ChipperTheDog
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U-238 fission
Does anyone know the cross-section for high energy neutron fission of Uranium 238? What are the daughter nuclei and their energy levels?
[Edited on 21-7-2004 by ChipperTheDog]
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Nevermore
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if i remember good is somewhat small, like a couple barn, less than 20 barn for sure for energy around 2-3MeV fastly dropping with energy rising.
Daughter nuclei are different, as you know, symmetric fission is higly improbable..
Also the energy of fission fragments depends about their mass, the energy of the incoming neutron, all the angles, the energy of the particles
expelled, and so on.
You can assume reasonably the mass of the fragments to be around 96 and 138.
Nevermore!
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JohnWW
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Re: U-238 fission
I thought that U-238 does not normally undergo fission induced by neutrons, because by itself it does not produce a sufficiently large neutron (or
alpha particle) flux to support a chain reaction in any mass. This is why it is necessary to concentrate the U-235 (a less stable shorter-lived
isotope, but much rarer, which unlike U-238 can decay by spontaneous fission as well as alpha-decay) in naturally-derived uranium, using UF6 put
through diffusion columns under pressure or centrifuges, to produce "enriched uranium".
When sufficiently enriched, above a "critical mass" of the order of 1 Kg if spherical, the U-235 undergoes a sustainable induced-fission
chain reaction, producing stable and long-lived isotopes of elements of lesser mass, with left-over neutrons (which arise because the lighter elements
need a lower neutron ratio in their nuclei) which propagate further fission. This is what happens in a controlled manner in "breeder"
nuclear reactors as in enriched-U power stations, and how the bomb used on Hiroshima 59 years ago worked. At the same time, the surplus neutrons also
react with the U-238 present to form U-239 (and smaller amounts of heavier isotopes), which immediately decays by beta-emission from the nucleus to
form Pu-239. This Pu-239 can be recovered by chemical dissolution and separation from spent fuel rods from "breeder" reactors, and cast into
pure Pu ingots for use in Pu bombs (like that used on Nagasaki) or in Pu power stations.
John W.
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Polverone
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U-238 does undergo fission from high-energy neutrons, such as those produced by fusion reactions. This means that unenriched or even depleted uranium
can be used as a cheap-and-dirty way to massively boost the yield of thermonuclear weapons. Many early thermonuclear tests/weapons developed more of
their energy from U-238 fission than from thermonuclear reactions; the fusion reaction functioned as a very powerful one-shot neutron generator. This
is also in part why early thermonuclear tests produced so much radioactive fallout, because most of the energy was actually coming from dirty fission.
If you look on the FTP, the file us_nuclear_weapons_the_secret_history.djvu has a relatively short but highly readable account of the development of
these weapons and the physics behind them.
PGP Key and corresponding e-mail address
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