How does neutron absorption turn uranium into plutonium?

  1. I keep reading that when a U-238 atom absorbs a neutron it turns into Pu-239. How does that happen? If I'm not mistaken Pu-239 is U-238 plus 2 protons and 2 electrons and minus 2 neutrons. When U-238 absorbs the neutron and becomes U-239 I can see that it can turn into Pu-239 if 2 neutrons decay into 2 protons + 2 electrons. Is that what happens? Are the electrons expelled like in regular beta decay or do they remain in the nucleus so that a neutral atom is formed rather than an ion?
    Last edited: May 22, 2011
  2. jcsd
  3. Astronuc

    Staff: Mentor

    U (Z=92), Np (Z=93), Pu (Z=94)

    U238 + n => U239 => Np239 + ß => Pu239 + ß (leaving out the electron-associated antineutrinos that accompany beta emission) The ß particles slow down in the vicinity of the decay. Electrons are quite mobile and they generally migrate toward positive charges or up the electric potential.

    The U239, Np239 and Pu239 can also absorb neutrons and elevate A to 240, which can also absorb neutrons to A=241. Pu239 is more likely to fission upon neutron absorption. Np-240,241 appear to have low n-capture cross-sections, or the cross-sections are not measured, and the half-lives are relatively short, so the isotopes aren't around long enough to capture neutrons. See figures.

    Attached Files:

    Last edited: May 22, 2011
  4. Astronuc

    Staff: Mentor

    Here are the principal transuranics in a uranium/plutonium based fissile system.

    Code (Text):
    Isotope   Half life (yr)
     Pu-236        2.858
     Pu-238       87.7
     Pu-239    24110
     Pu-240     6561
     Pu-241       14.29
     Am-241      432
     Pu-242    37500
     Am-243     7370

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