Re[2]: skin contamination to skin dose assessment

[BRZGV@ccmail.ceco.com]

     Here is a question and a little data calculated by Varskin2.
     
     This question is related to GM instrument response.  Each decay may 
     yield one or more beta particles.  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?...  
     
     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?...
     
     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
     
     If isotopic analysis isn't really an option for some smaller 
     facilities, then I would certainly think you could come up with some 
     rules of thumb that take instrument response and varskin-generated 
     dose factors into account.
     
     
     Sincerely and looking forward to feedback,
     
     Glen Vickers
     HP, Nuclear Power
     BRZGV@ccmail.ceco.com


______________________________ Reply Separator _________________________________
Subject: Re: skin contamination to skin dose assessment
Author:  radsafe@romulus.ehs.uiuc.edu at INTERNET
Date:    4/9/97 11:07 AM


At 10:23 AM 4/9/97 -0500, you wrote:
>with the exception of the case of a zero thickness source on bare skin 
>(absorber thickness = 7 mg/cm^2), where 7 +/- 20% only bounds this curve 
>up to a maximum beta energy of 750 keV or so.  For a zero-thickness source 
>on bare skin, the skin dose rate per micro-curie per cm^2 is bounded 
>between 8.5 and 9.2 for maximum beta energies between 750 keV and 3 MeV 
>(according to Healy).
>
     
This is a good rule of thumb for determining skin dose once the identities 
of the isotopes in the contamination are known. But that's the hard part. 
Operationally, determining the isotopes in the contamination can be very 
difficult, from obtaining a truly representative sample for analysis to 
dealing with isotopes that cannot be detected by gamma spectrum analysis 
(Sr-89, Sr/Y-90, et al.).
     
The detection efficiency of the standard frisker (15.5 cm2 thin window GM 
tube in all its many configurations) changes with beta energy in a way that 
looks like a GM plateau curve: basically, detection increases as beta 
energy increases up to the point where almost every beta emitted within the 
solid angle defined by the source and the detector  is detected, hence the 
plateau.
     
The skin dose rate per uCi also exhibits this same plateau curve versus 
beta energy (I have my own ideas why this happens, but I'd love to hear 
some opinions). As a result, it so happens that the standard 2 inch 
diameter frisker (15.5 cm2 detector surface area) is an almost energy 
independent skin dose rate meter. One can simply use any matched pair of 
detection efficieny and dose rate factor for a given isotope and calculate 
an accurate dose rate, without knowing the actual isotopes present.
     
For example, let's look at a frisker survey of 10,000 cpm from 
contamination on the skin. If we use (without benefit of analysis) the 
detection efficiency for Co-60 (10%) and the dose factor for Co-60 (4.1 
rad/h/uCi), the dose rate calculates to about 185 mrad/h (10,000 cpm = 
0.045 uCi). But suppose the contamination was all really Rh-106, max beta 
energy about 3.6 MeV, with a dose rate per uCi over double that of Co-60 
(9.4 rad/h/uCi). Well, using the detection efficiency for a hard beta beta 
like Rh-106 (25%), one gets 10,000 cpm/0.25 = 0.018 uCi, times 9,400 
mrad/h/uCi, yielding 171 mrad/h. I'll accept the difference between 185 and 
171 rather than try to do isotope identification any day.
     
If one does this calculation for a variety of isotopes from the Co-60 
energy to the Rh-106 neighborhood, one gets about 175 mrad/h/10,000 frisker 
cpm +/- 5%, which ain't bad for a hand held survey instrument!
     
     
Bob Flood
Stanford Linear Accelerator Center
(415) 926-3793     bflood@slac.stanford.edu
Unless otherwise noted, all opinions are mine alone.