Fermi, Enrico

I INTRODUCTION

Fermi, Enrico (1901-1954), Italian-born American physicist and Nobel Prize winner, who made important contributions to both theoretical and experimental physics. Fermi’s most well-known contribution was the demonstration of the first controlled atomic fission reaction. Atomic fission occurs when an atom splits apart (see Atom). Fermi was the first scientist to split an atom, although he misinterpreted his results for several years. He also had an important role in the development of fission for use as an energy source and as a weapon (see Nuclear Energy; see Atomic Bomb). He won the 1938 Nobel Prize in physics for his work in bombarding atoms with neutrons, subatomic particles with no electric charge. Initially, Fermi believed that this process created new chemical elements heavier than uranium (see Transuranium Elements), but other scientists showed that he actually split atoms to create fission reactions.

II FERMI’S LIFE

Fermi was born in Rome, Italy. At age 17 he earned a scholarship to the prestigious Scuola Normale Superiore in Pisa by writing an essay on the characteristics of sound. He went on to the University of Pisa, where he earned his doctorate in 1922. Fermi studied with German physicist Max Born in Göttingen, Germany, from 1922 to 1924.

In 1924 Fermi returned to Italy to teach mathematics at the University of Florence. He became professor of theoretical physics at the University of Rome in 1927. He was 26 years old—the youngest professor in Italy since 16th-century Italian scientist Galileo. In the 1930s dictator Benito Mussolini introduced anti-Semitic laws to Italy and Fermi feared for the safety of his wife, who was Jewish. In 1938, after traveling to Sweden to accept the Nobel Prize, Fermi immigrated to the United States rather than return to Italy. Fermi became a professor at Columbia University in New York in 1939, and in 1941 moved to Chicago, Illinois, for a professorship at the University of Chicago. During World War II (1939-1945) he was involved in the Manhattan Project, the American effort to develop an atomic bomb in Los Alamos, New Mexico. In 1945 Fermi became a U.S. citizen and returned to Chicago, where he remained until his death.

III FERMI’S WORK

Fermi’s first important contributions to physics were theoretical. In 1926 he devised a method for calculating the behavior of a system composed of particles that obeyed the Pauli exclusion principle. The Pauli exclusion principle, developed by Austrian-born Swiss physicist Wolfgang Pauli, states that no two particles can have identical quantum numbers. Quantum numbers identify properties of a particle such as energy, angular momentum, magnetic properties, and spin, or direction of rotation. The method that Fermi developed became known as Fermi statistics, and the particles that obey the Pauli exclusion principle became known as fermions. Fermions include all three of the particles that make up atoms (electrons, protons, and neutrons) as well as many other particles. British physicist P. A. M. Dirac independently developed an equivalent theory with a different approach several months later.

In 1933 Fermi published a theory that explained beta decay, or the transformation of a neutron into a proton, an electron, and a neutrino. Neutrinos are neutral particles related to electrons. Beta decay is a form of radioactivity, a process in which particles in atoms release energy and other particles. Fermi’s explanation of beta decay introduced a fundamental force called the weak force, or weak nuclear interaction. Scientists recognized three fundamental forces of interactions at that time: The gravitational force controls interactions between masses, the electromagnetic force controls the interaction of electric charges, and the strong force controls the interaction of particles in the nucleus of an atom. The weak force is more obscure and removed from everyday experience than the other forces. It allows particles to change into other particles under certain circumstances.

Fermi then turned to experimental physics. In 1933 French physicists Irène Joliot-Curie and Frédéric Joliot-Curie had artificially produced radioactive elements by bombarding aluminum and boron with alpha particles. Radioactive elements are elements composed of atoms that decay, or easily release particles and energy. Alpha particles are the nuclei of helium atoms, which contain two protons and two neutrons. In 1934 Fermi showed that single neutrons were even more effective than alpha particles at creating radioactive elements and isotopes. Isotopes of an element are atoms that contain the same number of protons (the number of protons in an atom determines which element it is), but different numbers of neutrons. Fermi discovered that shooting neutrons through paraffin wax at a sample of atoms slowed the neutrons down and increased the intensity of the radioactivity. He bombarded uranium samples with these slow neutrons and interpreted the results as the creation of elements heavier than uranium, or transuranium elements. In 1938, however, Austrian-born Swedish physicist Lise Meitner and Austrian-born British physicist Otto Frisch proposed and confirmed a theory that the uranium atoms were actually splitting apart instead of forming heavier elements. Fermi won the 1938 Nobel Prize in physics for his work with neutrons and radioactivity.

