Nuclear

Part 15 - Atom Bomb

Author's note:  Note: for a more complete history,  I recommend reading, The Making of the Atomic Bomb by Richard Rhodes.

By February of 1940, when the German army was about to over-run France, two German Jewish refugees in Sweden, Lise Meitner and her nephew, Otto Frisch, had realized that a nucleus of 235-U92 would fission (split into smaller elements) if it absorbed a neutron. One fission released up to three neutrons and an enormous amount of energy. 

 Frisch studied the absorption capture cross section for fast neutrons in 235-U92 and calculated that a bomb would need only one or two pound of 235-Uranium92 . . . about the size of a golf ball. 

Frisch returned to Britain in mid 1939 and worked with Rudolf Peierls who estimated the time for each fission would be about four millionths of a second and this exponential chain reaction would continue for several thousandths of a seconds, before the critical mass expanded, separating the atoms too much to permit fission. Several thousandths of a seconds would allow 80 generations (each doubling the number of neutrons 1, 2, 4, 8, 16, 32, 64 etc).

This was sufficient time to produce a temperature hotter than the sun's interior and pressure greater than the centre of the Earth. A five kilogram bomb might be the equivalent of several thousand tons of dynamite and the radiation would be fatal for a long time afterward. 

 They also calculated that 100,000 gaseous separation tubes could extract a few pounds of 235-Uranium92 in weeks. Frisch and Peierls discussed this with Mark Oliphant and convinced British authorities to back more research. James Chadwick began studying fast neutrons with his new cyclotron at Liverpool in Britain.

Meanwhile in Nazi Germany, Werner Heisenberg had decided to separate the isotope uranium 235-U92 (0.7%) from the more common and chemically identical 238-U92 (99.3%). He also needed heavy water, that is water with a higher than normal content of deuterium (an isotope of hydrogen which has a neutron in the nucleus in addition to the single proton). This would to slow the speed of neutrons, released in a nuclear fission, making them more likely to cause another 235-uranium92 atom to fission. Heavy water was a good moderator, as it did not absorb neutrons, but it was expensive. The Norwegians had a small stockpile of heavy water but refused to sell it to the Germans and quickly smuggled it to France. 

The Japanese were also working on the problems while also attempting to obtain heavy water from Norway.

In Russia, in 1940, Konstantin Petrzhak and Georgy Flerov, had noticed that other scientists, they knew to be working on fission, were no longer publishing.  Surmising that they were working on American military devices, Flerov wrote to Joseph Stalin urging the establishment of a nuclear research a program. (In 1942, Flerov wrote to the director at the Radium Institute in Leningrad, Igor Kurchatov, with plans for an atomic bomb).

Meanwhile Szilard was becoming frustrated by the lack of response from the U.S. uranium committee.

At the end of February 1940, Enrico Fermi, asked Alfred Nier to extract a pure sample of 235-uranium using an early mass spectrograph designed by Nier. This tiny sample was mailed to John R. Dunning's team who worked all night, with the Columbia University cyclotron, bombarding the sample with neutrons. This clearly showed that 235-U92 was responsible for the slow neutron absorption. No one noticed that 235-U92 also absorbed fast neutrons.

In May 1940, Louis Turner wrote to Szilard pointing out that neutrons absorbed by 238-uranium92 would create another element which might be fissionable. Meanwhile Edwin McMillan and Philip Abelson analyzed a sample of irradiated uranium and isolated a new element with atomic number 93 (later named neptunium) with a half life of 2.5 days.

Fermi was thinking about element 94 (later named plutonium). If it was fissionable with slow neutrons and could be chemical separated, it would be an ideal candidate for a bomb. 

In January 1941, Glen Seaborg, Arthur Wahl and Emilio Segré identified a new element, plutonium (element 94) by bombarding uranium with deuterons (the heavy isotope of hydrogen) in the 60-inch (150 cm) cyclotron at the Berkeley Radiation Laboratory at the University of California, Berkeley. It was produced when 238-uranium92 absorbed a neutron and transmuted into element 93 (239-neptunium93) which quickly decayed into 238-plutonium94. By March they had separated less than a millionth of a gram of 239-Neptunium93 and set it aside to decay completely to 238-Plutonium94. They then irradiated the tiny sample and found it had a massive cross section for neutrons which meant it would readily absorb neutrons and would indeed fission.

Meanwhile, Enrico Fermi had measured the neutron capture cross section of graphite. It would be a good moderator; it would slow down neutrons without absorbing them, thereby increasing the chance that the neutrons would be absorbed by a atom of 235-uranium92. 

(Paradoxically, German scientists had concluded that graphite was an unsuitable moderator because of impurities, particularly boron, which absorbed neutrons). 

Fermi found time to discussed the possibility of a fusion bomb with Edward Teller. A fission bomb would provide temperature high enough to fuse deuterium to form helium nuclei.

 However, in Kyoto, Japan, Tokutaro Hagiwara had reached the same conclusion in May 1941 one month after the Japanese army had authorized research for an atomic bomb.