From key@physics.utoronto.ca Sun Apr  6 20:40:29 2008
Date: Sun, 06 Apr 2008 20:40:30 -0400
From: Tony Key <key@physics.utoronto.ca>
To: Tony Key <key@physics.utoronto.ca>
Subject: Re: Problem-need help


Hi Stuart for part A, you need to calculate (1-exp(-mu_en)*distance) -
remember to multiply the mass energy absorption coefficient (0.026, the same
as tissue, etc) by the air
density and convert the distance to metres.  I think you have forgotten
that the question asks for the energy absorbed by the air - which is the
amount at the start minus the amount at the end, so to speak.  Your
formula looks fearsome - rather than applying a  formula, think before
you leap!

Hope this helps,

TonyK

> stuart.wiber@utoronto.ca wrote:
> 
> > Good saturday to you professor Key,
> > 
> > I hope that you will be able to respond to this letter, even though it
> > is in such a short notice.
> > 
> > I am working on the problem set due tomorrow and I am really lost as to
> > the method that is used in the Q$ N&R PS#6 (8) Dose rate from a
> > Cobalt-60 source: allowing for absorption in body. Before you jump to
> > any conclusions, hear me out.
> > 
> > Firstly, I am aware that it is assumed that the mass energy absorption
> > coefficient for air is presumed to be 0.026 cm^2*g^-1, the same as that
> > provided for the body.
> > 
> > Secondly, I was not able to derive the answer for part A, which is to
> > calculate the absorptive effect of the air, i.e, the fraction of the
> > energy of the gamma rays absorbed by the air. The method that I used
> > repeatedly gave me an answer of 2.55*10^-2. The correct answer provided
> > is 2.10*10^-2. As you can imagine, I am quite startled that I am not
> > able to derive the correct answer so soon to the test. The method I used
> > was as follows (thank you again for taking the time to look over this):
> > 
> > Part A Equation:
> > 
> > fraction of energy absorbed = [(R*en)/(4*pie*L^2) *
> > exp^(-0.026*0.001293*L) *0.026] / [(R*en)/(4*pie*L^2)]
> > Simply, the formula for the Dose in air at time T divided by the energy
> > at time T if the air absorbed nothing.
> > 
> > Part A-Your thoughts?
> > 
> > Part B
> > I derived a number of 4.9 mGy*min^-1. The masteringphysics accepted this
> > answer and rounded up to 5.01 mGy*min^-1. Clearly I am using the wrong
> > coefficients, but I don't know which ones or why.
> > 
> > Part B- Thoughts?
> > 
> > Part C
> > 
> > The question does not provide the height of the person. I don't know
> > what to use for a surface area. The SNIV notes provide the equation of
> > the difference in fluence rates multiplied by the area divided by the
> > mass. I can assume the height, assume the person is a cylinder, assume
> > that half their body is exposed, and use the density and depth of the
> > individual provided to calculate a surface that agrees with the given
> > information, using my assumptions. However, the masteringphysics does
> > not want to accept my answer, therefore I am once again stumped. To
> > calculate the fluence rate at the skin I am using the formula that
> > fluence rate ?(o) is equal to Dair rate divided by the mass energy
> > absorption coefficient of air.
> > 
> > Maybe, if this is too dense for you to process in this format, would you
> > please address it in class on Monday, if you can find the time. Or, can
> > we make an appointment on either Tuesday or Monday to work through this
> > confusion.
> > Thanks professor Key,
> > Stuart
> > P.S I have a gift for you that I will give to you on Monday.
> > Thanks
> > 
> 

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