Question 7

A photon with energy above 1.02 MeV has an energy greater than the rest energy of an electron-positron pair. Nevertheless the process

$$\gamma \rightarrow e^- + e^+$$

cannot occur in the absence of other matter or radiation. Why not?

In the presence of a stationary electron, a high energy photon can produce an electron-positron pair via the reaction

$$\gamma + e^- \rightarrow e^- + e^- + e^+.$$

Find the minimum photon energy for this reaction to occur.

[Numerical answer can be found here.]

[Hints can be found here, but do not follow this link until you have attempted the question on your own.]