SEGRè, EMILIO GINO

(b. Tivoli, Italy, 30 January 1905; d. Lafayette, California, 22 April 1989), nuclear and particle physics, history of science.

Segrè opened the world of antimatter to physical investigation with his discovery of the antiproton, discovered the first of the elements produced by a particle accelerator, provided the critical measurements that established the need for implosion in the ignition of nuclear weapons, and provided important resources for the historical understanding of modern physics. Born Jewish in Italy, he fled because of the Fascist anti-Semitic legislation as did his mentor, Enrico Fermi, whose papers he edited.

Education Born into an industrial family, Emilio Segrè had an early interest in mathematics and physics that was nurtured by his uncle, Claudio Segrè, who was a mining engineer. He was privately educated in addition to attending public school in Tivoli and Rome. After beginning engineering studies in the University of Rome at his father’s behest, he switched to physics after meeting Fermi and sneaking into an International Physics Conference at Como in September 1927. He became part of Fermi’s group at Rome, working with Franco Rasetti, Edoardo Amaldi, and Ettore Majorana, whom Segrè recruited from the engineering school. After completing his military obligation in 1930 he studied transition lines in the atomic spectra of alkaline metals, which should have been forbidden by the laws of quantum mechanics. At Peter Debye’s suggestion, he continued these studies with the Dutch physicist Pieter Zeeman in Amsterdam, which offered superior facilities to those he had in Rome. Perhaps as a result of Zeeman’s intervention, he won a Rockefeller Foundation fellowship that allowed him to study with Otto Stern at the University of Hamburg, who set him to finish an experiment on the dynamics of space quantization and schooled him in the precise experimental techniques that had established Stern’s reputation in molecular beams. Segrè returned to Rome where he worked with Fermi on the explanation of hyper-fine structures in spectra, which they demonstrated resulted from the nuclear magnetic moment as expressed in the Fermi-Segrè formula.

Nuclear Physics Segrè’s generation entered physics at the time quantum mechanics was consolidating thirty years of discovery in atomic physics and the new field of nuclear physics was opening up. Fermi regarded this as a great opportunity to establish a leading school on the frontiers of theoretical and experimental physics. After the Rome group acquired Geiger-Müller counters, a gamma ray spectrograph, a cloud chamber for experimental nuclear studies, and a source of radium, they held an international congress on nuclear physics in Rome in October 1931, which attracted the leading European nuclear physicists. Fermi soon joined them at subsequent international congresses. He quickly made an important contribution to nuclear theory with his theory of beta decay, which explained how some elements transform into others by emitting an electron from the nucleus.

After the discovery of artificial radioactivity by Irène and Frédéric Joliot-Curie, Fermi realized that low-energy neutrons would be an effective tool of nuclear physics. Discovered in 1932 by James Chadwick, neutrons could easily penetrate atomic nuclei since they were not repelled by the positively charged protons, having no charge of their own. Using neutrons produced when alpha particles struck beryllium, the Fermi group began to activate all the elements. Segrè noticed that paraffin, which was used in experiments bombarding silver with neutrons, greatly increased the activity produced. Fermi deduced from this observation that hydrogen atoms, which were abundant in paraffin, had been struck by neutrons, slowing the neutrons down, and that the slower neutrons, contrary to accepted opinion, actually interacted more effectively with the silver atoms. Further experiments showed this to be the case in a number of elements, and a search began for effective moderators of neutrons that would slow them down to increase their effect. At the same time, a patent application was made on the process of slow-neutron activation.

In the fall of 1935, Segrè was appointed to the physics chair at the University of Palermo in Sicily. The following summer, he and his bride, Elfriede, traveled to the United States, where Segrè had spent part of the summer of 1934 at Columbia University in New York City, and visited Ernest Lawrence’s Radiation Laboratory at the University of California, Berkeley. Lawrence had used high-energy neutrons to bombard elements in a program similar to the Fermi group. Returning from Berkeley with discarded fragments of Lawrence’s early cyclotrons, Segrè investigated the radioactive substances produced in them by accelerated deuterons and found them “a true mine of radioactive substances” (Segrè, 1993). More followed from Berkeley, leading Segrè to discover a new

element, technetium, which filled the empty space in the periodic table at atomic number 43, using a molybdenum foil that had been incorporated in a beam deflector. It was the first element created artificially.

