# Why and how does screening effect effect cross section for pair production?

The photon here does not interact with the nucleus; it just needs the nucleus for momentum conservation. Why does the screening effect then effect pair production cross section? Also, why does the cross section increase with atomic number?

• You have to give a specific reaction, a link to a paper, to make an intelligible question. for example, this does not help in understanding your question en.wikipedia.org/wiki/Screening_effect . The photon interacts with the fringe fields of the whole ATOM, not with the nucleus, and the whole atom takes the momentum balance. – anna v Oct 1 '16 at 17:48

I think that the curvature of the field is larger for larger atomic number $Z$. Therefore it is easier in the vicinity of a larger nucleus. Then the screening effect takes away the field strength again.