Re: I-125, 131 sources

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

>Does anyone have the address/phone of a company that manufactures "mock" I-125 
>and I-131 sources?  We need to generate a calibration curve for a thyroid neck 
>phantom, and would like sources with long half lives so that we can perform 
>regular calibrations.

Rather than "mock-" anything, I'd recommend getting single-nuclide analog 
sources, for the long term, and obtaining THE REAL THING for a one-time 
comparison calibration.  The people in Nuclear Medicine here gave me two 
pairs of I-131 and I-125 sources, in plastic tubes which I provided, so I 
was able to establish the energy window and efficiency corresponding to 
those.  By comparing these with the same parameters determined for rod 
sources of Ba-133 and I-129 (The single-nuclide analogs of I-131 and I-129, 
respectively), which are used for daily function checks also,  I now have 
efficiency ratios and window position relationships which will allow 
calibrating the instrument with the Ba-133 and I-129 sources in future.

My information will probably differ from yours, but here is what I determined:
   I-131 optimum window is identical to Ba-133 optimum window.
   I-131 efficiency is 1.0924 times Ba-133 efficiency.
   I-125 optimum window is "6 channels" (on the Ludlum 2218 threshold knob) 
below       the I-129 optimum window.

Mock sources rely on mixtures of nuclides, with different decay rates.  For 
example, a mix of I-129 and Am-241 for mockI-125.  This leads to the 
question, how to correct for the age of the source?  Sure I-129 lasts 
forever, but Am-241 decays about 1% every 6 years.  So how much has the 
window moved, or how has the gamma flux gone down, since it was new?  After 
several years, you'll probably prefer to dispose of the source and get a new 
one rather than attempt those corrections.

Speaking of mock I-125 sources, I think it fair to warn anyone using one not 
to get it wet.  We had one start leaking a couple of years ago, and had to 
decontaminate one of those wells in our acrylic/water phantom.  Maybe the 
glue used to hold the colored plastic to the clear plastic was water-soluble?

A followup question for the members:  What do they mix Ba-133 with to get 
"mock-I-131"?  And what proportions?  I've got a source I need to dispose 
of, but don't know what to declare it.  I've heard a rumor it's mixed with 
Cs-137, but nothing authoritative.
Albert Lee Vest    avest@magnus.acs.ohio-state.edu
health physicist        Office of Radiation Safety
(614)292-0122            The Ohio State University
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