Refugee During his second visit to Berkeley in the summer of 1938, he learned that, because of his Jewish origins, his professorship at Palermo had been revoked by Benito Mussolini’s government. At Lawrence’s invitation, he became a research associate at the Radiation Laboratory. There, he worked with Glenn Seaborg to develop the field of nuclear chemistry, and participated in the discovery of plutonium and the determination of its fissile properties.

Segrè’s work with Seaborg began with a search for short-lived technetium-isotopes that resulted in the observation of a nuclear isomerism that produced technicium-99, which is used millions of times annually in medical imaging research and diagnosis. Of the many isotopes that were the object of the Radiation Laboratory’s systematic search for medically useful radiopharmaceuticals, it remains the most effective. Segrè also collaborated with Chien-Shiung Wu in a search for another element missing in the periodic table, element 61. With Robert Cornog, Dale Corson, and K. R. McKenzie, he found element 85, astatine, in 1940.

Fission and Plutonium Although Fermi and his group had supposed that the slow-neutron bombardment of uranium had produced previously unknown elements of greater atomic weight and number, for which Fermi received the Nobel Prize in 1938, the discovery of nuclear fission by Otto Hahn and Fritz Strassmann in Berlin with help from the exiled Lise Meitner at the end of 1938 made it clear that fission had produced the results: elements of approximately half the atomic weight of uranium. The discovery was quickly verified around the world, which alarmed many refugees like Fermi (who had also emigrated to the United States at about this time) and Segrè. Edwin M. McMillan at the Radiation Laboratory found that fission in uranium foils bombarded by the cyclotron produced a true transuranium element, neptunium, and this launched a search for others, led by Segrè and Seaborg. At Fermi’s suggestion, Lawrence gave priority for the project on the 60-inch cyclotron. Assisted by Arthur Wahl and Joseph Kennedy, Segrè and Seaborg found element 94, plutonium, and subsequently showed that it was fissionable with both slow and fast neutrons, making it a potential source of nuclear energy. This work, completed in May 1941, stimulated Lawrence to urge the Briggs Committee, which had been founded as a result of Albert Einstein’s letter to President Franklin Delano Roosevelt, to accelerate research leading to the making of nuclear weapons. Plutonium would supply a nuclear fuel similar to the uranium isotope U²³⁵ for such weapons, and it could be made in a nuclear reactor, which Fermi and Leo Szilard were then developing at Columbia University. This project was subsequently transferred to the University of Chicago, where the first chain-reacting nuclear pile was completed in December 1942, after Roosevelt endorsed a full-scale, top secret program to develop nuclear weapons, which became the Manhattan Project.

Lawrence’s role in the project was to develop giant mass spectrographs based on cyclotron technology to separate uranium-235 from uranium-238. Segrè set up an analytical laboratory to perform isotopic analyses of the process. In addition, he began systematic studies of spontaneous fission in uranium and plutonium and, in the summer of 1942, of fission cross sections as well. This information was vital to the success of the atomic bomb program, since an explosive chain reaction would have to be instantaneous: if the chain reaction began too early, the material would be dispersed by the explosion before it could achieve significant energy. This required several generations of fission, each new generation producing exponentially more energy. Any stray neutron could thus predetonate the material and create a “dud” rather than a bomb.

Los Alamos When Seaborg went to the University of Chicago at the end of 1941 to develop means of refining plutonium for the Manhattan Project, Segrè and his associates Owen Chamberlain and Clyde Wiegand went to Los Alamos National Laboratory at the invitation of laboratory leader J. Robert Oppenheimer to continue their investigation of spontaneous fission in uranium and plutonium. They conducted exceedingly sensitive measurements in a Pajarito canyon cabin, remote from the main technical area at Los Alamos, where other experiments produced electric fields and stray neutrons that would spoil them. The higher altitude of Los Alamos allowed more cosmic rays to initiate spontaneous fission than had been the case at Berkeley, but it still occurred only a few times a month. Unfortunately, as additional supplies of plutonium arrived from Chicago, Oak Ridge, and Hanford, the count went up.

