Enrico Fermi


Enrico Fermi : biography

29 September 1901 – 28 November 1954

During their time in Rome, Fermi and his group made important contributions to many practical and theoretical aspects of physics. In 1928, he published his "Introduction to Atomic Physics" (), which provided Italian university students with an up-to-date and accessible text. Fermi also conducted public lectures and wrote popular articles for scientists and teachers in order to spread knowledge of the new physics as widely as possible. Part of his teaching method was to gather his colleagues and graduate students together at the end of the day and go over a problem, often from his own research. A sign of success was that foreign students now began to come to Italy. The most notable of these was the German physicist Hans Bethe, who came to Rome as a Rockefeller Foundation fellow, and collaborated with Fermi on a 1932 paper "On the Interaction between Two Electrons" ().

At this time, physicists were puzzled by beta decay, in which an electron was emitted from the atomic nucleus. To satisfy the law of conservation of energy, Pauli postulated the existence of an invisible particle with no charge that was also emitted at the same time. Fermi took up this idea, which he developed in a tentative paper in 1933, and then a longer paper the next year that incorporated the postulated particle, which Fermi called a "neutrino". His theory, later referred to as Fermi’s interaction, and still later as the theory of the weak interaction, described one of the four forces of nature. The neutrino would not be detected until after his death, and his interaction theory showed why it was so difficult to detect. When he submitted his 1934 paper to the British journal Nature, that journal’s editor turned it down because it contained speculations which were "too remote from physical reality to be of interest to readers". Thus Fermi saw the theory published in Italian and German before it was published in English.

After his difficult time with beta decay, Fermi decided to switch to experimental physics, using the neutron, which James Chadwick had discovered in 1932. The result would be a burst of more than twenty papers by Fermi and his collaborators. In 1934, Irène Joliot-Curie and Frédéric Joliot bombarded elements with alpha particles and induced radioactivity in them. Fermi suggested to the Via Panisperna boys that they perform the same experiment with neutrons, which would theoretically work better because neutrons had no electric charge, and so would not be deflected. They constructed a neutron source from radium and beryllium and started bombarding elements, starting with hydrogen, and working their way up the periodic table. Nothing registered on their Geiger counter until they reached fluorine and aluminium, which emitted an alpha particle and decayed into calcium.

They also noticed some unexplained effects. The experiment seemed to work better on a wooden table than a marble table top. Fermi decided to try placing some lead in the path of the neutron source, but then, remembering that Joliot-Curie and Chadwick had noted that paraffin wax was more effective than lead at slowing neutrons, he decided to try it instead. The paraffin induced a hundred times as much radioactivity in silver. He guessed that this was due to the hydrogen atoms in the paraffin, and confirmed this by repeating the effect with water. He concluded that slow neutrons were more easily captured than fast ones, and developed a diffusion equation to describe this, which became known as the Fermi age equation.

Fermi’s group systematically bombarded elements with slow neutrons. When they reached thorium and uranium, the natural radioactivity of these elements made it hard to determine what was happening, but they concluded that they had created new elements, which they called hesperium and ausonium. At that time, fission was thought to be improbable if not impossible, mostly on theoretical grounds. While people expected elements with higher atomic numbers to form from neutron bombardment of lighter elements, nobody expected neutrons to have enough energy to actually split a heavier atom into two light element fragments, and it was thought still more unlikely that slow neutrons could accomplish such a task. The chemist Ida Noddack did criticise Fermi’s work and suggest that some of Fermi’s experiments could have produced lighter elements, but was not taken seriously. At the time, Fermi dismissed the possibility on the basis of his calculations. However, he had not taken into account the "pairing energy" that would appear when a nuclide with an odd number of neutrons absorbed an extra neutron.

In 1938, Fermi received the Nobel Prize in Physics at the age of 37 for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". After Fermi received the Nobel Prize in Stockholm, he, his wife Laura, and their children did not return home to Italy, but rather continued to New York City, where they applied for permanent residency. The decision to move to America and become American citizens was primarily a result of the racial laws promulgated by Mussolini in order to bring Italian Fascism ideologically closer to German National Socialism. The new laws threatened Laura, who was Jewish, and put many of Fermi’s research assistants out of work.