Re: Comparison of Electronic Dosimeters to TLDs

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

> Has anyone compared Electronic Dosimeter results to TLD results in nuclear
> power stations. We have done so with one kind, and find that the ED
> under-responds by about 12%, but it varies a little depending the station
> and whether there is an outage going on or not. For our situation, the
> angular response of our TLD goes above 1.0 as the angle of incidence goes to
> 60* and beyond, but the Electronic Dosimeter's response goes below 1.0 at
> 60* and beyond, because of the additional material beside and behind the
> detector. Has anyone seen this kind of under-response before? Does anyone
> have field data that compares the response of TLDs and EDs?

John,

Several points to be considered, all depending on the type of ED 
worn:

(1) Depending on battery inside the ED, angularity can be an issue, 
resulting in severe under-response.

(2) EDs generally will under-respond to both the low energy photons 
(< 70 keV) and to the high energy photons (> 662) which is normally 
used to calibrate. Exposures to Co-60 energies generally result in 
under-response to about 15% .. again, dependent on the ED you use. 
Not all display these characteristics.

(3) Need to be evaluated how the ED is calibrated. I have assessed 
several EDs during a NVLAP on-site assessment. I rarely see a 
facility where they calibrate their TLDs and EDs independently, which 
is what each facility should do. Generally, they calibrate and enter 
a fudge factor to make the ED read within some % of the TLD response. 
This is not an appropriate calibration. The ED and TLD response 
characteristics are different, and each should be evaluated based on 
those different characteristics.

(4) How are the ED data points collected and maintained. The TLD 
responds and collects dose cummulatively. Does your ED program 
roundup to the next integer? Do you drop each small incremental entry 
through access control? Do you use the same ED throughout the day, or 
week or month. or do you use a different ED for each entry. Small 
incremental entries generally cause an over-response, unless small 
doses are dropped.

(5) A lot depends on the TLD as well. Is fade correction used for the 
TLD? If not, the TLD will generally under-respond. 

Final suggestion, don't look at the TLD to ED ratio and try and 
determine the problem. Look at the specific individuals who have 
outlying dose differential, and then look at the jobs they worked, 
length of the jobs and when the jobs took place within the wear 
period. Looking at the outliers will assist in the determining the 
causes of your differences.

What you see is not unusual. Mitigating factors can be implemented to 
minimize these differences.

Good Luck!

------------------------------------------------------------------------
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/scperle
ICN Worldwide Dosimetry Website: http://www.dosimetry.com

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