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?
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$\begingroup$ 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. $\endgroup$ – anna v Oct 1 '16 at 17:48
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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.
Something has to interact with the nucleus, otherwise it would not be able to absorb the momentum. So a virtual photon has to couple to the new particles and the nucleus. If there are more particles in the nucleus to couple to, this should increase the cross section.
answered Oct 2 '16 at 20:25
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$\begingroup$ What happens in the screening effect though? shoudln't a higher Z also mean more screening effect since there would be greater no of electrons shielding the nucleus? $\endgroup$ – Sabeeka Nazeer Oct 3 '16 at 14:55
