Pair production and observation of high energy photons

Question

Pair production occurs when a photon with sufficient energy produces a particle and an anti particle. For this to happen with an electron-positron pair the minimum required energy of the photon has to be great enough to produce an electron and positron which I get to be around $1.6\times10^{-13}$J which is a gamma photon with a frequency of $2.46\times10^{20}$Hz.

When the pair is produced it then annihilates again and produces at least 2 photons to conserve momentum. These photons will be at a lower frequency as the energy is shared between them. From what I have researched, the probability of pair production occurring increases with photon energy.

If this is the case how come we observe cosmic gamma rays? Surely the photons that are emitted from the source will undergo pair production and annihilation at some point while propagating causing none of the original photons to reach us. How is it possible that we observe these and in our observations is pair production taken into consideration when calculating estimates for the emitted energy of the source?

Answer 1

Observations of cosmic gamma rays demonstrate that pair production doesn't happen spontaneously in a vacuum. You can tell various stories about this fact. It wouldn't conserve momentum. It involves the photon energy, so in some inertial frames that energy is too low, but if it happens in any frame it happens in all frames. Choose the story you like.

A common way to provoke pair production is to put a lead sheet in the way of the photon. Lead nuclei are good at taking up the extra momentum, and the reaction occurs in the lead's inertial frame, so that's the frame where the photon energy matters.