γ-ray emissivities in the interstellar medium. The energy spectra of CR electrons and protons are taken from Casandjian (2015). The ambient gas number density is 1 cm⁻³. The background radiation fields consist of three components: (1) CMB with an energy density of 0.24 eV cm⁻³ and temperature of 2.7 K; (2) the optical/UV component which is modelled as grey-body radiation with an energy density of 2 eV cm⁻³ and temperature of 5000 K and (3) IR component which is approximated as a grey-body component with an energy density of 1 eV cm⁻³ and temperature of 100 K.