The Particle type can be set to render primary particles, secondary particles, both, or by a logical expression. The logical expression can be used to plot only particles of a certain species, like red blood cells or protons.
Figure 2-5: Neutralization of a H+ beam by a rarefied buffer gas of argon, showing all particles (upper left), H+ only (upper right), H only (lower left), and Ar+ only (lower right).
If you have the Molecular Flow Module, see Neutralization of a Proton Beam Through a Charge Exchange Cell, Application Library path Molecular_Flow_Module/Industrial_Applications_charge_exchange_cell.