Re: TLD Background subtraction

["Sandy Perle" <sandyfl@earthlink.net>]

Scott Schwahn

> Follow this scenario: someone walks by a badge rack with a radiation
> source in hand a couple times a day during the quarter.  I think for some
> facilities this is reasonable, even if not for ours.  Anyway, the people
> who do not normally wear their dosimeters store them in the rack, right? 
> So if the rack control badge is subtracted they are okay.  But the people
> who are more typically exposed to radiation sources are wearing their
> badge during the work day, so their dosimeter is not on the rack.  And
> they are getting dose.  If you then subtract the badge rack control, you
> "erase" their legitimate dose.  And you never knew it happened.

Scott,

I do agree with your comments, to a degree. That is the reason I 
believe that the rack dosimeters should be evaluated for the mean 
and variance and then be compared to the control dosimeters, 
comparing the same parameters. The scenario above is what 
actually happened at one of our reactor sites. In that case, the 
Eberline EPD was being used, and the readers were located on the 
ingress and egress aisles. At the end of the first month of use, 
there were a significant number of high false positives, in varying 
degrees. What the site learned from an evaluation was that the 
reader had an internal source for response checks. The variation in 
microR/hr was mapped out for the various TLD storage drawers, 
that were kept BELOW each of the readers. Those who never 
made any entries had higher doses, than those who were wearing 
their TLDs.

There is no simple answer. I do believe that if the rack dosimeters 
are compared to the control dosimeters, and there is statistical 
agreement, that the rack dosimeters are still more representative of 
what the actual background is. There will always require evaluation, 
and there is always the possibility, although a low probability, that 
the control badges can also have some negative causal agent 
affect them as well.

Interesting problem to work on, with no one true accurate 
methodology.


Sandy Perle
E-Mail: sandyfl@earthlink.net 
Personal Website: http://www.geocities.com/capecanaveral/1205

"The object of opening the mind, as of opening 
the mouth, is to close it again on something solid"
              - G. K. Chesterton -
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html