Re: Sandy's EPD Comments

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

I think a key point in this discussion has to do with changes in the
spectrum to which a dosimeter is exposed during a monitoring period. Any
dosimeter can be calibrated to a spectrum, and if that spectrum NEVER
changes, accurate dose measurements can be made. However, if the
calibration factors for different spectra are very different from each
other, a post-processing algorithm may be able to identify the spectra and
apply approriate corrections, but a simple one-factor calibration embedded
in the device will be inherently inaccurate and highly vulnerable to
failure of quality-related tests. Operationally, one can compensate by
using the most conservative calibration factor and thereby avoid breaking
the law, but how many of your department heads will be rewarded at year's
end with bonuses for higher man-rem totals? And while a conservative
calibration used in mixed fields may get you past the NRC or DOE inspector,
it won't get you past the accreditation tests and assessments.

Another characteristic is vitally important to the use of EPDs - cost. The
units cost far more than film or TLDs, and their use cannot be justified if
they are to be assigned permanently to each worker. That practice would
mean replacing a $25 dosimeter with a $350 dosimeter - poor economics.

To be viable, EPDs must be used as shared dosimeters, worn as needed by
many individuals. In this way, one need only have enough EPDs to monitor
everyone in the controlled area at any one time rather than maintaining an
inventory to monitor everyone who will need monitoring at some time. But
this shared dosimetry approach has an enormous trap door - if an EPD fails
its routine annual calibration by under-responding, how will you do the
dose investigations? That particular EPD may have been used by thousands of
people during the year, generating perhaps hundreds of thousands of
individual measurements. Even if this happens only once, do you have the
staff to cope with the workload of such dose investigations?

Sandy is right in saying the EPD is an excellent (if expensive) secondary
dosimeter for real-time exposure control. It's ability to interface with
computers adds functionality that cannot be obtained from any other
dosimeter (most notably, access control functions used in many nuclear
power plants, and the rate alarm feature for High Radiation Area entries).
But evaluated only by its radiation measuring abilities, a pocket chamber
can equal or beat EPD measurements, and a TLD or film plus pocket chamber
program can be operated cheaper than an EPD-only program. Justification of
the use of EPDs really has to be based on those additional functions they
provide rather than the measurement characteristics.

---------
Bob Flood
Dosimetry Group Leader
Stanford Linear Accelerator Center
(650) 926-3793
bflood@slac.stanford.edu

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