| Lecture
4B PhotonAbsorption ComptonScattering Nuclear Structure Neutron Decay Exercises |
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| Photon Absorption |
High energy photons incident upon matter can undergo three types of interactions:
If a beam of photons of intensity Io is incident upon a sheet of material of thickness X, then the intensity I that penetrates to the other side goes as
I = Io * exp(-µx)
where
µ = µPE + µPP + µC
While the exact formulas for these absorption coefficients are quite complicated, they are each proportional to a power of the atomic number Z of the material:
µPE
~ Z5
µPP ~ Z2
µC
~ Z
The dependence of the absorption co-efficients on the energy of the photon can also be obtained for each type of material. See the plots for
Silicon
and Germanium 
.
Simulation Suggestion
Prof. Lindblad's blackboard shows a slab of material of width X. On the left of the slab is a box whose height is proportional to Io.
On the right is a stack of 3 boxes, each of whose height is proportional to the corresponding exp(-µProcess *x)
The total width of the boxes on the right is then proportional to exp(-µx).
Your applet could loop (using a thread) through different values of Z, illustrating with this display how the three types of interactions vary in importance as the atomic mass increases.
Also, X could be varied for each Z to show how quickly the absorption grows with thickness.
Another enhancement would be to include the absorption vs photon energy using the coefficient plots discussed above.
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