# Why is the total interaction cross section of photons larger for incident particles with lower energy?

Why does the cross section continue to decease with increasing energy for the photoelectric effect? For energies higher than the binding energy of K shell, shouldn't the cross section still be high since k electrons should absorb the energy to cause ionization and rest should be simply converted to kietic energy?

• Is this for photons interacting with atoms? Sep 24 '16 at 15:39
• Sorry i forgot to specify, this is for photoelectric effect. Sep 24 '16 at 15:42

The energy must be transferred to a single electron, a single photon can only interact with a single electron. If you have a higher order Feynman diagram where the struck electron directly emits a virtual photon to another electron, you get a suppression factor of $1/137$ in there. This makes those things improbable and we can probably ignore them, at least in the leading order.
And there is a simple dimensional argument that it has to decrease eventually: At stupendously high photon energies, the only relevant energy scale is that of the photon. The cross section has dimension of length squared, that is $\text{Energy}^{-2}$. The relevant scale is the photon energy, so from this you get that $\sigma \sim E^{-2}$ for energies beyond any resonance or something else.