RE: TLD Background subtraction

["Walker, Scottie" <swwalke@sandia.gov>]

I would like to just mention an alternative to posting control dosimeters at
a designated location.  Our lab is fairly large and we do not have the
luxury of establishing one badge rack for a building or even one technical
area.  And, like many other R&D sites, many of our workers don't regularly
enter radiological areas during the monitoring period.  So, we use
statistical methods to remove the dosimeters having occupational doses and
evaluate the remainder for the background subtraction.  What we're left with
is several thousand measurements of the background radiation levels during
the monitoring period.

This method is obviously more labor intensive than just evaluating a few
control dosimeters, but it is much more pertinent to our site, since our
researchers roam over a pretty broad expanse of our laboratory.  If anyone
is interested in information on our background subtraction method, feel free
to contact me.

Scottie Walker, CHP
External Dosimetry
Sandia National Laboratories
(505) 844-5241
swwalke@sandia.gov

-----Original Message-----
From: Scott Schwahn [mailto:schwahn@jlab.org]
Sent: Wednesday, February 03, 1999 6:38 AM
To: Multiple recipients of list
Subject: TLD Background subtraction


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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