link to lecture transcript - UT-H GSBS Medical Physics Class Site

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Let us now summarize what we know about Compton scatter. First of all, if we base our determination of the attenuation coefficient on the free electron model, the attenuation coefficient is nearly constant to 100 keV. It falls less than 2 orders of magnitude out to 100 MeV. So the coefficient is proportional to hν raised to a power something smaller than -1, maybe -2/3, from 100 keV to 100 MeV. To a ballpark approximation we can say it’s roughly independent of energy. At low energies, the binding energy of the electrons affects the Compton scatter coefficient suppressing attenuation in high-Z materials such as lead. But this suppression only occurs at lower energies. 40
At higher energies there is no difference between any of the Compton attenuation coefficients so then σ/ρ is essentially independent of Z. The dependence of the Compton coefficient on atomic number is really an effect of the binding energies. At higher photon energies you can forget about the binding energies; we are basically dealing with free electrons. So in summary, σ/ρ, the total attenuation coefficient, is roughly independent of Z, and depends on the hν to something a little bit less than the magnitude of -1, say -2/3 or something like that. 41
Page 1 and 2: Today, we continue our talk about C
Page 3 and 4: We previously derived an equation f
Page 5 and 6: So σ 0 is the classical cross sect
Page 7 and 8: Let’s look at this graph. Notice
Page 9 and 10: What about the dependence of the Co
Page 11 and 12: The energy transferred to the elect
Page 13 and 14: If we plot the energy transfer coef
Page 15 and 16: The energy transfer coefficient has
Page 17 and 18: What about the scatter coefficient?
Page 19 and 20: Using this equation we can now plot
Page 21 and 22: This probability is represented by
Page 23 and 24: This figure is a plot taken from Jo
Page 25 and 26: We can always calculate what the ma
Page 27 and 28: Remember dσ/dE is relatively const
Page 29 and 30: Now we also look at the graph and s
Page 31 and 32: First, let’s recall that the atte
Page 33 and 34: What we now want to find out is how
Page 35 and 36: So far, our approximations have bee
Page 37 and 38: These values were tabulated by Hubb
Page 39: So what is the effect of binding en
Page 43 and 44: Note that this graph is for very lo
Page 45: The fraction of energy transferred

link to lecture transcript - UT-H GSBS Medical Physics Class Site

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