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In 1940, E. Macmillan (E.McMilan) and P. Abe Wilson (P.Abelson) identified the first transuranic element; they call this element neptunium -239 (Np 239). It seems that this element will decay into a long- lived isotope of element 94 . I visited New York around Christmas Day. During the period, Fermi and I exchanged in detail the views on the various expected properties of the atomic nucleus with a mass of 239 and an atomic number of 94. Later, this element was called é’š-239 (Pu 239). Our particular interest is the cause, the life of this substance may be longer, fission can occur in slow neutron bombardment. If this is the case, it is possible to provide a replacement for U235, eliminating the need for isotope separation. Of course, in order to raise enough new substances, we have to build a nuclear reactor to make this new material, and natural uranium may be the ideal fuel for this reactor.
The method can be implemented in order plutonium reality, necessary to prepare a sufficient amount of such new artificial element to clear out its nuclear properties, this element is difficult, particularly in the fission neutron bombardment slow Easy level. In the jargon, i.e. it knows fission cross section of slow neutrons. If you figure out that people can use it instead of U 235, you must produce enough of this atomic bomb. This is far from a trivial matter. On the contrary, alternative methods of separating isotopes are certainly extremely difficult and costly. In the first few months of 1941, J.W. J.W. Kennedy, G. T. G. T. Seaborg , A. C. A.C. Wahl and I, using Pulkley's cyclotron, prepared about 1 microgram of Pu 239, proving that the material could be used as a nuclear fuel.
To make an atomic bomb, there are two alternatives: separating the uranium isotopic cable, or using a sufficient amount of the ruthenium composition . Isotopic separation can be accomplished by constructing a large mass spectrometer or by gas diffusion; the production of niobium first requires a nuclear reactor that forms it, and the other is the chemical industry used to separate and purify the material. In order to produce a sufficient amount of plutonium by artificial acceleration of nuclear bombardment of charged particles it is impossible.
Explosive having a U 235 or Pu 239 becomes practical, people have to put them into a suitable form for producing bomb was OK.
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