Slide 26 of 48
Notes:
When a gamma-ray interacts to produce an electron-positron pair, it makes little difference to the gamma-ray whether it interacts with another photon or with a nucleus. Indeed in a laboratory situation, photon-nucleus pair-production is more commonly considered.
In both types of interaction, a similar cross-section is presented to the gamma-ray (about 1e-29 m^2). It is the number density of photons in space compared to the number density of particles which is important.
