  A gamma-ray line production calculation in astrophysics depends on i) the
composition and energy source spectrum of the energetic particles, ii) the
propagation model, and iii) the nuclear cross sections. The main difficulty for
model predictions and data interpretation comes from the fact that the spectrum
of the particles which actually interact in the ISM - the propagated spectrum,
is not the same as the source spectrum coming out of the acceleration site, due
to energy-dependent energy losses and nuclear destruction. We present a
different approach to calculate gamma-ray line emission, based on the
computation of the total number of photons produced by individual energetic
nuclei injected in the interstellar medium at a given energy. These photon
yields take into account all the propagation effects once and for all, and
allow one to calculate quickly the gamma-ray line emission induced by energetic
particles in any astrophysical situation by using directly their source
spectrum. Indeed, the same photon yields can be used for any source spectrum
and composition, as well as any target composition. In addition, these photon
yields provide visual, intuitive tools for gamma-ray line phenomenology.
