(1905-1989) Italian/American Experimentalist, Nuclear Physicist, Particle Physicist
Emilio Gino Segre opened the door to the modern exploration of the world of antimatter with his discovery of the antiproton in 1955. He and his colleague Owen Chamberlain were honored for this work with the 1959 Nobel Prize in physics. During World War II, Segre made a key contribution to the development of the atomic bomb by his confirmation that plutonium is fissionable.
He was born on February 1, 1905, in Tivoli, Rome, the son of Guiseppe Segre, an industrialist, and Amelia Treves. The Segres were a wealthy family of intellectuals, professionals, and businesspeople. After completing his secondary education in Rome, Emilio entered the University of Rome in 1922 to study engineering. Fascinated with the new developments in physics, he changed fields five years later. Segre received a doctoral degree in 1928 as the first doctoral student of the great enrico fermi, who remained his friend and collaborator for over three decades. After Fermi’s death, Segre became his biographer.
After being discharged from the Italian army, in which he served between 1928 and 1929, Segre became a research assistant at the University of Rome. A Rockefeller Foundation Fellowship in 1930 gave him the opportunity to work with otto stern in Hamburg, Germany, and pieter zeeman in Amsterdam. In 1932, he returned to Italy, where he was appointed associate professor at the University of Rome and began a period of intensive collaboration with Fermi and his group. Until 1934, except for a short period spent in investigating molecular beams, all his research focused on atomic spec-troscopy. Between 1934 and 1935, he played a pioneering role in the discovery of slow neutrons, which became an important component in the discovery of nuclear reactors.
Segre left Rome in 1936 to become director of the Physics Laboratory at the University of Palermo but was forced to leave Italy only two years later to escape Mussolini’s anti-Semitic decrees. He decided to immigrate to the United States and eventually became a U.S. citizen. His American career began where it would end many years later: at the University of California, Berkeley. Starting out as a research assistant at the Radiation Laboratory, he went on to become a lecturer in the physics department. His prewar period at Berkeley was one of his most productive times in nuclear physics, when he worked with Glenn Seaborg on methods of separating nuclear isomers, that is, pairs of isotopes of the same proton and neutron numbers, but in different quantum states.
When World War II erupted, as were many scientists of the time, Segre was concerned about Germany’s possible military application of the new discoveries in nuclear fission. He undertook a study to prove that a bomb based not on separated isotopes of uranium, but on plutonium, discovered in 1943, was feasible. The results of this work put him in the center of the uranium project, which later became the Manhattan Project to build the first atomic bomb. Moving his family to the Los Alamos Laboratory, from 1943 to 1946, he directed a group whose task was to study spontaneous fission of uranium and plutonium isotopes. Present at the first test of an atomic bomb at Alamogordo in July 1945, he described it as “an awesome sight, comparable to great natural phenomena, [which] had a sobering impact on the beholders.”
After the war, Segre had several offers from universities; drawn by the prospect of new accelerators and the exciting experimental possibilities they offered, he chose to return to Berkeley as a full professor of physics. There, in 1955 he and his colleagues, Owen Chamberlain, Clyde Weigand, and Thomas Ypsilantis, discovered the antiproton. The notion of antiparticles originated with paul adrien maurice dirac, who in 1931 had predicted the existence of a positron, or positive electron. Dirac’s prediction was motivated by his discovery that the mathematics describing the electron contained twice as many states as were expected. He proposed that the positive energy states described the electron, and that the negative energy states could be physically interpreted as describing a particle with a mass equal to that of an electron but with an opposite (positive) charge of equal strength, that is, an antiparticle of the electron. Independent experiments by carl david anderson and lord patrick maynard stuart blackett, in 1932 and 1933, confirmed that a positron could be produced by a photon. Dirac’s discovery of the positron led to the prediction of other antiparti-cles, such as the antiproton.
In 1954, the testing of this prediction could only be done by means of a high-energy proton accelerator, such as the Bevatron at Berkeley, which had reached its planned capacity of 6 billion electron volts (Bevs)—the energy required for the pair formation of protons-antiprotons. This, however, was only a starting point. Segre would later recall:
The chief difficulty of the experiment was the extraction of a reliable antipro-ton signal out of a huge noise produced by the many reactions occurring in the target. In fact, only about one in 50,000 of the negatively charged particles emerging from the target was an antiproton. Two lines of attack were possible: a determination of the e/m ratio for the particles produced, or an observation of the terminal event that was sufficiently detailed to identify the annihilation process.
Segre and Chamberlain’s ingenious methods of detection and analysis used both approaches to establish the existence of the antiproton. The colleagues shared the 1959 Nobel Prize in physics for this work.
Shortly afterward, another group at Berkeley discovered the antineutron, the antimatter counterpart of the neutron. From this point on, Segre’s major focus of research became antinucleons, the antimatter counterpart of the nuclear structure of matter. However, he would make no further basic discoveries. His group dispersed and Segre himself, who preferred “to keep apparatus as simple and inexpensive as possible [and] to have a solid theoretical foundation,” was never at home with the increasingly more powerful accelerators or with the larger and larger collaborations that became necessary in order to work with them.
Segre was married, first to Elfriede Spiro, with whom he had a son and two daughters, and later to Rosa M. Segre. During the 1960s and 1970s, he was editor of The Annual Review of Nuclear Science. After his retirement from Berkeley in 1972, he devoted much of his time to studying the history of physics and published From X-Rays to Quarks: Modern Physicists and Their Discoveries and Falling Bodies to Radio Waves: Classical Physicists and Their Discoveries. He died on April 29, 1989, of a heart attack.
Speaking of his 1964 text, Nuclei and Particles, which reflects the experience of his generation of physicists, Segre offered an important historical perspective: “Nuclei and particles were not yet separated, and indeed, a good fraction of the particle physicists came from the ranks of nuclear physicists or cosmic-ray physicists.” Segre’s own career both illustrated and furthered this progress. As a young physicist he probed the physics of the nucleus, then thought to consist only of neutrons and protons. In his mature work, through his experimental detection of the antiproton, he glimpsed the far greater complexity of the exotic world of elementary particles and became one of the first particle physicists.
