Re[2]: LLD for TLD and ED

["Jim Dunlap" <jim.dunlap@guardian.brooks.af.mil>]

     I recall that in one facility on which we were doing D&D the walls 
     where the TLDs were stored were constructed of material different from 
     that of the walls of the area where the control badge was stored.  The 
     TLD storage area walls read several mR/h (gypsum containing U and it's 
     progeny?) more than the walls where the control TLD was kept (I 
     believe it was wood paneling).  This difference can lead to errors in 
     the readings as well.  You're absolutely correct; the badges should be 
     stored together as a "group".  Good point.
     
     jim.dunlap@guardian.brooks.af.mil


______________________________ Reply Separator _________________________________
Subject: Re: LLD for TLD and ED
Author:  radsafe@romulus.ehs.uiuc.edu at guardian
Date:    2/1/99 4:35 PM


At 01:52 AM 2/1/99 -0600, you wrote:
>
>One could have an LLD with a low value, but with a very high statistical 
>uncertainty. So - one should also consider the determination limit before 
>just accepting the LLD at face value.

And be certain to avoid a mismatch between background monitoring and
personnel dosimeter background. Storing personnel dosimeters in badge racks
around a work site and monitoring background in the HP's office can result
in inaccurate background subtraction, which could be either conservative or
nonconservative in nature. The average background can be measurably
different, and the variability of the background measurements will almost
certainly be different (single location versus multiple location). If the
dosimeters are stored offsite (i.e., taken home), and background is
monitored onsite, the system may seriously underestimate the LLDs and
reporting thresholds, and give frequent false positives. The background
value of interest to a personnel dosimetry system is the average natural
background to which the personnel dosimeters are exposed.

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

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