Fermi and other scientists realized the potential power of fission, or the splitting of atoms. Atoms release energy in the form of heat and radiation when they split. Because fission is triggered by neutrons, and atoms release neutrons when they split, one fission reaction can start more reactions, creating a self-sustaining, or chain, reaction. The more fission reactions that occur, the more energy the system releases. In 1939 Fermi, Hungarian-born American physicist Leo Szilard, and German-born American physicist Albert Einstein went to U.S. President Franklin D. Roosevelt with the concern that fission chain reactions could be used as weapons, and that Germany might be developing such a weapon—an atomic bomb. In 1942, the Manhattan Project, the American effort to develop an atomic bomb, officially began. By the end of the year Fermi had designed and presided over the first controlled fission reaction, which occurred in an unused squash court in the basement of Stagg Field at the University of Chicago. In July 1945 the United States tested the first atomic bomb, and in August of that year the United States dropped atomic bombs on two cities in Japan, Hiroshima and Nagasaki.

Fermi eventually returned to the University of Chicago and continued to research radioactivity and neutrons. He also consulted on the construction of the synchrocyclotron, a large particle accelerator at the University of Chicago, completed in 1951. Particle accelerators increase the speed of subatomic particles to allow scientists to study the particles at high energies. Fermi used the particle accelerator to study what happens to atoms when they break up under great force. In 1954 Fermi received the Atomic Energy Commission Award, which was later renamed the Fermi Award. In 1955, a year after his death, the element fermium was named in his honor.

Contributed By: Stanley Goldberg

Fermi, Enrico

b. Sept. 29, 1901, Rome, Italy

d. Nov. 28, 1954, Chicago, Ill., U.S.

Italian-born American physicist who was one of the chief architects of the nuclear age. He developed the mathematical statistics required to clarify a large class of subatomic phenomena, discovered neutron-induced radioactivity, and directed the first controlled chain reaction involving nuclear fission. He was awarded the 1938 Nobel Prize for Physics, and the Enrico Fermi Award of the U.S. Department of Energy is given in his honour.

Education and early career

Fermi was the youngest of the three children of Alberto Fermi, a railroad employee, and Ida de Gattis. Enrico, an energetic and imaginative student prodigy in high school, decided to become a physicist. At the age of 17 he entered the Reale Scuola Normale Superior, which is associated with the University of Pisa. There he earned his doctorate at the age of 21 with a thesis on research with X rays.

After a short visit in Rome, Fermi left for Germany with a fellowship from the Italian Ministry of Public Instruction to study at the University of Göttingen under the physicist Max Born, whose contributions to quantum mechanics were part of the knowledge prerequisite to Fermi's later work. He then returned to teach mathematics at the University of Florence.

In 1926 his paper on the behaviour of a perfect, hypothetical gas impressed the physics department of the University of Rome, which invited him to become a full professor of theoretical physics. Within a short time, Fermi brought together a new group of physicists, all of them in their early 20s. In 1926 he developed a statistical method for predicting the characteristics of electrons according to Pauli's exclusion principle, which suggests that there cannot be more than one subatomic particle that can be described in the same way. In 1928 he married Laura Capon, by whom he had two children, Nella in 1931 and Giulio in 1936. The Royal Academy of Italy recognized his work in 1929 by electing him to membership as the youngest member in its distinguished ranks.

This theoretical work at the University of Rome was of first-rate importance, but new discoveries soon prompted Fermi to turn his attention to experimental physics. In 1932 the existence of an electrically neutral particle, called the neutron, was discovered by Sir James Chadwick at Cambridge University. In 1934 Frédéric and Irène Joliot-Curie in France were the first to produce artificial radioactivity by bombarding elements with alpha particles, which are emitted as positively charged helium nuclei from polonium. Impressed by this work, Fermi conceived the idea of inducing artificial radioactivity by another method: using neutrons obtained from radioactive beryllium but reducing their speed by passing them through paraffin, he found the slow neutrons were especially effective in producing emission of radioactive particles. He successfully used this method on a series of elements. When he used uranium of atomic weight 92 as the target of slow-neutron bombardment, however, he obtained puzzling radioactive substances that could not be identified.