Painstaking studies established plutonium-239 had too high a spontaneous fission rate to be assembled by firing one fraction of the critical mass into another, as was done in the uranium bomb. Segrè’s group determined this to be the result of an isotopic impurity, plutonium-240, which had not been observed in accelerator-produced plutonium but which was created in nuclear reactors by neutron bombarded plutonium-239. Since plutonium could not be purified of this contaminant in time to be of use, panic struck, and the entire laboratory was reorganized to develop a new technique for igniting the plutonium bomb, the implosion technique. Ironically, the compartmentalization of the Manhattan Project kept this news from reaching Chicago, where DuPont engineers and scientists at the Metallurgical Laboratory were designing reactors to produce plutonium at Hanford, an effort that would have been in vain had the implosion technique not been perfected at Los Alamos.

Oppenheimer seems to have appreciated the dogged determination exhibited by Segrè in measuring spontaneous fission, and sent him to the Oak Ridge laboratory where Lawrence’s calutrons had been installed to investigate the DA atomic electrons NG atomic electrons ER of accruing enough enriched uranium in one spot to set off a fission reaction unintentionally. Segrè was able to prevent a possible catastrophe by correcting the situation, which had not been anticipated in the design and construction of the plant. He also undertook a study of the effects to be expected from a nuclear explosion.

After the implosion technique was developed, it was decided that a test must be made of the resulting device at an army bombing range in central New Mexico on the Jornada del Muerto, a bleak desert between the Oscura Mountains and the Rio Grande, near the birthplace of Conrad Hilton at San Antonio, New Mexico. Segrè’s group designed a variety of instruments to diagnose the effect of the blast from the neutron and gamma radiation it produced. He observed the test with Fermi, who was able to estimate the force of the blast by dropping confetti as it passed them at a distance of 10 miles.

Berkeley Professor Offered positions at the University of Chicago, where Fermi headed a new Institute for Nuclear Studies, and Washington University, St. Louis, where many of the nuclear chemists with whom he had worked took jobs after the war, Segrè elected to negotiate a full professorship at the University of California and to return to his studies of elementary nuclear processes using the new generation of accelerators built after the war. McMillan’s principle of phase stability made possible the completion of the Radiation Laboratory’s 184-inch cyclotron as a synchrocyclotron, the building of a new electron-synchrotron opened up new areas of exploration at high energies, and Luis Alvarez’s linear accelerator, based on techniques he conceived after working on radar during the war, provided high-energy protons for studies of proton-proton interactions. Frustrated in his efforts to continue research in spontaneous fission and nuclear chemistry by the growing influence of Seaborg, who had convinced Lawrence to support a major effort in nuclear chemistry under his direction, Segrè chose to study high-energy interactions of nuclear constituents instead. This was to lead him to his most important discovery.

Segrè also resumed his efforts, begun with C. S. Wu before the war, to measure the effect of atomic electrons nuclear K-capture by beta decay. This work was completed in 1947 and was the last of the unfinished work that he was able to complete, because many of the radioactive materials needed for studies of heavy elements were now controlled by the newly formed Atomic Energy Commission and rationed to workers like Seaborg, whom Segrè believed would not collaborate with him as an equal.

The field of nucleon interactions seemed to offer opportunities to revive Ernest Rutherford’s studies of nuclear scattering at much higher energies. Segrè’s group spent several years studying interactions between neutrons and protons and between protons and protons. Polarized proton beams from the 184-inch synchrocyclotron supplemented these data, which were also used to compute the wave phase shifts in nuclear interactions with the Los Alamos computer, MANIAC. In order to make it widely available, along with other unclassified work in the field of nuclear physics, Segrè edited Experimental Nuclear Physics and beginning in 1952, the Annual Review of Nuclear Science.

The death of his mentor Fermi in 1954 sundered the last significant connection with his prewar work, and Segrè paid homage to him by editing Fermi’s Collected Papers(1958) and by writing a scientific biography, Enrico Fermi, Physicist (1970).

