From key@physics.utoronto.ca Mon Mar 24 09:54:31 2008
Date: Mon, 24 Mar 2008 09:54:31 -0400
From: Tony Key <key@physics.utoronto.ca>
To: irisyi.wu@utoronto.ca
Subject: Re: question about"inversely proportional to the square of the distance"

Good question, Yi.  I'll cover this in my lectures this week.  The short
answer is that you use both the inverse square law, and the exponential fall
off with distance to calculate the fluence etc. at some distance from the
source of the radiation.  In the case of travel through air, the exponential
fall of is so slight, that the inverse fall off has more effect.  In tissue
the opposite it true.  So in many cases we can consider the result of only
one of these effects, but both are always operating.

TonyK

irisyi.wu@utoronto.ca wrote:
> Hi Professor,
> 
>   I have a question about the mastering physics this week, question  No.
> 5: since 60Co is a point source,so the dose is "inversely  proportional to
> the square of the distance". I remembered we did  another MP problem
> before, in that problem, exposure is also  "inversely proportional to the
> square of the distance".
>   I'm confused about this, because in most situation we use  "..exp(-ux)"
> to do calculation. So under wut situation we should use  "inversely
> proportional to the square of the distance"?
> 
> Thanks!!
> 
> Yi
> 

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