Fermi , Enrico

(1901–1954) Italian–American physicist

Fermi was without doubt the greatest Italian scientist since Galileo and in the period 1925–50 was one of the most creative physicists in the world. Unusually in an age of ever-growing specialization he excelled as both an experimentalist and a theoretician.

He was born in Rome and brought up in the prosperous home of his father who, beginning as a railroad official, progressed to a senior position in government service. Fermi's intelligence and quickness of mind were apparent from an early age and he had little difficulty in gaining admission in 1918 to the Scuola Normale in Pisa, a school for the intellectual élite of Italy. He later completed his education at the University of Pisa where he gained his PhD in 1924. After spending some time abroad in Göttingen and Leiden, Fermi returned to Italy where, after some initial setbacks, he was appointed to a professorship of physics at the University of Rome. This in itself was a considerable achievement for one so young, considering the traditional and bureaucratic nature of Italian universities. It was no doubt due to the reputation he had already established with the publication of some 30 substantial papers, and the support of O.M. Corbino, the most distinguished Italian physicist at the time and also a senator. Corbino was determined to modernize Italian physics and had the good sense to see that Fermi, despite his youth, was the ideal man to advance his cause.

Fermi began by publishing the first Italian text on modern physics,Introduzione alla Fisica Atomica (1928; Introduction to Nuclear Physics). Soon his reputation attracted around him the brightest of the younger Italian physicists. But the growth of fascism in Italy led to the dispersal of its scientific talent. By 1938 Fermi, with a Jewish wife, was sufficiently alarmed by the growing anti-Semitism of the government to join the general exodus and move to America.

However, before his departure, his period in Rome turned out to be remarkably productive, with major advances being made in both the theoretical and the experimental field. His experimental work arose out of attempts to advance the efforts of Irène and Frédéric Joliot-Curie who had announced in 1934 the production of artificial radioactive isotopes by the bombardment of boron and aluminum with helium nuclei (alpha particles). Fermi realized that the neutron, discovered by James Chadwick in 1932, was perhaps an even better tool for creating new isotopes. Although less massive than an alpha particle, the neutron's charge neutrality allowed it to overcome the positive charge of a target nucleus without dissipating its energy.

Fermi reported that in 1934 he had impulsively and for no apparent reason interposed paraffin between the neutron source and the target. “It was with no advance warning, no conscious prior reasoning ... I took some odd piece of paraffin” and placed it in front of the incident neutrons. The effect was to increase the activation intensity by a factor that ranged from a few tens to a few hundreds. Fermi had stumbled on the phenomenon of slow neutrons. What was happening was that the neutrons were slowing down as the result of collisions with the light hydrocarbon molecules. This in turn meant that they remained in the vicinity of the target nucleus sufficiently long to increase their chance of absorption.

The production of slow neutrons was later to have a profound impact in the field of nuclear energy, both civil and military. However, Fermi's immediate task was to use them to irradiate as many of the elements as possible and to produce and investigate the properties of a large number of newly created radioactive isotopes. It was for this work, for “the discovery of new radioactive substances ... and for the discovery of the selective power of slow neutrons” that Fermi was awarded the 1938 Nobel Prize for physics.

He did however miss one significant phenomenon. In the course of their systematic irradiation of the elements Fermi and his colleagues naturally bombarded uranium with slow neutrons. This would inevitably lead to nuclear fission, but Fermi thought that transuranic elements were being produced and in his Nobel address actually referred to his production of elements 93 and 94, which he named ‘ausonium’ and ‘hesperium’. In 1938 Otto Frisch and Lise Meitner first realized that nuclear fission was taking place in such reactions.

On the theoretical level Fermi's major achievement while at Rome was his theory of beta decay. This is the process in unstable nuclei whereby a neutron is converted into a proton with the emission of an electron and an antineutrino (n - p + e^– + ν). Fermi gave a detailed analysis which introduced a new force into science, the so-called ‘weak’ force. An account was published in Italian in 1933 as an original English version was rejected by the journal Nature as being too speculative.

In America Fermi soon found himself caught up in the attempt to create a controlled nuclear chain reaction. In 1942 he succeeded in building the first atomic pile in the stadium of the University of Chicago at Stagg Field. Using pure graphite as a moderator to slow the neutrons, and enriched uranium as the fissile material, Fermi and his colleagues began the construction of the pile. It consisted of some 40,000 graphite blocks, specially produced to exclude impurities, in which some 22,000 holes were drilled to permit the insertion of several tons of uranium. At 2.20 p.m. on 2 December 1942, the atomic age began as Fermi's pile went critical, supporting a self-supporting chain reaction for 28 minutes. In an historic telephone call afterwards Arthur Compton informed the managing committee that “the Italian navigator has just landed in the new world,” and that the natives were friendly.

Fermi continued to work on the project and was in fact present in July 1945 when the first test bomb was exploded in the New Mexico desert. He is reported to have dropped scraps of paper as the blast reached him and, from their displacement, to have calculated the force as corresponding to 10,000 tons of TNT.

After the war Fermi accepted an appointment as professor of physics at the University of Chicago where he remained until his untimely death from cancer. His name has been commemorated in physics in various ways. Element 100, fermium, and the unit of length of 10^–13 centimeter, the fermi, were named for him, as was the National Accelerator Laboratory, Fermilab, at Batavia, near Chicago.