additional information on the life and science of Willy Fowler can be found in the book Essays in Nuclear Astrophysics, edited by Barnes, Clayton, and Schramm and published in 1982 by Cambridge University Press. There also exist, at the American Institute of Physics, transcriptions of taped interviews of an autobiographical nature recorded in 1972 and 1973 and the Caltech Archives have transcripts of interviews with Fowler that are part of the Caltech Oral History Project, as well as part of his archival collection.

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