Fermi, Enrico

Italian-born US physicist who was awarded the Nobel Prize for Physics in 1938 for his proof of the existence of new radioactive elements produced by bombardment with neutrons, and his discovery of nuclear reactions produced by low-energy neutrons. This research was the basis for studies leading to the atomic bomb and nuclear energy. Fermi built the first nuclear reactor in 1942 at Chicago University and later took part in the Manhattan Project to construct an atom bomb. His theoretical work included the study of the weak nuclear force, one of the fundamental forces of nature, and beta decay.

Neutron bombardment and the Nobel Prize
Following the work of the Joliot-Curies, who discovered artificial radioactivity in 1934 using alpha particle bombardment, Fermi began producing new radioactive isotopes by neutron bombardment. Unlike the alpha particle, which is positively charged, the neutron is uncharged. Fermi realized that less energy would be wasted when a bombarding neutron encounters a positively charged target nucleus. He also found that a block of paraffin wax or a jacket of water around the neutron source produced slow, or ‘thermal’, neutrons. Slow neutrons are more effective at producing artificial radioactive elements because they remain longer near the target nucleus and have a greater chance of being absorbed. He did, however, misinterpret the results of experiments involving neutron bombardment of uranium, failing to recognize that nuclear fission had occurred. Instead, he maintained that the bombardment produced two new transuranic elements. It was left to Lise Meitner and Otto Frisch to explain nuclear fission in 1938.

Nuclear reactors and the atomic bomb
In the USA, Fermi continued the work on the fission of uranium (initiated by neutrons) by building the first nuclear reactor, then called an atomic pile, because it had a moderator consisting of a pile of purified graphite blocks (to slow the neutrons) with holes drilled in them to take rods of enriched uranium. Other neutron-absorbing rods of cadmium, called control rods, could be lowered into or withdrawn from the pile to limit the number of slow neutrons available to initiate the fission of uranium. The reactor was built on the squash court of Chicago University. On the afternoon of 2 December 1942, the control rods were withdrawn for the first time and a self-sustaining nuclear chain reaction began. Two years later, the USA, through a team led by Arthur Compton and Fermi, had constructed an atomic bomb, in which the same reaction occurred but was uncontrolled, resulting in a nuclear explosion.

Beta decay and the neutrino
Fermi's experimental work on beta decay in radioactive materials provided further evidence for the existence of the neutrino, predicted by Austrian physicist Wolfgang Pauli.