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He-3 Neutron absorption

  1. Oct 11, 2015 #1
    I'm curious, why does Helium-3 fission into H-3 and H upon neutron absorption? Surely the He-4 nucleus configuration is more stable?
    I get that the energy release may be what splits the nucleus, because it releases quite a lot of energy if it were to simply absorb the neutron.
     
  2. jcsd
  3. Oct 11, 2015 #2

    mfb

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    How does the energy get released if it does not split up?
    That reaction is possible, but I would expect it to be unlikely as it cannot proceed via the strong interaction alone.
     
  4. Oct 11, 2015 #3
    I find the cross-section quoted as 55 microbarns - compared to the cross-section of proton emission, at 5500 barns.
    Looking around the light isotopes, those which can release the energy by strong interaction - He-3, Li-6, B-10 and N-14 - have cross-section in barns to kilobarns. Those which cannot and only are capable of electromagnetic interaction and gamma ray emission - all H isotopes, Li-7, Be, B-11, C, N-15, O - have cross-section in microbarn to millibarn range.
     
  5. Oct 11, 2015 #4
    Ok, so you're saying that its just much more likely to split up. Actually, now that you mention it, it makes sense now. So if I'm understanding correctly, the He-4 does form, but is only a short lived intermediate product, proceeding to decay into other smaller sub-components.
    n + He-3 --> (He-4) --> H-3 + H-1 (or H-2 + H-2?)
    What promotes the one fission reaction to be more probable than the other? ( Assuming of course D+D is a valid product)
     
  6. Oct 11, 2015 #5

    mfb

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    I'm not sure if the intermediate state lives long enough to call it He-4.
    The emission of a single nucleon (or alpha particle for heavier nuclei) is more likely than a split into two similar and bounds parts.
     
  7. Oct 12, 2015 #6
    I see. So just proton emission then. Thanks a lot. :)
     
  8. Oct 12, 2015 #7
    Nuclear reactions have to conserve both energy and momentum. When you only have one product it's not possible to conserve both except when the reactants have the right kinetic energies. Thus nuclear reactions with only 1 product tend to be rare. For instance why the d+d = He-4 reaction is also rare.
     
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