Re: Mass Energy Absorption Coefficient

[aenns@unixg.ubc.ca (Alan Enns)]

On Thursday Aug. 3 Craig Harmon wrote:

>So an absorption cooefficient is the measure of energy transfered to the 
>medium through inelastic collisions.  I've never heard it termed 'mass 
>energy aborption coefficient' by the way.  Just absorption coefficient.
>
>> Does this include only photoelectric interactions?? > 
>
>No, it is the sum of all the inelastic collisions the photon/particle 
>is likely (energetic enough) to experience.

Not to be picky, but as David Simpson wrote:

>... mass energy absorption coefficient:...the product of the mass energy 
transfer >coefficient and (1-g) where g is the fraction of energy of 
secondary charged >particles that is lost to bremsstahlung.  

>... mass energy transfer coefficient is defined as  the fraction of 
incident particle >energy that is transferred to kinetic energy of charged 
particles by interactions, >in traversing a distance of material.

For X- and gamma rays, the mass energy transfer coefficient is the sum of 
three terms (its hard to type them in this world of limited fonts).  It is 
simpler to type the following if I  multiply everything by the material 
density, rho.    Then the energy transfer coefficient per unit volume of 
absorbing material is (pls. bear with me here):

mu_k (ie mu sub k) = tau_a + sigma_a + kappa_a

where

tau_a = tau(1- delta/hv)  where tau is the photoelectric attenuation 
coefficient, delta is the average energy emitted as fluorescent radiation 
per photon absorbed, and hv is the photon energy.

sigma_a = sigma*E/hv  where sigma is the total Compton attenuation coef. and 
E is the average energy of the Compton electrons

kappa_a = kappa(1 - 2mcc/hv)  where kappa is the attenuation coef. for pair 
production and mcc (mc squared) is the rest mass of the electron

Finally, the term attenuation coef. is a general term which could refer to 
mass attenuation coef. (number of particles removed from beam) or mass 
energy attenuation coef. (energy removed from beam).

hope this helps...
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Alan Enns
Radiation Safety Assistant,
Department of Health, Safety and Environment
University of British Columbia,
Canada.
aenns@unixg.ubc.ca
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