Explaining the difference between Electronic Dosimeter

["Sandy Perle" <sandyfl@earthlink.net>]

I receive this type of question often, and decided to post my reply, 
removing the information regarding the source of the latest question 
and facility. Since the question is raised often, it is probably a 
question that others ask themselves, but never put it into writing. 
If this can be of assistance, then it's worth taking the time to make 
the dialogue available to everyone. Of course the discussion 
addresses a TLD and an Electronic Dosimeter. However, the philosophy 
is valid when comparing two devices of any type, that are different 
and unique.

My reply to this particular individual follows:

There are many differences. Each dosimeter has its own 
characteristics, and needs to be dealt with independently, even in 
the manner in which they are calibrated. Some thoughts:

(1) The TLD accumulates dose from the time it is annealed, prior to 
shipment, until the time it is processed (heating to 300 degrees C). 
Therefore, all exposure, occupational, natural background and transit 
dose (higher natural background) is accumulated. The Control Badge is 
used to define what is natural and not occupational dose, and is then 
used to report the net dose on the personnel dosimeter.

The EPD only measures exposure while the unit is on.

(2) The TLD uses an algorithm that allows the device to measure and 
report dose for all types of radiation, i.e., gamma, beta, x-ray and 
neutron, and, dose ranges based on specific methods of calibration.

The EPD is generally calibrated to Cs-137, and highly under-responds 
to much higher energy photons and low energy x-rays, and will not 
respond to beta (unless you have Siemens or perhaps, the new MGP 
EPD). In any event, the SDE is not as accurate as the DDE.

When a facility is in an outage, and there is much work performed in 
high energy photon araes, where the average energy is around 1 MeV 
(due to Co-60), the TLD is highly accurate, where the EPD will under-
respond to this high energy photon component, by ~ 10 to 15%. This 
causes the TLD to report a higher dose than the EPD. It is very 
important to know the fileds one has worked in when attempting to 
compare a TLD / EPD ratio.

(3) Many calibrate their EPD to match the TLD. This is not a correct 
method. They act characteristically different, and need to be 
assessed differently. Many EPDs have set points, and how these are 
manipulated determines how the EPD reports dose. The EPD generally 
estimates a dose rate and then converts to a cumulative dose reading. 
If there is a spike in the reading, there may be the potential for an 
over-response for the cumulative dose.

(4) TLD / EPD correlations. The TLD only reports dose > MRD, and in 
ICN's case, as is the industry standard, we report dose > 10 mrem. 
The EPD generally measures dose rate in 0.1 mr/hr (so they say) and 
will report a 1 mrem cumulative dose. The real issue is in how the 
facility issues and reads EPD dose for personnel. Do you give a 
person an EPD and they wear it for the whole day, a certain time 
period, or do they exchange for a different EPD on each entry into 
the radiation controlled area? If there is a new EPD used for each 
entry, there is the issue of EPD accuracy and precision, and, the 
small incremental dose that is received, and rounded up on each 
entry. This could lead to higher EPD over-response.

(5) The EPD is susceptible to external factors, such as RF, cell 
phone etc. These do cause spikes in many EPDs used in the power 
reactor world. This is getting better, but not there yet.

(6) The TLD using LiF is tissue equivalent. Therefore, the dose is 
easier to measure and report. The EPD is not tissue equivalent, and 
is only as good as the source used to perform the calibration. As 
individuals move through the plant, they are exposed to all kinds of 
radiation, and energies, due to primary and scattered radiation. The 
TLD algorithm accommodates this. The EPD can not, and will assume all 
response due to Cs-137 equivalent energy, 0.662 MeV. All of the low 
energy x-ray will under-respond, and the high energy gamma, > 0.662 
MeV will under-respond. How the TLD / EPD ratio works at the end of a 
monitoring period is dependent on these mixtures and time frames of 
exposure.

(7) The EPD is highly geometry dependent, and the batteries do skew 
the data sometimes. The TLD is more tolerant to geometry, and is not 
nearly as much a problem.

(8) EPD accumulated dose in ROM does not fade. TLD dose will fade 
based on the TL material used in the dosimeter. If it is a Panasonic 
TLD, which incorporates both Lithium Borate and Calcium Sulfate 
elements, each element fades independently of the other. If it is a 
Lithium Fluoride TLD, it too has a specific fade characteristic 
(similar to Lithium Borate). We incorporate fading correction in our 
algorithm. In that we can not determine the specific fade for each 
specific dose received by the individual, we use the mid-point of the 
wear period for calculating the fade correction factor. In other 
words, for a quarterly wear period, the fade used is based on 45 
days. The variance around that point is not significant. What is 
significant is when the individual receives ALL of the dose at the 
very beginning of the monitoring period, or at the very end of the 
period. In the first case, the TLD under-responds, since the dose 
requires a fade correction of 90 days, and we used a 45 day fade. For 
the latter case, the fade to be used may only be on the order of a 
few days, but the 45 day fade will cause an over-response. These are 
issues that can be addressed between the processor and facility PRIOR 
to the dose calculation process.


------------------------------------------------------------------------
Sandy Perle					Tel:(714) 545-0100 / (800) 548-5100   				    	
Director, Technical				Extension 2306 				     	
ICN Worldwide Dosimetry Division		Fax:(714) 668-3149 	                   		    
ICN Biomedicals, Inc.				E-Mail: sandyfl@earthlink.net 				                           
ICN Plaza, 3300 Hyland Avenue  		E-Mail: sperle@icnpharm.com          	          
Costa Mesa, CA 92626                                      

Personal Website:  http://www.geocities.com/capecanaveral/1205
ICN Worldwide Dosimetry Website: http://www.dosimetry.com

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