William M. Brobeck, with whom Segrè had collaborated in the magnet design of the 184-inch cyclotron before the war, designed a proton synchrotron in 1946, and after the completion of the postwar accelerators, Ernest Lawrence succeeded in winning Atomic Energy Commission funding to build a 6.2-billion-electron-volt (BeV) proton synchrotron at the Radiation Laboratory in 1948. Although the accelerator was delayed by the construction of a one-quarter-scale model (used to test the aperature required) and the laboratory’s involvement in the race for the hydrogen bomb, which led to a second laboratory in Livermore, California, the 6.2 BeV machine finally became operational in 1954.

The energy of the machine had been designed to make antiprotons, which had been predicted by nuclear theorists in the 1930s, through proton-proton interactions. Several groups in the Radiation Laboratory competed to develop the appropriate experiment to identify the particle, which would be concealed in a beam of fifty thousand other particles of negative charge produced by the interaction. The new machine was surrounded by their experimental setups, all of which represented variants on two themes: (1) electromagnetic detection and (2) photographic detection of the rare particles.

Segrè’s group, which now included Tom Ypsilantis as well as Chamberlain and Wiegand, won the competition. The experimental setup included a mass spectrograph, which used a magnetic field to separate negatively charged protons from other particles, and time-of-flight detectors and a Cerenkov counter, which measured the velocity of the particles thus separated. These detectors had been used in different configurations by other groups involved in the search. Arranging them in sequence with quadrupole focusing magnets allowed the Segrè-Chamberlain group to deduce the mass of the particle from the momentum and velocity. They found their first evidence for antiprotons in September 1955.

Another branch of Segrè’s group, led by Gerson Goldhaber, sought to detect antiprotons in a different way, through the use of photographic emulsions, which had long been employed in cosmic-ray physics in the search for antiprotons. Amaldi’s group at the University of Rome collaborated with this group in processing the resulting films, and in November 1955 the Amaldi group found a single event in their film corresponding to the theoretical mass of the antiproton. Other Radiation Laboratory groups confirmed the discovery within a month, when a complete track of an antiproton ending in an annihilation by collision with a proton was found in a stack of emulsions. These results were collectively published: thirty-five events by eighteen authors! Segrè and Chamberlain were awarded the Nobel Prize in Physics for the discovery in 1959.

Oreste Piccioni, a Brookhaven physicist who had discussed his plans for an antiproton experiment with Segrè and Chamberlain in December 1954, subsequently filed a lawsuit alleging that they had used those plans in the design of their experiment, from which he had been excluded. Because he filed his suit after the relevant statute of limitations permitted, the suit was never heard. Although many scientists claim credit for work done by others who receive the Nobel Prize, this was the first such claim ever filed in a court of law, and continued to cast a shadow on the discovery for many years. Piccioni was a member of the group that subsequently discovered the antineutron, and Segrè’s group confirmed that it resulted from a process called charge exchange whereby the two particles switch identity.

Antiprotons have become important research tools in high-energy particle physics since the invention of stochastic cooling, which makes it possible to store, concentrate, and direct antiproton beams. The Center for European Research in Nuclear Physics (CERN) built a 300 BeV proton-antiproton colliding-beam accelerator, the Super Proton Synchrotron, in the early 1980s. Fermi-lab’s colliding-beam accelerator has energies over 1 trillion electron volts (TeV) produced in the collision of beams of protons and antiprotons. CERN plans a 14 TeV machine. Slowing down antiprotons has also been accomplished and has led to the assembly of antiatoms.

Antiprotons have also figured prominently in cosmology. Although speculation that galaxies made up of antiprotons, antineutrons, and positrons might exist followed the particles’ original discovery, this has not been supported either by observations of antiprotons in cosmic rays or by any convincing theory. Nevertheless, the search goes on since the big bang theory accepted by most cosmologists indicates that, at least in the first few seconds of the universe, the number of particles and antiparticles was equivalent. Proposed experiments using magnetic spectrometers placed in orbit around the earth seek to solve the mystery of antimatter.

