It's understood that the PRESENCE of a heavy nucleus is necessary for conservation of linear momentum in pair creation. What I can't understand is why it must occur in ADJACENCY of the nucleus. Is there something I'm neglecting?
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Related: http://physics.stackexchange.com/q/13513/2451 and links therein. – Qmechanic Nov 08 '15 at 06:52
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Being "near by" means that there is (unavoidably) an interaction between them.
The problem is no longer $$ \gamma \to e^+ + e^- \,,$$ but $$ \gamma + A \to e^+ + e^- + A\,,$$ (here $A$ represents the spectator nucleus) and there is now a way to share out the energy and momentum. The interaction is generally thought of as mediated by the electromagnetic interaction.
