High energy photons
incident upon matter can undergo three types of interactions:
- Photoelectric Effect (PE) - knock out an electron
Production (PP) - convert to electron-antielectron
- Compton scattering (C)- elastic collision off
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
= Io * exp(-µx)
= µPE + µPP + µC
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:
µPP ~ Z2
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
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
The total width
of the boxes on the right is then proportional to
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.
could be varied for each Z
to show how quickly the absorption grows with thickness.
would be to include the absorption vs photon energy using the coefficient
plots discussed above.