TLD Background subtraction

[Scott Schwahn <schwahn@jlab.org>]

Sandy Perle wrote:

> ....What I would like to add is, that the rack monitors are
> in effect, the Control Badges, and should be the dosimeters used
> to make the background subtraction from the personnel
> dosimeters, in that they are located where the personnel
> dosimeters are stored, when they are not being worn. The Control
> Badges that are maintained in the Dosimetry Program Office
> should only be processed, and compared to the various dose
> responses from the rack dosimeters. This is useful to determine if
> there is a higher than expected background in the dosimeter
> storage location. Even if there is a higher background, this is what
> should be subtracted since the rack dosimeters do represent the
> background. I would not use the Control Badges for any
> subtraction, for they only represent the background in an area
> where dosimeters are not stored...

Sorry, Sandy, I must disagree with you on this point.  What you suggest is good in principle only if the badge racks are stored in an area that you know is secure and cannot be exposed to any sources, known or unknown.  This condition is true on only a few of our badge racks.

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.

We've been through these and similar scenarios with our DOELAP assessors,  and we have all come to agree that the best solution is to keep several control badges in a known low-background, secure area for subtraction.  If the badge rack is exposed, their controls show up and appropriate measures can then be taken.


--
Scott O. Schwahn, CHP
External Dosimetry Technical Lead
Thomas Jefferson National Accelerator Facility
schwahn@jlab.org


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