Discovery of Nuclear Fission

On December 22, 1938, German chemists Otto Hahn and Fritz Strassmann published a paper in Naturwissenschaften reporting that bombarding uranium with neutrons produced barium, a much lighter element. This unexpected result provided the first experimental evidence of nuclear fission, a process later explained by Lise Meitner and Otto Frisch. The discovery fundamentally altered both physics and world history.

Historical Context

By the 1930s, physicists had identified the neutron and were using it to probe atomic nuclei. Enrico Fermi and others had bombarded uranium with neutrons, hoping to create elements heavier than uranium (transuranics). Instead, they observed confusing products. In Nazi Germany, Hahn and Strassmann continued this work at the Kaiser Wilhelm Institute in Berlin, while their long-time colleague Lise Meitner, an Austrian Jewish physicist, had been forced to flee in July 1938 due to anti-Semitic persecution. She settled in Sweden, but remained in close contact with Hahn. The prevailing assumption was that neutron bombardment would only cause small nuclear changes, not split the atom.

What Happened

Hahn and Strassmann bombarded uranium with neutrons and used radiochemical methods to analyze the products. They found that one product was chemically identical to barium, an element with roughly half the atomic mass of uranium. This contradicted the expectation that neutron capture would lead to heavier elements. After repeated experiments, they concluded that the uranium nucleus had split into lighter nuclei. They published their findings on December 22, 1938, describing the formation of barium and noting that the process released a large amount of energy. Hahn sent their paper to Meitner, who, with her nephew Otto Frisch, worked out the theoretical explanation over the Christmas holidays. They coined the term "fission" and calculated that the process released about 200 million electron volts per split, confirming the immense energy release.

Significance & Legacy

Nuclear fission revolutionized physics and had profound historical consequences. Meitner and Frisch's explanation in early 1939 confirmed that splitting the atom released enormous energy, making nuclear power and nuclear weapons theoretically possible. The discovery came just months before the outbreak of World War II. Fears that Nazi Germany might develop an atomic bomb prompted Albert Einstein's 1939 letter to President Franklin D. Roosevelt, which led to the Manhattan Project. Hahn received the 1944 Nobel Prize in Chemistry for the discovery, though Meitner's crucial theoretical contribution was controversially overlooked. Fission later became the basis for nuclear reactors and atomic bombs, shaping geopolitics and energy production for decades.

Conclusion

The discovery of nuclear fission on December 22, 1938, stands as a pivotal moment in science and world history. It transformed humanity's understanding of the atom and unleashed both immense promise and peril. Hahn and Strassmann's careful chemical work, combined with Meitner and Frisch's theoretical insight, opened the nuclear age.