The Segrè group lost much of its collegiality after their success, as Chamberlain and Wiegand sought greater independence. Although the group held together for another decade, Segrè no longer figured as the intellectual leader. He wrote a treatise on the field in 1964, and continued to provide professional leadership through his editorship of Annual Review. He became a senior statesman in science rather than the experimentalist he had been since the 1930s. He gave lectures all around the world, served as a trustee to the National Accelerator Laboratory (later called Fermilab) during its formative period, and received a number of honorary degrees and appointments. After reaching retirement age he returned to Rome where he was given a chair in physics for one year—when he reached Italy’s retirement age.

The premature death of Fermi in 1954 and Segrès’ subsequent involvement in preserving and recounting Fermi’s life and work reinforced Segrè’s interest in history of physics. In addition to publishing a two-volume history of twentieth-century physics, Segrè taught courses in history of physics at the University of California annually before and after his retirement and was an active participant in the field. In addition to his participation in Berkeley’s hiring the soon-to-be-world-famous historian of science Thomas Kuhn in 1956 and assisting Kuhn in the preparation of the Archive for the History of Quantum Physics, Segrè was the Sarton Lecturer at the Tenth International Congress of the History of Science. He was one of the initiators of the Lawrence Berkeley Laboratory history project, and welcomed opportunities to advise, as well as criticize, the work of historians of modern physics.

Segrè was a man of strong views and seldom hesitated to express them. Early in his career, he earned the title of “Basilisk” because, like the mythical beast, his glance made a deep, if not lethal, impression. Segrè’s wife thought him a careful judge of character, although he was not always cautious in expressing his judgments. His eldest son, Claudio, wrote a memoir of his life with his father, which captured Segrè’s difficulty in communicating with him. Although not abusive, he was often remote, preoccupied with work and critical rather than encouraging of his children.

In 1993 his autobiography, A Mind Always in Motion, provided a candid personal view of physics in Rome and Berkeley. Segrè regarded himself as an outsider at the Radiation Laboratory throughout his career, although his use of its accelerators was among the most fruitful for high-energy physics. This detachment not only made him among the more successful physicist-historians of his age, it allowed him to remain aloof from the exigencies of big science and to avoid being captured by the research programs that subsume hundreds of physicists in huge collaborative experiments. It also reflected his perspective, forged in Rome in his work with Fermi, on the human importance and intellectual elegance of doing physics.

Segrè married Rosa Mines in 1972 after the death of his first wife, Elfriede, and remained in good health, living in Lafayette, California, until 1989, when on 22 April he collapsed and died while walking with her, leaving his son, two daughters, and five grandchildren.

BIBLIOGRAPHY

WORKS BY SEGRÈ

“Some Chemical Properties of Element 43.” Journal of Chemical Physics5 (1937): 712–716.

With Glenn T. Seaborg. “Nuclear Isomerism in Element 43.”Physical Review54 (1938): 772.

With Joseph W. Kennedy, Glenn T. Seaborg, and Arthur C.Wahl. “Properties of 94²³⁹.” Physical Review70 (1946): 555–556.

Editor. Annual Review of Nuclear Science1–27 (1952–1977).

Editor. Experimental Nuclear Physics. 3 vols. New York: Wiley,1953–1959.

With Owen Chamberlain, Clyde Wiegand, and Thomas Ypsilantis. “Observation of Antiprotons.” Physical Review100 (1955): 947–950.

Nuclei and Particles: An Introduction to Nuclear and Subnuclear Physics. New York: W. A. Benjamin, 1964. Rev. ed. Menlo Park, CA: Benjamin/Cummings Pub. Co., 1977.

Enrico Fermi, Physicist. Chicago: University of Chicago Press,1970.

From X-rays to Quarks: Modern Physicists and Their Discoveries.New York: W.H. Freeman, 1980.

From Falling Bodies to Radio Waves: Classical Physicists and Their Discoveries. New York: W.H. Freeman, 1984.

A Mind Always in Motion: The Autobiography of Emilio Segrè.Berkeley: University of California Press, 1993.

OTHER SOURCES

Segrè, Claudio G. Atoms, Bombs & Eskimo Kisses: A Memoir of Father and Son. New York: Viking, 1995.

Seidel, Robert. “Accelerating Science: The Postwar Transformation of the Lawrence Radiation Laboratory.” Historical Studies in the Physical Sciences 13, part 2 (1983): 375–400.

Robert Seidel