Re: Screening for contaminated dosimeters

[Brian Gaulke <Brian_Gaulke.HWC@hc-sc.gc.ca>]

On 97/11/20 Jim F. Herrold wrote:

> We got our dosimetry report back from last month with a higher number of
> low x- or gamma-ray exposures (10 to 20 mrem) where we shouldn't have
> seen any. The control badge was returned, and, as far as I know,
> subtracted. The possibility of one or more contaminated badges sent back
> with the group came up. The dosimetry company routinely screens incoming
> badges for contamination and did not see anything unusual (they didn't
> report it to us, anyway). It raised a question, however. We are not in
> the practice of checking our badges for contamination before sending
> them off. In the past ten years I can't recall this being a problem.
>
> My question is: Do other institutions check their badges routinely for
> contamination? If so, or if not, why? What else could have caused
> abnormal counts on maybe 10 percent of the badges?

We recently demonstrated that contamination with natural or depleted
uranium at a level on the order of 0.1 Bq on dosimeter plaques, i.e., what
goes inside the holder, can result in false positive readings of up to 2
mSv for a quarterly wearing period, depending on the distribution of the
contamination relative to the TLD chips.  This level of contamination is
not detectable using standard portable survey equipment.  It is around the
detection limit with a ten minute count with an unshielded thin end window
GM detector (background 20 - 25 cpm).

The problem this brings to light is that any screening process for a high
volume dosimetry service will either be impractical due to the required
counting times, or will be ineffective at detecting low levels of
contamination which can still result in significant false positive dose
estimates.

Brian R. Gaulke, CHP
Head, Dosimetry Section
Radiation Protection Bureau
Health Canada
e-mail: brian_gaulke.hwc@hc-sc.gc.ca