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Gamma rays induced fission of silicon

  1. Jul 24, 2014 #1
    Hey everyone i'm new here and this is my first thread, although i have great interest in chemistry and physics my knowledge of these fields is very basic( i'm graduated in economics) so dont be surprised if i ask something that may look silly.

    so here are my firsts questions.

    -Is it possible to cause the nucleus of a silicon atom to split into 2 new ones of silimilar mass by shooting high energy gamma rays at that nucleus?

    -If so would a gamma ray with the energy equal to the atom's nuclear binding energy sufice?

    Thank you
  2. jcsd
  3. Jul 24, 2014 #2
    Yes. Although I think it is never very likely.
    It would suffice - and it would much more than suffice.

    A gamma ray with energy equal to the nuclear binding energy (for the stable silicon isotopes, ranging from 236 MeV of silicon 28 to 255 MeV of silicon 30) has sufficient energy to split the nucleus into 28 to 30 free nucleons, all slow moving. It would also be massively unlikely to do so. It would be much more likely to split the nucleus into a few free nucleons, but also one or several big nuclei remaining with a lot of binding energy, and kinetic energy.

    For silicon 28, it might be split into 2 new ones of equal mass, both being nitrogen 14. This would take 27,2 MeV. Alternatively, the silicon 28 might be split into nuclei of similar but unequal mass, namely oxygen 16 and carbon 12. Then this takes just 16,8 MeV.

    But I suspect that for 16,8 MeV energy, splitting into slow oxygen 16 and carbon 12 is not awfully likely, because alternatively that silicon 28 might be split into α and magnesium 24, which takes just 10 MeV, so the remaining 7 MeV are left for kinetic energy of α. Or alternatively the silicon can be split into aluminum 27 and proton, which takes 11,6 MeV.

    Can anyone have more informed comment on what photonuclear spallation branching ratios are?
  4. Jul 25, 2014 #3


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    This process is called photodisintegration.

    According to "The Photodisintegration Rate of ^{24}Mg" (Couch, R. G. & Shane, K. C., page 414), the splitting of Mg24 to Ne20 + α is dominant for photon energies relevant in stars. I would expect that Si28 to Mg24 + α does not look completely different.
    principles of stellar evolution and nucleosynthesis seems to support this.
    This presentation has typical timescales for proton and alpha emission as function of the temperature in a star, and mentions that those two processes dominate (where proton emission is more frequent).

    I think that gives a very consistent picture what happens. Other processes are possible, but extremely rare.
  5. Sep 12, 2014 #4
    Thanks guys ;)
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