You Might Want to Know: What, exactly, is nuclear fission? When was it discovered? What does it do?
“Atom” comes from a Greek word that means “uncuttable.” Until 1938, that was believed to be the nature of atoms. They could not be divided into anything smaller.
In the early twentieth century, scientists had discovered particles that were smaller than atoms. At the center of an atom, they had learned, was a “nucleus,” (from a word meaning “kernel”). The nuclei contained particles called “protons” that carried a positive electrical charge. The number of protons in a nucleus determined what chemical element the atom was. The protons were bound together in the nucleus with a number of “neutrons” (particles that had no electrical charge). Outside the protons and neutrons in the nucleus were “electrons” (very much smaller particles that were negatively charged) which usually matched the number of protons in the nucleus. If there wasn’t a match, the atom was an “ion” and unstable as it sought to balance out its charge.
It had also been discovered that atoms with a certain number of protons might carry different numbers of neutrons, producing different “isotopes” of a particular element. These isotopes could be radioactive, emitting energy or particles until they attained a stable form.
Before 1938, scientists had shown that bombarding uranium nuclei with neutrons might add protons to the nucleus. This would create new, even heavier, “transuranic” elements. But the nucleus itself had continued to be thought uncuttable.
The fact that a nucleus could be, not “cut” exactly, but made to divide into smaller nuclei was discovered in 1938. At the end of that year, the German radiochemist Otto Hahn and his assistant Fritz Strassmann had bombarded some uranium with neutrons and in a chemical analysis afterwards found elements in the sample that had not been there before, elements that had not more but fewer protons than uranium did.
At first, because the idea that the nuclei of atoms could be split was so radical, Hahn was hesitant to claim that this is what had happened. He was a chemist, not a physicist, which also might have made him hesitate. But he was a very good chemist. He knew he had found in the uranium atoms with a smaller number of protons. He knew those atoms weighed a little less than the uranium had weighed. He knew also that energy had been released and how much.
At the beginning of 1939, what Hahn had done was interpreted fully by his friend the physicist Lisa Meitner and her physicist nephew Otto Frisch, both Jewish refugees from Nazi Germany, to whom Hahn had quickly sent his results. Meitner and Frisch recognized that the energy that had been released corresponded to the amount of mass that had disappeared. The amount of energy was predicted by the formula derived from Einstein’s work, E=mc2. That is, the amount of energy released equaled the amount of mass that had disappeared multiplied by the speed of light squared. Multiplied, that is, almost thirty-five billion times.
Otto Hahn called the process “fractionization.” Frisch borrowed the term “fission” from cell biology and Hahn adopted it.
The news of Hahn’s discovery was brought to the United States early in 1939 by the Nobel Prize winning Danish physicist, Niels Bohr, who had come to the United States in January to lecture at Princeton University and attend a conference in Washington, D.C. At the end of January, chain-reacting fission in uranium was confirmed by the Hungarian physicist, Leo Szilard and others in a table-top experiment at Columbia University.
In February of 1939, Hahn had predicted that the fission process would release neutrons. In March of 1939, a French physicist, Frederic Joliot, and his team found the number neutrons released to be 2+. Any number over 1 made possible a chain reaction in uranium that might release vast amounts of energy.
In August 1939, Leo Szilard got Albert Einstein, another refugee physicist in the U.S., to sign a letter--drafted by Szilard, it is thought--addressed to President Franklin Roosevelt. The letter alerted Roosevelt to the possibility that “vast amounts of power and large quantities of new radium-like elements” could be produced by the fission of uranium, something that was “almost certain” to be achieved, the letter said.
The letter went on to say that the “new phenomenon” might also lead to the construction of “extremely powerful bombs of a new type,” though this, it said, was “much less certain.”
The letter mentioned that Germany had “stopped the sale of uranium from the Czechoslovakian mines which she has taken over.” Germany was where fission had been discovered. The implication was obvious. The Germans also knew what the letter was alerting Roosevelt to.
FDR received the letter on October 1, 1939. After a slow start and after the United States had entered World War II, the Manhattan Engineering Project was established, near the end of 1942, to try to build the extremely powerful bomb of a new type that would use the energy released in the nuclear fission of uranium.
The first explosion on earth of such a bomb occurred in southern New Mexico on July 16, 1945. In August, two such bombs were dropped on Hiroshima and Nagasaki. By now, many thousands of such bombs exist, held in the arsenals of nine different Nuclear Weapons States. So far, the bombs dropped on Hiroshima and Nagasaki are the only two bombs of this type to be used in combat.
Coming: What was the hardest part about making the first bomb? And What’s the difference between a nuclear bomb and a nuclear reactor?
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