Extremity Doses and Iodine

[John Harvey <harveyj@mcmail.CIS.McMaster.CA>]

Once again I come looking for advice.

At McMaster, we produce curie quantities of I-125 for commercial use. The
iodine is produced in a simple manner, and there is virtually no handling
of the iodine. No labelling is performed. One of our production workers
recently had a very large reported quarterly finger dose, the order of 70
rem. Based on measured dose rates, procedures, and timing, this is not
credible. His quarterly body dosimeter was 100 mrem, and this was
confirmed by his EPD. His coworker received a finger dose of less than the
detectable limit. Naturally, one wishes to rationalize this reported dose.
There are several possible explanations, and I would like the benefit of
your collective experience to rule out the unlikely ones and perhaps to
receive some alternative explanations. The worker receives routine thyroid
counts, and there was no uptake during this period and so he personally
was not exposed to significant I-125 concentrations. He wore the ring in
an unusual fashion for comfort reasons; it was worn over one pair of
disposable gloves and under one or two other pairs of disposable gloves. 
It is possible for the ring to be contaminated or exposed to iodine
without exposure to skin. 

1. Contamination. Crude calculations indicate that it would take perhaps a
uCi for half the period to generate this dose, assuming the conversion
electrons and Augers don't get to the phosphor. I would assume that this
quantity of I-125 would be detected by the vendor. Is it possible for
I-125 contamination to oxidize in air over a period of time, and so
essentially disappear from the ring by the time it is returned?

2. Rather than simple contamination, can the I-125 bind temporarily to the
plastic holder, irradiate the badge, and over time abrade off or be
displaced? 

3. Can LiF absorb some quantity of I2 or CH3I vapor and convert some small 
fraction to LiI? In this case, a large "dose" could be obtained from a 
relatively small quantity. The dosimeter was stored in an area where the 
average concentration was about 20-40 Bq per cubic meter, and thus there 
was a chronic exposure.

4. Considerable work is done in a glove box. We know that iodine diffuses
through the gloves. Is it reasonable to consider that perhaps high local
concentration results in some diffusion through the outer layer of
disposable hand gloves, and so iodine reaches the dosimeter, but not the
skin which is still under one more pair of disposable gloves? 

Obviously we are reaching for an explanation. Only contamination seems to
be credible, but our regulatory agency is under the impression that the
vendor checks each dosimeter for contamination, and if a result is
reported, no contamination was detected. Ergo, the dose is personal. We
will make measurements, but suggestions about where to start based on
experiences at other institutions would be nice. 

Thanking you in advance.
 
=======================================================================
John Harvey                            | McMaster University
Senior Health Physicist                | NRB-110
(905) 525-9140 ext 24226               | 1280 Main St. W.
harveyj@mcmail.cis.mcmaster.ca         | Hamilton, Ont Canada L8S 4K1