Pair Production

Claire84

I'm pretty much okay with pair production but in the eqt we have that a photo gives an electron and a positron. If we do it the other way round we have an electron and a positron giving 2 photos in order to conserve momentum. Why do we not have 2 photos of energy for pair production ir order to conserve momentum?

mathman

You are correct (up to a point). Pair production in a vacuum does require two photons to interact. However, when a high energy photon passes near a nucleus of an atom (the heavier the better), it can give rise to pair production alone, with the nucleus taking care of the momentum balance.

Claire84

Is that to do with the recoil momentum of the nucleus? I've heard it mentioned before so just wondering if that's what is going on here.

mathman

Your description is correct. It also explains why heavier nuclei are better. It is easier for them to absorb the recoil momentum.