RE: Low Positive Doses in Personnel Monitoring

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

More on this interesting topic.

First, my statement regarding a 5% false positive level was made by typing 
without engaging brain and incorrect. The 5% false positive level is the 
critical level. The detection level (which can be used as LLD) is the critical 
level plus the 95% single-sided normal distribution confidence times the 
standard deviation of the signal at the detection level, which gives a 5% 
chance of not reporting a true positive at the critical level. An in-depth 
explanation can be found in Health Physics, Vol 62 No 1, January 1992, pp. 2 - 
9.

I agree with Sandy completely that the variance of the system is the most 
important issue, primarily because it is the hardest to control. It is 
relatively easy to weed out and correct bias, since it can be introduced at 
fewer points. The variance of the system is used to calculate the LLD, so the 
lower the LLD, the better the system. However, the LLD will also depend on the 
type and energy of the radiation, the mixture ratios of radiation types, and 
the actual background level (and thus the age of the dosimeter and the 
location where it is used).

One can calculate an LLD for controlled conditions, but the "true" LLD for 
badges exposed in the field will likely be higher by  some unknown amount that 
changes from client to client. You can take the calculated LLD and apply a 
"fudge factor" to allow for this, until you get to the point where your 
minimum reportable dose is high enough that there are no false positives. The 
disadvantage to this is a higher number of false negatives and higher 
potential (but truly zero) dose "hidden" in the summing of minimum 
reportables. I contend that unless there are some false positives, your 
statistical limits are set too high. 

The example of one badge reading 9.9 mrem being reported as less than MRD and 
one reading 10.1 reported as 10 is a case in point. Since there is no 
significant statistical difference between the two, one of them will be 
reported as a false reading. In this case I think 9.9 reported as 10 ("false 
positive") would be preferred from a dosimetry practitioner point of view. 

While I am forced by my position to be concerned over the fate of 0.1 mrem 
occupational dose, I encourage HP's managing dosimetry programs to teach their 
users the true significance of an occasional 10 or 20 mrem showing up on their 
report.

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