RE: Sandy's EPD Comments

[Mike Lantz <mlantz33@cybertrails.com>]

I am shocked and saddened by the sudden passing of Charlie Willis.  He was a wonderful Health Physicist who will be missed by all.  His support of this list alone was amazing considering all of the efforts he was involved in.  In that light, I can attempt to honor him by increasing my support of the Health Physics Society and this list.  And that I will do.  


I will quit lurking on this list and start supporting it.   I will begin by listing a few of the ways that ED's have measured doses low and therefore could create the scenario that Sandy Perle had presented.  He certainly was not being an alarmist, simply an HP who has tested ED's in various situations.

1) Exposure to high energy photons from underneath, such as standing on a high integrity container of rad waste, TLD = 600 mrem, SRD = 575 mrem and ED = 250 mrem because of significant angular dependence.  This factor of more than 2 increases to a factor of 4 underresponse if the transmitter is added to the ED. 

2) ED's have underresponded by a factor of 3 to noble gas submersion (dominated by Xe-133) even though testing shows the ED responds accurately to 80 kev photons at perpendicular incidence.  Again, a serious angular dependence.

3) Low energy photons from the side, top or underneath have been measured to respond a factor of 14 low (!) in laboratory testing by the vendor.  In my article a few years ago, I worried if these ED's were being used by flouroscopists because of the likelihood of significant underresponse. 

4)  The vast majority of ED's in use in the US will significantly underrespond to any photons below approximately 60 keV at perpendicular incidence, and worsens dramatically as the angle moves from 0 degrees.

5) ED's will saturate and underrespond in VHRA's; and the underresponse will be relatively unknown.  We recently tested a brand new dosimeter type that responded well at 90 R/hr, but at 120 R/hr indicated NO response, nothing!  Its secondary dead time correction in its complicated dose rate to dose conversion algorithm, which is known to NO users, failed and the dosimeter stopped working until the dose rate was reduced to below 100 R/hr!  We also tested ED's to NVLAP Proficiency Testing and 2 dosimeters read 54% and 65% lower than expected because they were irradiated to a high dose rate.

6) Drills, motors, and magnetic fields have placed ED's into a latent state where they are completely UNRESPONSIVE to radiation, regardless of the dose rate.

7) The energy response characteristics of individual pin diode detectors have been documented to be much more variable than I have ever seen in TLD or film testing.  Variable responses, certainly more notable as the energy of photons is decreased, have been found, possibly related to dopants in the pin diode materials; errors up to 60%.

8) ED's have been found to change calibration factors by a factor of 2 in the field; and then correct themselves.

9) The current crop of ED's are fairly new.  I find this the hardest part to swallow that people think they just work; and will continue to work or be supported by the vendor for years, even as the vendors move to newer designs.  As they have aged, loss and repair rates have been significant.  Speakers for the alarms fail.  And so little has been published. 

10) Computer software and hardware that is not in a mature state yet controls the tracking of all doses to be recorded by ED's.  Examples of data loss (ie, dose) include a recent situation where a terminal to verify that the ED was turned on actually turned off random ED's.  Other software problems continue to lose dose by ED's.

This is part of the list of problems with EDs that I wrote about in 1996.  One of our many duties must be to continue to set standards for personnel dosimetry that guarantee our facility personnel high quality dosimetry that works in all spectra, angles and environments that they will encounter.  The vendors have continued to improve their products and we should be proud that we were part of the push for that improvement.

Mike Lantz, CHP