From key@physics.utoronto.ca Tue Apr  1 18:31:29 2008
Date: Tue, 01 Apr 2008 18:31:29 -0400
From: Tony Key <key@physics.utoronto.ca>
To: hanfei.luo@utoronto.ca
Subject: Re: PHY138 Problems

Hi Hanfei - There is no direct relationship, since activity and exposure
apply to different things.  Activity refers only to radioactivity, exposure
only to X-rays.  Both give you some idea about the amount of radiation, but
that's about it.

Of  course you are correct about the fact that the geometry will be
different and very complicated if you take all that into effect. Of course
the actual energy fluence - photons per sq. metre - can be calculated
exactly at any particular point in space.  However, as you say its value
will be different at points that are at different distances.  To the level
of our calculation in this course, these small differences can be ignored -
in fact if we are trying to calculate dosage, the uncertainties in these
calculations are probably worse than any geometric approximation.

Energy fluence is calculated differently depending on the context.  The
relationship with exposure is true for X-rays, without much meaning for
radioisotopes.  I gave an example in this week's class of a calculation of
energy fluence at a distance from a radioisotope.

tonyK

hanfei.luo@utoronto.ca wrote:
> Dr. Key:
>   It's Hanfei again. I have to ask you a couple more questions.
>   The problem is Suggested Extra Problems SNIV #4. What is the  
> relationship between activity and exposure? And, since the person is
> standing 3.5 metres from a gamma ray source, the radiation arrive at  the
> person in the form of a plane or in the form of a sphere surface  section?
> If it's a sphere surface section than the whole problem gets  really
> complicated since some rays are no longer arriving at the  person in right
> angles; they arrive at some other angles, and that  makes calculation
> really hard.
>   By the way, to calculate energy fluence from exposure, we always  use 
> that D air formula. Is there any other way to calculate fluence  without
> the D air formula?
>   Thanks
> 
> Hanfei
> 
> P.S.: Do we need to solve differential equations for the test? Because  if
> we do, we are in trouble since for MAT135 they only taught us  really
> elementary things about differential equations (a brush at the  surface),
> nothing more.
> 
> 


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