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RE: LLD for TLD and ED
Jim and Sergio make some excellent points about: 1) the many factors that we have to deal with to evaluate TLD readings at low doses above background; and 2) the good statistics of electronic dosimeters at low doses. I always agree with them over good technical issues. So I won't spend a lot of time reminding everyone that good TLD's are spectrometers, but that most ED's have only a single sensitive element that will only measure personnel dose appropriately if calibrated to the representative photon spectra. Sandy Perle and Bob Flood clarified that. Or that EDs must continue to get smaller to deal with the significant angular dependence issues. Or that peer-reviewed information on EDs remains minimal.
However, I would like add to a minor point to Jim's note that the ED LLD that he is discussing is still a PER ENTRY LLD. And if a facility would choose to record 0.1 mrem increments, there could be other problems. For one, they could be setting themselves up for extra plant dose; i.e., natural background recorded as occupational dose. Our workers wear their EDs for eight to 12 hours and would incur 0.1 mrem each from natural background.
IF the EDs do not spike, as these electronic devices have been doing today at low and high doses because of many factors (RFI, welding, motors, shock, etc), and the EDs operate correctly without incident, you would obtain a reading of 0.1 mrem for practically everyone in the RCA. Maybe we could train our workers to believe that THEIR 0.1 mrem per shift is not related to the radiation fields at our facility, and maybe not. If not, it would add up to a great deal of recorded dose for our many thousands of entries each year; about 30 Man-Rem last year.
However, still more importantly, let's see YEARS of stable ED low dose measurements first before we think about recording tenths of a mrem for every entry --- years without 0.5 mrem spikes and 13 mrem spikes and 1857 mrem spikes. Then we maybe we can deal with this natural background subtraction issue. I know that my TLDs integrate those 0.1, 0.2, 0.3 mrem entries very well, whether it's dose above background or the dose that is natural background.
Very glad to see the manufacturers of these dosimeters watching, listening and making better products.
Mike Lantz, CHP
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From: Jim Kost
Sent: Tuesday, February 02, 1999 4:23 PM
To: Multiple recipients of list
Subject: RE: LLD for TLD and ED
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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