Nuclear fission: Encyclopedia II - Nuclear fission - History

Nuclear fission - History

The results of the bombardment of uranium by neutrons had proved interesting and puzzling. First studied by Enrico Fermi and his colleagues in 1934, they were not properly interpreted until several years later.

On January 16, 1939, Niels Bohr of Copenhagen, Denmark, arrived in the United States to spend several months in Princeton, New Jersey, and was particularly anxious to discuss some abstract problems with Albert Einstein. (Four years later Bohr was to escape to Sweden from Nazi-occupied Denmark in a small boat, along with thousands of other Danish Jews, in large scale operation.) Just before Bohr left Denmark, two of his colleagues, Otto Robert Frisch and Lise Meitner (both refugees from Germany), had told him their guess that the absorption of a neutron by a uranium nucleus sometimes caused that nucleus to split into approximately equal parts with the release of enormous quantities of energy, a process that they dubbed nuclear "fission."

The occasion for this hypothesis was the important discovery of Otto Hahn and Fritz Strassmann in Germany (published in Naturwissenschaften in early January 1939) which proved that an isotope of barium was produced by neutron bombardment of uranium. Bohr had promised to keep the Meitner/Frisch interpretation secret until their paper was published to preserve priority, but on the boat he discussed it with Léon Rosenfeld, but forgot to tell him to keep it secret. Rosenfeld immediately upon arrival told everyone at Princeton University, and from them the news spread by word of mouth to neighboring physicists including Enrico Fermi at Columbia University. As a result of conversations among Fermi, John R. Dunning, and G. B. Pegram, a search was undertaken at Columbia for the heavy pulses of ionization that would be expected from the flying fragments of the uranium nucleus. On January 26, 1939, there was a conference on theoretical physics at Washington, D. C., sponsored jointly by the George Washington University and the Carnegie Institution of Washington.

Fermi left New York to attend this meeting before the Columbia fission experiments had been tried. At the meeting Bohr and Fermi discussed the problem of fission, and in particular Fermi mentioned the possibility that neutrons might be emitted during the process. Although this was only a guess, its implication of the possibility of a chain reaction was obvious. A number of sensational articles were published in the press on this subject. Before the meeting in Washington was over, several other experiments to confirm fission had been initiated, and positive experimental confirmation was reported from four laboratories (Columbia University, Carnegie Institution of Washington, Johns Hopkins University, University of California) in the February 15, 1939, issue of the Physical Review. By this time Bohr had heard that similar experiments had been made in his laboratory in Copenhagen about January 15. (Letter by Frisch to Nature dated January 16, 1939, and appearing in the February 18 issue.) Frédéric Joliot in Paris had also published his first results in the Comptes Rendus of January 30, 1939. From this time on there was a steady flow of papers on the subject of fission, so that by the time (December 6, 1939) L. A. Turner of Princeton wrote a review article on the subject in the Reviews of Modern Physics nearly one hundred papers had appeared. Complete analysis and discussion of these papers have appeared in Turner's article and elsewhere.

A major focus of early fission research was on producing a controllable chain reaction, which would mark the first harnessing of nuclear power. This led to the Manhattan project to develop a nuclear weapon and the development of Chicago Pile-1, the world's first man-made critical nuclear reactor (which used uranium, the only natural nuclear fuel available in macroscopic quantities). But producing a fission chain reaction in uranium fuel is far from trivial. It requires separation of the rare ²³⁵U isotope from the far more common ²³⁸U isotope, and inclusion of extremely chemically pure neutron moderator materials such as deuterium, beryllium, and graphite (The high purity is required because many chemical impurities such as boron are very strong neutron absorbers and poison the chain reaction). Up to 1940 the total amount of uranium metal produced in the USA was not more than a few grams and even this was of doubtful purity, of metallic beryllium not more than a few kilograms, concentrated deuterium not more than a few kilograms, and carbon had never been produced in quantity with anything like the purity required of a moderator, so the problem of producing and purifying materials was a major one.

The problem of producing large amounts of high purity uranium was solved by Frank Spedding using the thermite process. Ames Laboratory was established in 1942 to produce the large amounts of uranium that would be necessary for the research to come.

Unknown until 1972, when French physicist Francis Perrin discovered the Oklo Fossil Reactors, nature had beaten humans to the punch by engaging in uranium fission some 2,000 million years in the past.

For more detail on the early development of nuclear reactors and nuclear weapons, see Manhattan Project.

Adapted from the Wikipedia article "History", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki