RE: Low Positive Doses in Personnel Monitoring

["Michael Bielby" <Mike_Bielby@msn.com>]

Badge reading is a stochastic process. The minimum reportable dose is a trade 
off between false positives and false negatives. NVLAP currently requires 
processors to use a p value of 0.95 in determining the lower limit of 
detectablity, and, at least in our case, we use the LLD as the minimum 
reportable for competitive reasons. Thus one would expect 5% of the unexposed 
badges to show a false reading. If you are seeing a higher number than that in 
badges you know weren't exposed to occupational sources, then there may be a 
systemic problem, probably relating to use of the control.

Practically, you could ask your vendor to raise your minimum reportable dose, 
say to a p value of 0.99. This would result in a higher number of undetected 
doses, but really, doses in this range are of no consequence anyway. I suspect 
that most people who would be concerned over a dose of 30 mrem would feel 
better if it just "went away", and not ask "Why?".

Said with tongue barely in cheek,

Mike Bielby
TLD Technical Manager
Radiation Detection Company
mike_bielby@msn.com