Re: Re[2]: skin contamination to skin dose assessment

[Bob Flood <bflood@SLAC.Stanford.EDU>]

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.