Fermi and the first atomic pile

The story

The boy from Rome

Enrico Fermi was born in Rome in 1901 and from boyhood showed an extraordinary talent for mathematics and physics. At seventeen he won a competition to enter the Scuola Normale in Pisa, and he graduated a little after turning twenty. At only twenty-five he became professor of physics at the University of Rome, where, in the old institute in Via Panisperna, he brought together a group of brilliant young scholars, whom their friends called “the Via Panisperna boys”: Edoardo Amaldi, Emilio Segrè, Franco Rasetti, Ettore Majorana and others. Fermi, nicknamed “the Pope” because he seemed never to make a mistake, was a teacher who explained every problem in simple words.

In 1934 Fermi and his group discovered something important: by bombarding atoms with neutrons, particles with no electric charge, new radioactive elements could be produced. They also noticed that the effect became much stronger if the neutrons first passed through paraffin or water, which slowed them down. For this work Fermi received the Nobel Prize in Physics in 1938.

Escape from Italy

In 1938 the Fascist regime passed the racial laws. Fermi's wife, Laura, was Jewish, so the family was at risk. When Fermi went to Stockholm to collect the Nobel Prize, he never went back to Italy: from Sweden he sailed with his wife and two children to the United States, where he got a post at Columbia University in New York. Many European scientists, fleeing persecution, ended up in America, and their work was extremely important.

The discovery of fission

At the end of 1938 two German chemists, Otto Hahn and Fritz Strassmann, found that when uranium was bombarded with neutrons, the atom split into two smaller pieces, releasing a great deal of energy. The phenomenon was explained by the Austrian physicist Lise Meitner and her nephew Otto Frisch, and was called nuclear fission. Scientists realised at once that fission could produce a chain reaction: the neutrons released by one atom could split others, and so on, releasing an enormous amount of energy.

It was the beginning of the Second World War. Scientists feared that Nazi Germany might build an atomic bomb. In August 1939 the Hungarian physicist Leo Szilárd and Albert Einstein wrote to the American president Roosevelt to warn him. So the United States decided to start a huge secret programme, which would become the Manhattan Project.

Under the Chicago stadium

To build a bomb, it first had to be shown that a chain reaction could be controlled. Fermi, who had moved to the University of Chicago, led a team that built an “atomic pile”, that is, a reactor, in a very strange place: an old squash court under the stands of the university stadium. The pile was a huge mass of graphite blocks, a form of carbon that slows neutrons, with pieces of uranium inside. To control it there were cadmium rods, a metal that absorbs neutrons: pulled out, the reaction started; pushed back in, it stopped. Fermi and his colleagues built the pile layer by layer, for weeks, and measured everything with great care.

On 2 December 1942 the team gathered in silence for the experiment. Fermi had the rods drawn out, a little at a time, checking with his slide rule how the number of neutrons grew. At twenty-five past three in the afternoon the chain reaction kept itself going: the pile worked! Fermi let it run for a few minutes (about half an hour in all from the start), then shut it down. It was the first time that human beings had controlled the energy of the atom. To celebrate, the physicists drank red wine from paper cups, and one of them phoned a project official: “The Italian navigator has landed in the new world.” “How did he find the natives?” asked the other. “Very friendly.”

After the experiment

Fermi's work helped to build the first atomic bomb at Los Alamos, in the New Mexico desert, which was tested on 16 July 1945 and used on 6 and 9 August 1945 against two Japanese cities. Fermi, who took part in the project, was after the war one of the first to remind people of the dangers of nuclear weapons. He died in Chicago in 1954, aged fifty-three.