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Question about gamma spectroscopy here. If I suspect that on a gamma spectrum I have a line which is a consequence of the presence of Ge-78 - which is being produced via the neutron activation of a stable isotope of Ge (namely Ge-74) - should I then expect to see evidence of the presence of the isotopes Germanium 75,76 and 77? My logic being that for Ge-78 to be produced via neutron activation of Ge-74, then we must have these other isotopes being produced in between. Particularly in the case of Ge-77, which has a much longer half life than Ge-78, I would have thought I should expect to see evidence of it's presence?

My source for the half-lives and other info is here: http://environmentalchemistry.com/yogi/periodic/Ge-pg2.html#Nuclides

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  • $\begingroup$ You'd need four neutron captures to get to Ge-78 from Ge-74 (or two to get there from Ge-76). You'd need a huge neutron flux to expect that and you should see lots of evidence in the form of other Germanium isotope decays. Have you look for those other lines? $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Sep 10, 2015 at 18:37
  • $\begingroup$ I'm not sure why I didn't answer this comment at the time. The gamma spec I'm looking at is taken from a hall in which neutron spallation is taking place from a neutron source (IE yes, we have a huge neutron flux). There is also much evidence of the other germanium isotopes. Although my original question still stands. $\endgroup$
    – Matt
    Commented Sep 16, 2015 at 10:11

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Natural germanium contains about 7% Ge-76. To get to Ge-78 would require two neutron captures on the same nucleus within the 11 hour half-life of Ge-77. According to the NNDC there is not data on neutron capture cross section for Ge-77, so if you are in fact seeing two-neutron capture you are in a position to make a very interesting measurement. I would expect to possibly see some evidence for two-neutron capture in the core of a high-flux reactor like the HFIR at Oak Ridge, but not at a pulsed spallation source unless you are actually looking at the spallation target.

To address your question directly: if you are starting with stable germanium, you're going to make many orders of magnitude more Ge-77 than Ge-78; the factor of 10 in lifetime difference is not nearly enough to make Ge-78 visible and Ge-77 hidden.

If your neutron spallation target were uranium, as at the defunct IPNS at Argonne, you might be interested to know that a neutron-induced uranium fission has probability $7\times10^{-5}$ to produce Ge-78 (data also from NNDC). If that's your source you would also expect signals from other fission products. If you have a germanium detector near a beamline at a user spallation source like LANSCE or SNS, I wouldn't expect any Ge-78 at all.

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