RE: LLD for TLD and ED

[Jim Kost <jkost@mgpi.com>]

George:  An electronic dosimeter is a continuously integrating device
with a detector sensitivity typically 300 cps per R/h.  This means that
it takes about 1000 pulses from the detector for a 1 mrem dose.  If you
want to be at least 85% sure of the delivered dose you should count for
at least a number of pulses that will give you a standard deviation not
bigger than 10% of the counts.  One sigma for 100 counts is 10.  It
meets this criteria and this is the reason we chose  0.1 mrem as the
minimum reportable dose.  Now this figure IS NOT time or rate dependent
(within operational limits of course which in the case of the DMC-2000
is 2 seconds for a display update and 1000 R/h for dose rate).  The
dosimeter does not care how long the 100 pulses take to be accrued or
how long after the exposure you read it, you can stop a dosimeter from
accumulating dose if you want to, so you could assign the 0.1 mrem to a
specific exposure circumstance.  Not an easy thing to do with a TLD
unless you have your own readers and one very tight quality and
calibration program.
 
This compares favorably with a TLD which can typically generate only 3
to 5 light events for the same 1 mrem exposure.  50 to 200 times less
signal for the same exposure.

On the Electronic Dosimeter the LLD is ONLY a factor of the detector
sensitivity and the accuracy depends on the calibration of the device.
In the TLD case, which is only a relative measurement device, the LLD
depends on many factors like the background of you reference TLD's,
calibration and sensitivity factor accuracy, noise of the TLD reader,
thermal history of the TLD, fading, time of exposure, time from exposure
to read, TLD material type, etc.

Please let me know if I can be of any further help.

Sergio Lopez
Jim Kost
jkost@mgpi.com


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