Pair Production: 2 Photos or Electron/Positron?

[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.