Re: Dosimeter Placement

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

Actually, good practice and the law agree on this one. Bear in mind that
the dose quantity to be measured is deep dose equivalent, not effective
dose equivalent. The NRC says that the dosimeter must determine the deep
dose at the location on the body receiving the highest dose. In practice,
one wears the single whole body dosimeter on the chest, representing a
mid-torso location for reasonably uniform exposure. But, when surveys
indicate that this would not measure the highest dose, one of two courses
of action would be followed. First, if the location of the highest dose can
be predicted (source above the head, below the feet, the work forces the
worker into a specific position, etc.), the single whole body dosimeter
would be relocated to the part of the body that will receive the highest
dose. However, if the maximum dose location cannot be predicted with
confidence, then supplemental dosimeters will be used at multiple body
locations. The dose assigned should be the maximum measured among the
several dosimeters.

There is an ANSI standard on use of multiple dosimeters (commonly called
multibadging), but it applies to determining effective dose equivalent.

Dan Reece, now at Texas A&M, has done some very nice modeling of the
angular dependence of effective dose equivalent for photons, and included
some work on various multibadging schemes. He found that a 2 dosimeter
method (chest and back) using a simple algorithm provided as good an
estimate of EDE as an 8 -dosimeter system. But that's EDE again.

The courts: If your survey records show that the source of radiation was in
a position to deliver a higher dose somewhere other than the chest, but the
dosimeter was worn at the chest, then the plaintiff's expert's speculation
about the "real" dose will get as much air time in court as the dosimeter
measurement. It used to be that just about every plaintiff introduced a
rather wild speculative dose calculation into evidence to counter the
defendant's claimed dose. A decision by an appeals court in a case
involving Pilgrim (I don't have the legal reference) held that when the
defendant introduced a dosimeter-based determination of dose as evidence
(basically, measurements routinely used to demonstrate compliance with
federal regulations), the plaintiff cannot introduce its own dose
calculation unless they show that their's has as good a basis as the
measured dose or that the defendant's dose measurements are flawed.
Difficult to do in these days of NVLAP accreditation, unless you show that
the dosimeter wasn't worn properly. That's where placement becomes an
arguing point.

For my nickel, the ultimate expert in this area is Dave Weidis of Jose and
Weidis. I don't have a phone number or address for him, but perhaps someone
else on Radsafe can help with that.


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