Re[4]: skin contamination to skin dose assessment

[BRZGV@ccmail.ceco.com]

     Bob,
     
     I believe I have heard that you wrote a pretty good article some time 
     back about many of the things you are talking about.  Might you be 
     able to kindly give a reference so that I might be able to obtain a 
     copy?  I think your comments are worth a thorough evaluation.  In the 
     end, the goal is to get the dose right and secondly develop a method 
     that is simple so that it can be performed consistently and quickly.
     
     Sincerely,
     Glen Vickers
     brzgv@ccmail.ceco.com


______________________________ Reply Separator _________________________________
Subject: Re: Re[2]: skin contamination to skin dose assessment
Author:  radsafe@romulus.ehs.uiuc.edu at INTERNET
Date:    4/16/97 6:22 PM


At 11:53 AM 4/16/97 -0500, you wrote:
>     This question is related to GM instrument response.  Each decay may 
>     yield one or more beta particles.
     
Actually, some may yield less than one per decay, and other more than one. 
It depends on the isotope
     
>     Would the GM response 
>     characteristics of the GM count all of them as one pulse/avalanche?  I 
>     would think that any delayed emissions from a decay would have to be 
>     substantial to fall outside of the typical 50-100 microsecond dead 
>     time.  Any thoughts?...  
>     
Delay is not the issue for dead time. Dead time affects the accuracy of the 
count or count rate when betas reach the detector too close to each other, 
i.e., a second beta arrives during the time the first betas effect is being 
processed by the instrument. During this processing time, the device is 
blind to newly arriving radiation. For a typical 15.5 sq cm GM, the 50 msec 
dead time means that count rates over about 10,000 cpm need deadtime 
correction (deadtime error exceeds a few percent). Note that those higher 
range rate meters that can display up to 500,000 cpm can lead to whopping 
corrections - more than you would want to defend in court! I recommend you 
have a smaller, less efficient GM as a backup detector for high count rate 
situations.
     
The avalanche occurs for each DETECTED beta (or photon). 
     
>     Does it not seem unlikely, from a kinetics/electrical field repulsion 
>     point of view, that all of the betas from a single decay would all be 
>     emitted in the same direction?  A random distribution of 
>     betas/energies striking the GM at any one instant would create a 
>     randomly changing detector efficiency.  I wouldn't go so far as to say 
>     that it would produce a noticeable effect by someone using a GM in the 
>     field, but it is just an interesting theoretical observation just the 
>     same.  Any thoughts?...
>     
Emissions do occur in randon directions in three dimensions (also know as 4 
pi geometry). Only those emitted within the volume defined by the source 
and the detector have a chance of being detected. (I am assuming that, in 
the long run, the number of betas scattered out of this volume equals the 
number from outside the volume that are scattered into it.) Variations 
within the detection volume due to the randomness of emissions within the 
"detection volume" are too small to notice unless the emission rate and 
count rate are extremely low, at which point the dose rate becomes of no 
interest.
     
>     Here is some data compiled from a few varskin runs. These are typical 
>     contributors in a nuclear power plant setting and you can see that 
>     there can be a significant difference in the actual dose factors.  We 
>     have the facilities to perform isotopic analysis easily and the mix of 
>     these contributors can vary greatly, so isotopic-specific dose factors 
>     are the rule.  The cases use a 1 microCi-hr point source on the skin: 
>     
>                Gamma   Beta
>                (mrem)  (mrem)
>     Co-60      185     3700
>     Fe-59      96      3890
>     Zr-95      88      4010
>     Sb-125     85      2400
>     Co-58      122     137
>     Nb-95      88      797
>     Ag-110m    269     1450
>     Sb-124     170     4950
>     Hf-181     67      11100
>     
Isotope-specific dose factors are useful only if you have isotope-specific 
detection efficiencies to use with the GM count rate and an algorithm of 
using them. Alternatively, the gamma analysis can be used, but this won't 
account for self-absorption in the source the way a beta-sensitive 
instrument will. That will result in a potentially huge overestimate of the 
dose (who gets bonus points for higher dose?).
     
What you will find is that the detection efficiency for beta-emitting 
isotopes increases with beta energy in the same fashion as skin dose rate. 
(The detection efficiency for photons is very poor, but the contribution to 
the skin dose rate by photons is small, also.) Thus, I could use the 
detection efficiency and dose factor for Hf-181 to calculate dose, even if 
it's realy from a Co-60 source, and get a more accurate dose estimate than 
I can produce from gamma analysis and isotope-specific dose factors. All 
this, and it's less work, too.
     
     
Bob Flood
Stanford Linear Accelerator Center
(415) 926-3793     bflood@slac.stanford.edu
Unless otherwise noted, all opinions are mine alone.