Re: I-125 bioassay

[avest@magnus.acs.ohio-state.edu (Albert Lee Vest)]

>Reply-to: Roger.Moroney@p0.f13.n233.z1.fidonet.org (Roger Moroney)
>Fido-To: hps
...
>The next question is in 
>using NUREG CR 4884 intake retention fractions (IRF) :( . There is no IRF 
>listed for I-125 in the thyroid. In the text, it suggests using the IRF for 
>the stable isotope and multiplying by the decay factor. Well, I looked for 
>IRF for stable I-127 in the thyroid but no such luck. 

Our copy of 4884 has I-127 listed first, before the radioactive iodines. So 
it isn't in numerical order, between I-126 and I-128 for example, but 
somewhat before I-123. Please look again.

>Upon closer inspection, I discovered that the phys. 1/2 life was used and 
not the >effective 1/2 life in calc the decay factor. It does make some 
difference as I >went back and used the effective half life and compared the 
results. Which should >it be? 

Since the IRF data for I-127 includes the effects otherwise characterized as 
"biological decay", with a "biological half life", the only remaining 
difference between nuclides is the nuclear decay rate.  So the difference 
you find should be the physical half life.

I did a similar comparison of I-131 vs I-127 data from this book.  The 
difference was accounted for by physical decay only, up to a certain point 
(about 80 days, if I remember right), after which the I-131 data dropped 
even more sharply.  Any idea what's going on - maybe DOE's FORTRAN program 
crumbled after several layers of round-off error?  Wish I knew.

BTW, we use an investigational level of 10 nCi for both I-125 and I-131, 
with followup as deemed appropriate by the RSO.
Albert Lee Vest    avest@magnus.acs.ohio-state.edu
health physicist        Office of Radiation Safety
(614)292-0122            The Ohio State University
My employer did not review or approve this message.