S. J. Brawley

Electron-like scattering of positronium.

The atom formed from an electron and positron interacts with molecules almost as if it were a free electron. Positronium (Ps), a hydrogen-like atom composed of an electron and its antimatter partner, the positron, is formed in considerable quantities whenever positrons interact with matter. It has unexpectedly been found to scatter from a wide variety of atoms and molecules in a way very similar to that of a bare electron moving at the same velocity, despite Ps being neutral and twice the mass.

Electron-like and resonant scattering of positronium

Recent progress on the study of positronium (Ps) scattering from atoms and molecules at University College London is reviewed. Following our findings (Brawley et al. Science 330 2010) concerning the similarity in shape and magnitude of the total cross-sections for equivelocity Ps atoms and electrons, enhancements have been observed in the Ps total cross-section for CO2 at velocities close to those at which resonances occur in the case of electrons. Further comparisons are also made between theoretical results for Ps scattering and experimental electron total cross-sections.

Ionization in positron- and positronium- collisions with atoms and molecules

Recent progress in the experimental study of positron- and positronium-induced ionization of atoms and molecules is outlined. Investigations include integral and differential cross-sections, as well as formation of positronium in the first excited state. Future prospects are discussed.

Positronium Formation and Scattering from Biologically Relevant Molecules

Recent progress in our experimental studies of positronium formation and scattering from simple atomic and molecular systems are reviewed. The former are used to highlight key features of ionizing collision by positrons before considering recent phenomena observed in the case of molecular targets, including positron impact excitation-ionization and the electron-like scattering of positronium. The guiding theme of this review arises from the role that repeated cycles of formation and dissociation of positronium are expected to play in the accurate description of positron interaction with matter.

Fragmentation in positronium collisions with atoms

Recent positronium collision experiments are reviewed in which the cross-sections for projectile- and/or target-fragmentation processes are determined. In contrast with theory, the likelihood for the latter process appears to be significant for positronium collisions with Xe at only ~3 eV above threshold. Measurements of the total direct ionization cross-section by positron impact, performed to test the reliability of the experimental apparatus and method, are also presented.

Experimental aspects of ionization studies by positron and positronium impact

© Cambridge University Press 2013. 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.