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RE: Luxel experiences

We are currently investigating what appears to be a problem with proper
interpretation of dose when there is a mixed gamma/beta field with a shallow
to deep dose ratio of ~3:1 or greater.  We are seeing the shallow dose being
under-estimated, and the deep dose being over-estimated.  We have seen
this problem in a test irradiation with Cs-137 gammas and Kr-85 betas, and
we have also had problems with exposures in the field, with Mo-99/Tc-99m
(unshielded) as the source.  The problem with Mo/Tc was discovered when badge
readings were compared with Siemens EPD results, where the badge and EPD 
were worn side by side.  A Victoreen 450 ion chamber gave shallow/deep dose
rate ratios that matched the EPD fairly well, but not the badge.

We are working with the dosimetry processor to resolve the issue, but are 
not there yet.  Anyone dealing with photon and medium to high energy beta 
fields who is using this new dosimeter should keep an eye out for this.
My best guess as to the cause of this problem is that the filters are too
thin, and the current algorithm(s) are not able to differentiate between
a low energy photon dose and a mixed beta/gamma dose.  Some betas get through
one or both filters, depending on energy, and are interpreted as photon dose.
The film badges used previously had much thicker filters.

I will post a follow-up status report once this issue is resolved.

John Laferriere, CHP
DuPont Pharmaceuticals Co.
Medical Imaging Division
john.r.laferriere@dupontpharma.com
(978) 671-8316

Standard disclaimers apply.

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