Fermi's colleagues were inclined to believe that he had actually made a new, "transuranic" element of atomic number 93; that is, during bombardment, the nucleus of uranium had captured a neutron, thus increasing its atomic weight. Fermi did not make this claim, for he was not certain what had occurred; indeed, he was unaware that he was on the edge of a world-shaking discovery. As he modestly observed years later, "We did not have enough imagination to think that a different process of disintegration might occur in uranium than in any other element. Moreover, we did not know enough chemistry to separate the products from one another." One of his assistants commented that "God, for His own inscrutable ends, made everyone blind to the phenomenon of atomic fission."

Late in 1938 Fermi was named a Nobel laureate in physics "for his identification of new radioactive elements produced by neutron bombardment and for his discovery of nuclear reaction effected by slow neutrons." He was given permission by the Fascist government of Mussolini to travel to Sweden to receive the award. As they had already secretly planned, Fermi and his wife and family left Italy, never to return, for they had no respect for Fascism.

Meanwhile, in 1938, three German scientists had repeated some of Fermi's early experiments. After bombarding uranium with slow neutrons, Otto Hahn, Lise Meitner, and Fritz Strassmann made a careful chemical analysis of the products formed. On Jan. 6, 1939, they reported that the uranium atom had been split into several parts. Meitner, a mathematical physicist, slipped secretly out of Germany to Stockholm, where, together with her nephew, Otto Frisch, she explained this new phenomenon as a splitting of the nucleus of the uranium atom into barium, krypton, and smaller amounts of other disintegration products. They sent a letter to the science journal Nature, which printed their report on Jan. 16, 1939.

Meitner realized that this nuclear fission was accompanied by the release of stupendous amounts of energy by the conversion of some of the mass of uranium into energy in accordance with Einstein's mass-energy equation, that energy (E) is equal to the product of mass (m) times the speed of light squared (c²), commonly written E = mc².

Work in the United States

Fermi, apprised of this development soon after arriving in New York, saw its implications and rushed to greet Niels Bohr on his arrival in New York City. The Hahn-Meitner-Strassmann experiment was repeated at Columbia University, where, with further reflection, Bohr suggested the possibility of a nuclear chain reaction. It was agreed that the uranium-235 isotope, differing in atomic weight from other forms of uranium, would be the most effective atom for such a chain reaction.

Fermi, Leo Szilard, and Eugene Wigner saw the perils to world peace if Hitler's scientists should apply the principle of the nuclear chain reaction to the production of an atomic bomb. They composed a letter, which was signed by Einstein, who, on Oct. 11, 1939, delivered it to Pres. Franklin D. Roosevelt, alerting him to this danger. Roosevelt acted on their warning, and ultimately the Manhattan Project for the production of the first atomic bomb was organized in 1942. Fermi was assigned the task of producing a controlled, self-sustaining nuclear chain reaction. He designed the necessary apparatus, which he called an atomic pile, and on Dec. 2, 1942, led the team of scientists who, in a laboratory established in the squash court in the basement of Stagg Field at the University of Chicago, achieved the first self-sustaining chain reaction. The testing of the first nuclear device, at Alamogordo Air Base in New Mexico on July 16, 1945, was followed by the dropping of atomic bombs on Hiroshima and Nagasaki a few weeks later.

Having satisfied the residence requirements, the Fermis had become American citizens in 1944. In 1946 he became Distinguished-Service Professor for Nuclear Studies at the University of Chicago and also received the Congressional Medal of Merit. At the Metallurgical Laboratory of the University of Chicago, Fermi continued his studies of the basic properties of nuclear particles, with particular emphasis on mesons, which are the quantized form of the force that holds the nucleus together. He also was a consultant in the construction of the synchrocyclotron, a large particle accelerator at the University of Chicago. In 1950 he was elected a foreign member of the Royal Society of London.

Fermi made highly original contributions to theoretical physics, particularly to the mathematics of subatomic particles. Moreover, his experimental work in neutron-induced radioactivity led to the first successful demonstration of atomic fission, the basic principle of both nuclear power and the atomic bomb. The atomic pile in 1942 at the University of Chicago released for the first time a controlled flow of energy from a source other than the Sun; it was the forerunner of the modern nuclear reactor, which releases the basic binding energy of matter for peaceful purposes. Element number 100 was named for him, and the Enrico Fermi Award was established in his honour. He was the first recipient of this award of $25,000 in 1954.