  Diffuse gamma-ray line emission traces freshly produced radioisotopes in the
interstellar gas, providing a unique perspective on the entire Galactic cycle
of matter from nucleosynthesis in massive stars to their ejection and mixing in
the interstellar medium. We aim at constructing a model of nucleosynthesis
ejecta on galactic scale which is specifically tailored to complement the
physically most important and empirically accessible features of gamma-ray
measurements in the MeV range, in particular for decay gamma-rays such as
$^{26}$Al, $^{60}$Fe or $^{44}$Ti. Based on properties of massive star groups,
we developed a Population Synthesis Code which can instantiate galaxy models
quickly and based on many different parameter configurations, such as the star
formation rate, density profiles, or stellar evolution models. As a result, we
obtain model maps of nucleosynthesis ejecta in the Galaxy which incorporate the
population synthesis calculations of individual massive star groups. Based on a
variety of stellar evolution models, supernova explodabilities, and density
distributions, we find that the measured $^{26}$Al distribution from
INTEGRAL/SPI can be explained by a Galaxy-wide population synthesis model with
a star formation rate of $4$-$8\,\mathrm{M_{\odot}\,yr^{-1}}$ and a spiral-arm
dominated density profile with a scale height of at least 700 pc. Our model
requires that most massive stars indeed undergo a supernova explosion. This
corresponds to a supernova rate in the Milky Way of $1.8$-$2.8$ per century,
with quasi-persistent $^{26}$Al and $^{60}$Fe masses of
$1.2$-$2.4\,\mathrm{M_{\odot}}$ and $1$-$6\,\mathrm{M_{\odot}}$, respectively.
Comparing the simulated morphologies to SPI data suggests that a frequent
merging of superbubbles may take place in the Galaxy, and that an unknown but
strong foreground emission at 1.8 MeV could be present.
