When a gamma ray $($a high$-$energy photon$)$ collides with a heavy atomic nucleus, it can produce an electron $($e$)$positron $($e$+)$ pair. Determine the minimum energy $($in MeV$)$ that a gamma ray must have in order for this pair$-$production event to occur. $($The masses of an electron and a positron are both $9\mathrm{.}111031$ kg$.)$