  Secondary nuclear production physics is receiving increased attention given
the high-quality measurements of the gamma-ray emissivity of local interstellar
gas between ~50 MeV and ~40 GeV, obtained with the Large Area Telescope on
board the Fermi space observatory. More than 90% of the gas-related emissivity
above 1 GeV is attributed to gamma-rays from the decay of neutral pions formed
in collisions between cosmic rays and interstellar matter, with lepton-induced
processes becoming increasingly important below 1 GeV. The elementary
kinematics of neutral pion production and decay are re-examined in light of two
physics questions: does isobaric production follow a scaling behavior? and what
is the minimum proton kinetic energy needed to make a gamma-ray of a certain
energy formed through intermediate pi0 production? The emissivity spectrum will
allow the interstellar cosmic-ray spectrum to be determined reliably, providing
a reference for origin and propagation studies as well as input to solar
modulation models. A method for such an analysis and illustrative results are
presented.
