Why are some nuclei more resistant to pair production?

[babemagnetics]

I've gotten interested in this pair production stuff, and I'm wondering: Why is it that some nuclei can absorb more energy than the energy needed for pair production, while other nuclei result with the incoming energy creating pair production? Is it because heavier nuclei can distribute the energy among their nucleons more?

Gracias

 

[mathman]

I'm not sure what your point is. However, pair production (photon to electron-positron pair) cross-section is an increasing function of atomic weight.