Re: Panasonic UD-716 badge reader

[Bob Flood <bflood@SLAC.Stanford.EDU>]

At 08:02 AM 5/9/97 -0500, you wrote:

Let me add a few things:

>1. What type of a TLD are you processing? What are the various 
>     phosphors used in the TLD?
>2. You state that the backup elements did not indicate an exposure.  
>    The question is, what elements did you actually process?
>3. Which of the phosphors, or was it just ONE element that displayed 
>     a high response?

These are extremely important questions. For example, the UD-802 dosimeter
is designed to provide a dramatic over-response of element 3 to low energy
photons, while element 4 shows a flatter, less energy-dependent response.
If E3 was read alone, and the individual had been exposed to low energy
photons (took the dosimeter to the doctor's office, perhaps?), it would
look like a high dose that cannot be confirmed by reading E4 or E2.

>4. What kinds of chemicals did the wearer work with? Hydrogen Sulfide 
>     gas can provide high element response?

If this happened, it will be obvious from the glow curves - they won't look
like radiation readouts.

>5. Did the wearer use any powder, as in talc?
>
This is an important and relatively common effect. Dust of almost any type
on the lithium borate elements can cause a false reading. If dust happens
to be in a location that is heated during processing, the dust may get hot
enough to incandesce, giving off enough glow to be counted. For calcium
elements, the number of counts is usually trivial (but not always).
However, if the element being read is lithium borate, these counts are
amplified by the application of the phosphor sensitivity ratio (multiplied
by a factor of about 30). As with chemical effects, the glow curve will
usuually look pretty flaky (no real glow peak, more like noise).

Another possibility is a strong impact against the dosimeter. We recently
had a case of a dosimeter that was worn during an automobile accident in
which the wearer's air-bag inflated. The impact broke the dosimeter hanger
and the element casing (and left a lovely impression of the clip in the
wearer's chest, but he's OK). Examination of the elements found they had
been hammered pretty bad - loose phosphor grains everywhere. I processed
the dosimeter by placing the element plate from the damaged badge inside a
different dosimeter case. The lithium elements read with reasonable
accuracy (about 30 mrem, the accumulated background since the badge was
annealed), but the calcium elements read very high (10,000 mrem). Glow
curves from calcilum sulfate at such doses are normally very smooth (lots
of counts), but these readings were very much unsmooth - noisy. I don't
have any specific data to back this up, but it seems reasonable that a
smaller effect could be seen as a result of a lighter (but significant)
impact on the dosimeter.


Bob Flood
Stanford Linear Accelerator Center
(415) 926-3793     bflood@slac.stanford.edu
Unless otherwise noted, all opinions are mine alone.