Introduction"I am aware of the tragic impact of the atomic bomb. It is a terrible responsibility which has come to us. We thank God it has come to us instead of to our enemies and we pray He may guide us in using it in His footsteps and to His purposes." Harry S. Truman, August 9th, 1945.
Monday, August 6th, 1945 began as a beautiful and sunny day in Hiroshima. The city had fared rather well up to then. Just like in the rest of Japan, the city suffered from shortages of raw materials and food stuffs but generally speaking, the citizens were rather content. Up to that moment, Hiroshima had been spared the massive American air raids other cities in Japan suffered so much from but all that was soon to change.
Nuclear fissionOn July 17th, 1939, three Hungarian scientists, Leo Szilárd, Eugene Wigner and Edward Teller paid a visit to Albert Einstein in his summer residence at Peconic Bay on Long Island. The gentlemen talked for a few hours. In August, Szilárd and Teller would be visiting Einstein once more. The conversation was held in German mostly as Einstein was not very fluent in English yet. The Hungarian scientists and Einstein discussed a matter that takes us back to 1932.
In 1932, British physicist Sir James Chadwick had discovered the neutron, a particle without charge, only mass, in the nucleus of an atom. During the following years, a number of physicists conducted experiments, firing neutrons at the nucleus of an atom. In a similar experiment in December 1938 in Berlin, conducted by German scientists Otto Hahn and Fritz Strassman, the new element barium (Ba) was formed of about half the mass of a uranium atom. Two other German scientists, Lise Meitner and her nephew Otto Frisch therefore drew the conclusion that the atom had been split. Meitner, a Jewess originally from Austria, had escaped to Sweden via the Netherlands in 1938. Frisch passed Meitner’s idea to Danish scientist Niels Bohr. He investigated the idea further.
Bohr discovered, among other things, that a uranium isotope (U-235) would be easier to split than the natural version of uranium (U-238). U-235 however was and is very rare: a mere 0.7% of the mined uranium is U-235. Isotopes are atoms occupying the same position in the table of chemical elements as the element they belong to but with a different mass. This means that the isotopes of an atom contain the same number of protons (particles with a positive charge, as opposed to electrons which have a negative charge) but a different number of neutrons. Bohr discussed the findings of Meitner and Frisch during a scientific conference which was held in America on January 26th, 1939. In this way, Szilárd and his fellow physicists came to know about the idea of nuclear fission and its consequences.
Each physicist immediately appreciated the possibilities of nuclear fission. During fission, two atoms are formed with a positive charge repelling each other. Moreover, during fission a small amount of mass is "lost". This mass is being transformed into energy, according to the well known formula, discovered by Einstein: E=mc2, which would mean an extremely powerful explosion. As more neutrons are set free during fission than are being used for the fission, a chain reaction will be triggered freeing more neutrons splitting more atoms and so on. If one could succeed in starting such a chain reaction, it would be possible to produce a bomb with a terrible explosive power.
The three Hungarian scientists had drafted a letter they wanted to hand over to the President of the U.S. Franklin Roosevelt (Bio Roosevelt). In it, they pointed to recent developments that would enable the production of a nuclear bomb, at least in theory, and they warned of the danger of Nazi Germany developing such a weapon. At the time, this fear was certainly not unfounded. During the interbellum, Germany was the pioneer in the scientific field and did have the required raw material at its disposal: the uranium mines in recently annexed Czechoslovakia. In their opinion, the president of the U.S. should finance research into the development of a nuclear bomb so the country could stay ahead of the Germans in the construction thereof. They asked Einstein and a number of other prominent scientists to co sign this letter, which they did. On October 11th, 1939, banker Alexander Sachs handed the letter to Roosevelt. His exact reaction is unknown. After having read the letter, the president did remark something like "so we’ll have to somehow prevent them from blowing us up."
Further researchOn October 21st, 1939, a committee was formed in the U.S.: the Advisory Committee on Uranium, chaired by scientist Lymann Briggs. After just one month, an advice was presented to Roosevelt, urging for more research. Unfortunately, only a small amount of money was made available for this, some $ 6,000 which was spent on a number of experiments by Fermi and Szilárd at Columbia University. After this, research in America petered out somewhat. The U.S. were not yet involved in the war and recognition of its importance did not penetrate yet.
In particular the British insisted on more action by the Americans. Two physicists at Birmingham University, Otto Frisch and Rudolf Peierls published a memorandum in March 1940 indicating that a relatively small amount of uranium was required to produce a bomb. Initially, for instance in Szilárd’s letter, a critical mass of a few tons was reckoned with. Frisch and Peierls came to the conclusion that a few pounds would be sufficient to trigger a chain reaction. Consequently, the British formed MAUD (Military Application of Uranium Detonation) of which all important British physicists were members. Research was done into the enriching of uranium and other technical problems inherent to the construction of an atom bomb. Enriching of uranium was possible, according to research by Franz Simon.
In March 1941, MAUD published a report, stating that for a U-235 bomb, a critical mass of some 26 lbs was sufficient and that such a bomb would have an explosive power of 18 kilotons. The report was sent to the U.S. and ended up, unread, in a drawer of Briggs’ desk. Only after a journey to the U.S. in August by British physicist and MAUD member Marcus Oliphant, the matter swung into high gear. Oliphant discussed the matter with a number of American physicists including James Conant and Ernest Lawrence. Again, a number of investigative committees were founded in the U.S. In October 1941, science advisor Vannevar Bush submitted the MAUD report to the president. Subsequently, Roosevelt ordered the National Academy of Sciences to conduct research into the bomb project. The investigation gained more priority after the attack by Japan on Pearl Harbor, December 7th, 1941 and the subsequent declaration of war against the United States by Germany on December 11th. In December, Bush set up the Office of Scientific Research and Development in order to coordinate the various research projects and to start new and more intensive research.