Photon-nucleus pair production

• In the laboratory, it is more usual to consider photon-nucleus production. So why do we ignore it in space?

• Photons and nuclei have a similar cross-section, and the g-ray does not differentiate much between another photon or a nucleus.

• Then we must compare the photon density with the particle density in space.

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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.