photon-photon absorption

photon-photon absorption

The absorption of gamma-ray photons on collision with ambient ‘gas' photons to produce electron-positron pairs (see pair production). The optical depth for this process can be written as: τγγ ≊ R σ(E T) N T (E T) dE T if the density of the target photons, N T, is assumed constant throughout the source region of size R . σ(E T) is the cross-section for pair production with a target photon of energy E T .