RE: LLD for TLD and ED

[Mike Lantz <mlantz33@cybertrails.com>]

Phil's list is excellent.  I would simply add that old TLD damage can present itself as TL today, combined with poor low dose algorithm assessments.  

I just gave a class at the HPS Midyear and the #1 point was to ALWAYS be able to obtain the element readings from your dosimetry processor.  A vendor may call the tld readings ( 0   12   0   0 ) --- 12 mrem.  But you can interpret it differently and explain to your worker that it probably is a false positive on the low sensitivity element because the high sensitivity elements (3 and 4) show nothing.
	
Please forgive my problemmatic email system.  Never had this problem except these notes to Radsafe.  Anyone know the answer?

Mike Lantz, CHP

----------
From: 	Fulmer, Philip
Sent: 	Friday, January 29, 1999 12:47 PM
To: 	Multiple recipients of list
Subject: 	RE: LLD for TLD and ED

Because of the nature of dosimeters, the phenomenon of false positives
can and will happen on occasion.  There are several possible reasons why
a TLD in particular may give a false positive result.  Just a partial
list...

1.  The TLD may not have entirely annealed during the previous cycle,
although one hopes that this isn't the case.
2.  The TLD may have been exposed to some chemical vapor that causes a
high response in one or more of the elements.
3.  The background/control TLDs may have read a little lower than usual,
causing a personnel badge to exceed the dose reporting threshold.
4.  If workers are allowed to wear their TLDs away from the workplace,
the dosimeters could be exposed to elevated background radiation in or
near their homes.
5.  The TLD may have been exposed to moisture (something as simple as a
person who profusely perspires).
6.  The TLD just statistically exceeds the threshold (The choice of a
critical level at 95% confidence implies a 5% false positive rate from
statistical considerations alone).
7.  Improper placement of the background/control TLDs in a location that
is shielded from the typical background dose rate.

..and the list goes on.  I'm sure others can add many more things
they've seen that cause false positives.

What is crucial in these circumstances is to review all results that
come back.  There may be a pattern in some instances, but it is always
necessary to ensure that the responsible HP be comfortable with the dose
assignment.  

It's also important to remember that the TLD that may be believed to
give a false positive could be telling the truth; so overriding a TLD
result should not be taken lightly.  When it is technically sound to
override the TLD results based on other workplace and worker information
(e.g., a worker assigned 20 mrem for a period in which the worker was on
leave of absence), documentation is everything!

Philip

________________________
Philip C. Fulmer, PhD, CHP
TetraTech NUS
900 Trail Ridge Road
Aiken, SC 29803
(803) 649-7963
fulmerp@ttnus.com


-----Original Message-----
I have another question regarding TLD badges. When I receive an exposure
report back from the dosimeter company. Very often, there has been one
or
two badges that come back with a reading ( though it is a low reading)
that
cannot be explained. i.e. the wearer is not a user, nor have they been
in
any radiation field whatsoever. Can these false readings be explained?
Is
this some sort of statistical anomaly? Has anyone else experienced this
problem? Thank you for your comments.
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