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Re: I-125



NCRP 58 gives the total xray and gamma emissions from I129 as 77.8%,
excepting an 8 percenter at 4 keV. It's not clear what you mean by: "the
other 21%". If that's all the (significant) photons there are, that's all
there are. The energy, when they're not emitted, is carried by the betas
and/or neutrinos, no?
I don't know what your budget is like, but I suggest purchasing a true I125
standard, to meet your needs. For one thing, I125 is electron capture, not
beta decay (although the max E of I129 is 153 keV, about the same as C14, so
if you have much of a cover over it, or over your detector, e.g., the
aluminum closure of an NaI crystal, efficiency will be miserable). Then, I125
has 146% emissions circa 30 keV, or about twice as many as I129, so, if you
use I129, as a calibration standard to simulate I125, you're underestimating
your efficiency by nearly a factor of two. Plus, I125 has a sum peak circa 60
keV, which can further enhance your efficiency, if your counting window is
set wide enough to include it.
You could contact Isotope Products Laboratories for info about the sort of
standard(s) you need: (818)843-7000. I suggest that you ask for the full
catalog, which has lots of glossy photos, which make it easier to think about
what you need/can afford.

Chris Alston
ccja@aol.com