Re: To Luxel Users

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

At 09:22 AM 3/27/00 -0600, you wrote:
>          I have asked to not see values less than 10 mrem so 
>          I could avoid all the questions resulting from the conversion from 
>          the old film badge to the new luxel.  

There is an important concept in the above reply - a decision by the
customer based on specific goals. Each dosimetry program (vendor-based or
in-house) should determine its LLD using their own data - not a generic
value supplied by a vendor (ANY vendor). The LLD for your program is a
function of the average background, the variability of background
measurements, and the variability of low dose occupational exposure
measurements. In other words, how different does a measurement have to be
from YOUR normal range of background measurements before YOU can say with
confidence that the measurement is due to occupational exposure?

The magnitude of background is a physical characteristic of your facility
and not something that can be adjusted. However, background varies around a
site, and can vary dramatically if the program allows (requires?) users to
take dosimetry home with them. If you haven't made measurements to
characterize the background in the places where your dosimetry are stored,
then you can't really have any idea what your LLD is. We do not emply badge
racks here at SLAC, and wearers routinely take dosimetry home with them. A
background study of the San Francisco bay area found that the true LLD for
our program is about 3 times what we would estimate using on-site
background data only. A conscious decision (i.e., an INFORMED decision) was
made to accept and implement the higher LLD rather than pursue the
extraordinary site and program changes that would have been necessary to
keep dosimeters on-site at all times.

Not all such decisions have to be based on local measurements. The case
above is a good example - a decision was made to maintain continuity with
the previous dosimeter system to avoid user confusion (and seems to carry
an implied decision that the lower dose values aren't necessary to worker
safety, with which I agree).

Just because something CAN be done doesn't always mean it SHOULD be done.
Adding a capability to your program is worthwhile if it adds something of
value. Otherwise, the work and expense of making the changes only got your
something different. How much work do we want to do just to have something
different?

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

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