GRB Hypothesis for Terrestrial C14 and Be10 Spike

  1. This may be (probably is) a stupid question but in a popular current story in astronomical news, scientists have put forth the idea that a spike in C14 and Be10 isotopes found in tree rings in Japan and a few other locations in 775 AD could have been due to a short GRB 3kly to 12kly away.

    http://phys.org/news/2013-01-8th-century-gamma-ray-irradiated.html

    What is puzzling me is that short GRB's have most of their flux in gamma rays in my understanding and I thought that C14 could only be produced by n + N14 -> p + C14 where the neutron comes from cosmic ray collisions with nucleii in the upper atmosphere. How could gamma rays induce C14 production? Similarly for Be10?
     
  2. jcsd
  3. mathman

    mathman 6,479
    Science Advisor
    Gold Member

    I don't have a definite answer, but I would guess the gamma ray burst led to a sharp increase in the number of neutrons available for the reaction you described.
     
  4. I suppose that is possible, not being a nuclear physicist I don't know but can gamma rays even induce nuclear reactions?
     
  5. mfb

    Staff: Mentor

    That is possible - a photon can hit a nucleus, and a neutron (or something else) can be emitted.
     
  6. Interesting, is there some example or discussion of such a process that can be linked to?
     
  7. mfb

    Staff: Mentor

  8. Found this on Wikipedia: interesting

    High energy photoneutron
    Neutrons (photoneutrons) are produced when photons above the nuclear binding energy of a substance are incident on that substance, causing it to undergo giant dipole resonance after which it either emits a neutron (photodisintegration) or undergoes fission (photofission). The number of neutrons released by each fission event is dependent on the substance. Typically photons begin to produce neutrons on interaction with normal matter at energies of about 7 to 40 MeV,

    Good info on photoneutron cross section here: http://prola.aps.org/abstract/PR/v91/i3/p659_1 Interesting if the most common species for phototransmutation is the N14 that also produces the C14.
     
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