Fermi, Enrico (1901–1954)
Enrico Fermi was both a brilliant theorist and an unusually gifted experimentalist — a combination of talents seldom found among twentieth-century physicists. Born in 1901 in Rome, Fermi obtained his doctor's degree in physics magna cum laude from the University of Pisa at the age of 21, with a dissertation on x-rays.
After two years of post-doctoral research at Max Born's Institute in Göttingen, and then with Paul Ehrenfest in Leiden, Fermi taught for two years at the University of Florence, where he soon established his reputation by developing what are now known as the Fermi-Dirac statistics. In 1926 he was appointed to a full professorship in physics at the University of Rome, where he quickly gathered around him a group of talented young faculty members and students, who helped him make a name for Rome in the fields of nuclear physics and quantum mechanics. His theoretical work culminated in a 1933 theory of nuclear beta decay that caused a great stir in world physics circles, and is still of major importance today.
Fermi had been fascinated by the discovery of the neutron by James Chadwick in 1932. He gradually switched his research interests to the use of neutrons to produce new types of nuclear reactions, in the hope of discovering new chemical elements or new isotopes of known elements. He had seen at once that the uncharged neutron would not be repelled by the positively-charged atomic nucleus. For that reason the uncharged neutron could penetrate much closer to a nucleus without the need for high-energy particle accelerators. He discovered that slow neutrons could be produced by passing a neutron beam through water or paraffin, since the neutron mass was almost equal to that of a hydrogen atom, and the consequent large energy loss in collisions with hydrogen slowed the neutrons down very quickly. Hence these "slow" or "thermal" neutrons would stay near a nucleus a longer fraction of a second and would therefore be more easily absorbed by the nucleus under investigation. Using this technique, Fermi discovered forty new artificial radioactive isotopes.
In 1934 Fermi decided to bombard uranium with neutrons in an attempt to produce "transuranic" elements, that is, elements beyond uranium, which is number 92 in the periodic table. He thought for a while that he had succeeded, since unstable atoms were produced that did not seem to correspond to any known radioactive isotope. He was wrong in this conjecture, but the research itself would eventually turn out to be of momentous importance both for physics and for world history, and worthy of the 1938 Nobel Prize in Physics.
Fermi's wife, Laura, was Jewish, and as Hitler's influence over Mussolini intensified, anti-Jewish laws were passed that made Laura's remaining in Italy precarious. After accepting his Nobel Prize in Stockholm, Fermi and his wife took a ship directly to the United States, where they would spend the rest of their lives. Enrico taught at Columbia University in New York City from 1939 to 1942, and at the University of Chicago from 1942 until his death in 1954.
In 1938 Niels Bohr had brought the astounding news from Europe that the radiochemists Otto Hahn and Fritz Strassmann in Berlin had conclusively demonstrated that one of the products of the bombardment of uranium by neutrons was barium, with atomic number 56, in the middle of the periodic table of elements. He also announced that in Stockholm Lise Meitner and her nephew Otto Frisch had proposed a theory to explain what they called "nuclear fission," the splitting of a uranium nucleus under neutron bombardment into two pieces, each with a mass roughly equal to half the mass of the uranium nucleus. The products of Fermi's neutron bombardment of uranium back in Rome had therefore not been transuranic elements, but radioactive isotopes of known elements from the middle of the periodic table.
Fermi and another European refugee, Leo Szilard, discussed the impact nuclear fission would have on physics and on the very unstable state of the world itself in 1938. The efforts of Szilard in 1939 persuaded Albert Einstein to send his famous letter to President Franklin D. Roosevelt, which resulted in the creation of the Manhattan Project to contruct a nuclear bomb. Fermi was put in charge of the first attempt to construct a self-sustaining chain reaction, in which neutrons emitted by a fissioning nucleus would, in turn, produce one or more fission reactions in other uranium nuclei. The number of fissions produced, if controlled, might lead to a useful new source of energy; if uncontrolled, the result might be a nuclear bomb of incredible destructive power.
Fermi began to assemble a "nuclear pile" in a squash court under the football stands at the University of Chicago. This was really the first nuclear power reactor, in which a controlled, self-sustaining series of fission processes occurred. The controls consisted of cadmium rods inserted to absorb neutrons and keep the reactor from going "critical." Gradually the rods were pulled out one by one, until the multiplication ratio of neutrons produced to neutrons absorbed was exactly one. Then the chain reaction was self-sustaining. To proceed further would run the risk of a major explosion. Fermi had the reactor shut down at exactly 3:45 P.M. on December 2, 1942, the day that is known in history as the beginning of nuclear energy and nuclear bombs.
Fermi lived only a little more than a decade after his hour of triumph. He spent most of this time at the University of Chicago, where, as in Rome, he surrounded himself with a group of outstanding graduate students, many of whom also later received Nobel Prizes. Fermi died of stomach cancer in 1954, but his name remains attached to many of the important contributions he made to physics. For example, element 100 is now called Fermium.
Fermi's overall impact on physics is well summarized by the nuclear physicist Otto Frisch (1979, p. 22): "But occasionally one gets a man like Enrico Fermi, the Italian genius who rose to fame in 1927 as a theoretician and then surprised us all by the breathtaking results of his experiments with neutrons and finally by engineering the first nuclear reactor. On December the second, 1942, he started the first self-sustaining nuclear chain reaction initiated by man and thus became the Prometheus of the Atomic Age."
Bibliography
Allison, S. K., et al. (1955). "Memorial Symposium in Honor of Enrico Fermi held on April 29, 1955 at meeting of the American Physical Society in Washington, D.C." Reviews of Modern Physics 27:249–275.
Fermi, E. (1962–1965).Collected Papers, 3 vols., ed. E. Segrè, et al. Chicago: University of Chicago Press.
Fermi, L. (1954). Atoms in the Family: My Life with Enrico Fermi. Chicago: University of Chicago Press.
Frisch, O. (1979). What Little I Remember. Cambridge: Cambridge University Press.
Segrè, E. (1970). Enrico Fermi, Physicist. Chicago: University of Chicago Press.
Segrè, E. (1971). "Fermi, Enrico." In Dictionary of Scientific Biography, ed. Charles Coulston Gillispie, Vol. 4, pp. 576–583. New York: Scribner.
Smyth, H. D. (1945). Atomic Energy for Military Purposes. Washington, DC: U.S. Government Printing Office.
Wattenberg, A. (1993). "The Birth of the Nuclear Age." Physics Today 46(January):44–51.
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