Introduction

As well as probing matter–antimatter interactions, positrons (as positive electrons) have been employed to highlight charge and mass effects in the dynamics of collisions, including those resulting in the ionization of atoms and molecules (see, e.g., [1]). Positronium (Ps), the hydrogenic atom formed from the binding of a positron and an electron, is readily produced in the scattering of positrons from matter. Ps is quasi-stable with a lifetime against annihilation dependent upon its spin: ground-state para-Ps (1 1S0) has a lifetime τ ≃ 125 ps, whilst ortho-Ps (1 3S1) is considerably longer lived (τ ≃ 142 ns). The beam employed for the scattering work discussed in this chapter consists solely of ortho-Ps atoms. In a dense medium, Ps may undergo several cycles of formation and break-up prior to the annihilation of the positron (see, e.g., [2–6]). A quantitative understanding of this cycle is important also for practical applications such as nanodosimetry relating to positron emission tomography (PET) [e.g., 4].

In this chapter, we consider experimental methods employed to investigate positron and positronium impact ionization and fragmentation in collision with atoms and molecules, and associated results. In the case of positrons, an extensive database now exists of integral cross sections for the inert atoms (see, e.g., [5]), less so for molecules (see, e.g., [6]); differential data remain sparse (e.g., [5]). Our focus will be on the latter two topics as well as studies with positronium projectiles.