Nuclear fission discovered

Splitting of atomic nuclei to release large amounts of energy

Desirable and undesirable practical applications of science are rooted in attempts to understand and control nature. Although nuclear fission was discovered accidentally, during investigations whose purpose was to create transuranium elements, it was at once recognized as a reaction by which a large amount of energy could be released.

Leading to the development of atomic bombs and nuclear reactors, the 1939 discovery of nuclear fission was mostly the result of an international effort. It was preceded by the 1932 discovery of neutrons by James Chadwick in England and the 1933 discovery of artificial radioactivity by Irène Joliot-Curie and Frédéric Joliot in France. In 1934, when uranium was the last known element in the periodic chart, a team of Italian scientists headed by Enrico Fermi started bombarding uranium with neutrons. They wanted to produce artificially elements “beyond uranium.”

German chemists Otto Hahn, Lise Meitner, and Fritz Strassmann also bombarded uranium with neutrons for several years, investigating properties of reaction products. At first, they suspected these products were radium, actinium, and thorium. However, in a January, 1939, paper, published without the coauthorship of an exiled Meitner, they wrote that these products might be barium, lanthanum, and cerium. This was not a definite statement, however; the authors hesitated because they had no idea how such elements could be produced.

Credit for understanding the mechanism reactions belongs to Meitner, who was in Sweden when the paper was published. She was the first to realize that an atomic nucleus could split into two large fragments. Meitner coined the term “nuclear fission” and subsequently published a paper in which the amount of energy released in fission was calculated on the basis of Albert Einstein’s E = mc² formula. Two scientists, Otto Robert Frisch in Denmark and Joliot in France, provided experimental evidence for the magnitude of released energy. This was at once confirmed in early nuclear fission experiments in the United States.

Another important discovery, made in 1939 by Joliot and his collaborators Hans von Halban, from Austria, and Lew Kowarski, from Russia, was the emission of several energetic neutrons during each fission event. This was a clear indication of the possibility of an explosive chain reaction. Basic principles of future applications of nuclear energy were already known when World War II started. However, technical details leading to practical applications were not clear. Fearing that Nazi Germany was planning to develop nuclear weapons, Leo Szilard, one of many refugees from Europe, persuaded Einstein to write a letter to President Franklin D. Roosevelt. The large-scale U.S. nuclear bomb project (1942-1945) can be traced to that letter.

Prior to the discovery of fission, early American nuclear-science efforts focused on accelerators, such as the Columbia Universitycyclotron built by John Dunning. The first American nuclear fission experiments were conducted using this accelerator in January and February of 1939. Fermi, by then one of many European refugees, joined Dunning’s team. After confirming the high energy of fission fragments and the possibility of a chain reaction, the team showed that only a rare uranium isotope, uranium 235, and not the common uranium 238, was responsible for fission fragments. This important American discovery opened the path toward both military and peaceful applications of nuclear energy.

Reacting to Einstein’s letter, President Roosevelt asked the director of the National Bureau of Standards to organize a secret Advisory Committee on Uranium. The first meeting of that body took place on October 21, 1939. A modest sum of six thousand dollars was at once budgeted to support basic chain-reaction research. The United States was still a neutral country at that time. The Manhattan Project, whose purpose was to develop atomic bombs, was created in August, 1942, and the enrichment of uranium with uranium 235 became its high-priority task.

Impact

The first atomic bomb was exploded in Alamogordo, New Mexico, in July, 1945. One month later, two atomic bombs were dropped on Hiroshima and Nagasaki, Japan, killing or injuring several hundred thousand people. The bombs prompted Japan to surrender unconditionally to the Allies, thereby ending World War II. The obvious value of nuclear weapons in combat prompted an international arms race during the ensuing Cold War. However, the possibility of mutual annihilation in an all-out nuclear war helped prevent the use of such weapons.

Bibliography

Bird, Kai, and Martin J. Sherwin. American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer. New York: A. A. Knopf, 2005.

Dahl, Per F., ed. From Nuclear Transmutation to Nuclear Fission, 1932-1939. London: Taylor & Francis, 2002.

Graetzer, Hans G., and David L. Anderson. The Discovery of Nuclear Fission. New York: Van Nostrand Reinhold, 1971.

Sime, Ruth. Lise Meitner: A Life in Physics. Berkeley: University of California Press, 1996.