  Several years of neutral particle measurements by the NASA/IBEX mission have
yielded direct observations of interstellar neutral helium and oxygen. The data
indicate the presence of secondary neutral helium and oxygen, which are created
within the heliosphere by charge exchange involving helium or oxygen ions. This
contribution describes a detailed conserving calculation method based on
Keplerian orbits that has been developed to characterize helium distribution
functions throughout the heliosphere, in particular in the innermost
heliosphere, while accounting for loss and production of neutral particles
along their path. Coupled with global heliosphere models of plasma
distributions, this code is useful for predicting the fluxes of heavy neutral
atoms at spacecraft detectors, so enabling inferences on the characteristics of
the interstellar medium.
