Red Cross nurse with atomic bomb victim in Nagasaki, 10 August 1945.

Red Cross nurse with atomic bomb victim in Nagasaki, 10 August 1945.

Atomic bombings of Hiroshima and Nagasaki, during World War II, American bombing raids on the Japanese cities of Hiroshima (August 6, 1945) and Nagasaki (August 9, 1945) that marked the first use of atomic weapons in war. Tens of thousands were killed in the initial explosions and many more would later succumb to radiation poisoning. On August 10, one day after the bombing of Nagasaki, the Japanese government issued a statement agreeing to accept the Allied surrender terms that had been dictated in the Potsdam Declaration.

Early atomic research

nuclear fission

The impact of a slow (low-energy) neutron splitting the nucleus of the uranium isotope U-235 into two new nuclei. These can be nuclei of any of 30 or more elements ranging in atomic number from 30 to 64. Krypton and barium are examples. Energy and neutrons (2 or 3 for an average of about 2.5) are also produced.

Otto Hahn

The turning point in the quest for atomic energy came in January 1939, eight months before the start of World War II. German scientists Otto Hahn and Fritz Strassmann, following a clue provided by Irène Joliot-Curie and Pavle Savić in France (1938), proved definitely that the bombardment of uranium with neutrons produced radioisotopes of barium, lanthanum, and other elements from the middle of the periodic table.

Lise Meitner and Otto Hahn

Physicist Lise Meitner and chemist Otto Hahn at the Kaiser Wilhelm Institute of Chemistry, Berlin-Dahlem, Germany, 1913.

Neils Bohr

Niels Bohr.

Observe an animation of sequential events in the fission of a uranium nucleus by a neutron

Sequence of events in the fission of a uranium nucleus by a neutron.

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The significance of this discovery was communicated by Lise Meitner and Otto Frisch, two Jewish scientists who had fled Germany, to Niels Bohr in Copenhagen. Bohr had been preparing to journey to the United States, and he arrived in New York on January 16, 1939. He discussed the matter with Albert Einstein, John Archibald Wheeler, and others before announcing to the world on January 26 the discovery of a process that Meitner and Frisch had termed fission. Enrico Fermi proposed to Bohr that neutrons might be released during the fission process, thus raising the possibility of a sustained nuclear chain reaction. These revolutionary suggestions triggered a flurry of activity in the world of physics. Subsequent studies by Bohr and Wheeler indicated that fission did not occur in uranium-238, the isotope of uranium most commonly found in nature, but that fission could take place in uranium-235. Gradually many of the riddles surrounding fission were resolved, and by June 1940 the basic facts concerning the release of atomic energy were known throughout the scientific world.

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