The energy-dependence of the secondary electron yield by electron impact on clean materials is shown, through comparisons with both experimental data and previous semi-empirical models, to be described well by a log-normal distribution. The maximum yield and corresponding energy are calculated in terms of the fit parameters. The function also agrees with the 'universal curve' obtained by expressing the experimental data in terms of the position and magnitude of maximum yield. The positron- and ion-induced secondary electron yields also exhibit a log-normal dependence and this is used to extend their energy ranges without prior knowledge of the position and magnitude of their maxima. In addition to their intrinsic usefulness, the results provide further support for the statistical description of inelastic processes advanced by Laricchia et al. (2018).

The total cross section of positronium (Ps) scattering from molecular oxygen has been measured in the velocity range 0.27-1.50 a.u. (energy range 2-61 eV) and has been found to be close to the correspond-ing equivelocity electron cross section above 0.87 a.u. (20 eV), as previously found by Brawley et al., [Science 330, 789 (2010)]. However, below this value the cross section for positronium is observed to exceed that for electrons by up to a factor of 4 at the lowest energy. Measurements are compared to the predictions of low-energy resonant peaks in the elastic-scattering cross section calculated within a free-electron-gas model refined by applying corrections to the correlation energy for the interaction between Ps and the electron gas. Additionally, cross sections for O-2 formation and positronium breakup have been calculated using a classical -trajectory Monte Carlo approach. Comparisons are made with earlier calculations and discussed in terms of both experimental and theoretical uncertainties.

Experimental determinations of the detection efficiency for positrons impacting a channel electron multiplier with incident energies between 0–1400 eV are presented. A log-normal dependence with energy is established and used to compute the positron-to-positronium detection efficiency ratio as a function of positronium energy, as required for determining quantities involving the ratio of positron and positronium rates. A log-normal energy-dependence is also observed in results of previous work with electrons, protons and ions.

Experimental determinations of the absolute differential positronium-formation cross sections near 0 degrees for Ne, Ar, Kr, and Xe are presented and compared with theory. The degree of forward collimation, expressed by the ratios of the differential-to-integral positronium-formation cross sections, is also computed and compared with theories and other targets. Trends among targets and structures at low energies emerge when considered as a function of the reduced total energy.

Quantum physics is undoubtedly the most successful theory of the microscopic world, yet the complexities which arise in applying it even to simple atomic and molecular systems render the description of basic collision probabilities a formidable task. For this reason, approximations are often employed, the validity of which may be restricted to given energy regimes and/or targets and/or projectiles. Now we have found that the lognormal function, widely used for the probability distribution of macroscopic stochastic events (as diverse as periods of incubation of and recovery from diseases, size of grains, abundance of species, fluctuations in economic quantities, etc.) may also be employed to describe the energy dependence of inelastic collisions at the quantum level (including ionization, electron capture and excitation by electrons, positrons, protons, antiprotons, etc.), by allowing for the relevant threshold energy. A physical interpretation is discussed in this article by analogy with the heat capacity of few-level systems in solid state physics. We find the generality of the analysis to extend also to nuclear reactions. As well as aiding the description of collision probabilities for quantum systems, this finding is expected to impact also on the fundamental understanding of the interface between the classical and quantum domains.

In order to clarify the physics underlying the observations of the electronlike behavior of positronium (Ps) and its resonant scattering from CO2, we have measured the Ps + N-2 total cross section and found it also to exhibit significant structure. Analysis of the resonances reveals that Ps is distorted in the collisions and classical trajectory Monte Carlo calculations indicate that the electron is on average closer to the target than the positron, which may in turn bind resonantly to the ensuing temporary negative ion. This description of the nature of Ps resonances agrees with long-standing theoretical predictions.

Recent findings on the similarity between electron and positronium scattering at the same velocity [Brawley et al., Science 330, 789 (2010)] have guided us towards the realization of a detectable flux of positronium atoms at beam energies five times lower than previously obtained, enabling total cross sections to be measured in the energy range ∼(1-7)  eV for the first time. In collision with Ar and Xe, the total cross sections of positronium are found to be smallest at the lowest energy probed, approaching those of the Ramsauer-Townsend minima for electron projectiles. Additional structure has been observed in the case of positronium scattering at incident energies around 5 eV.

The first absolute experimental determinations of the differential cross sections for the formation of ground-state positronium are presented for He, Ar, H2, and CO2 near 0°. Results are compared with available theories. The ratio of the differential and integrated cross sections for the targets exposes the higher propensity for forward emission of positronium formed from He and H2.

We are now able to produce a positronium beam at energies in the range 1 – 400 eV, significantly lowering the previously achievable minimum of ∼ 7 eV and opening up the possibility of investigating subtle quantum mechanical effects such as those which give rise to low energy electron scattering phenomena (e.g. resonances and 'barrier transparency').

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].

The total cross section of positronium scattering from a variety of atoms and molecules has been found to be unexpectedly close to that of a bare electron moving at the same velocity, despite Ps being neutral and having twice the mass [Science 330, 789 (2010)]. This behaviour appears to extend to velocities where effects such as the Ramsauer-Townsend minima and shape resonances occur for electrons. The latter has now been observed in collisions with CO2. Results are presented along with recent production efficiency measurements of Ps at high energies.

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.

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.

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.

The total cross sections of positronium (Ps) scattering from a carbon-dioxide molecule have been measured over the range (7-400) eV incident-Ps energy. For the first time in Ps collisions, a resonantlike structure is observed. For the present target, it occurs around 9.5 eV followed by a broader peak at ∼60 eV. Following Brawley et al. [Science 330, 789 (2010)] who have observed similarities between the total cross sections of positronium and of electrons incident upon a given target at the same velocity, a corresponding comparison is made for CO2. The comparison suggests that the former peak corresponds to the well-known 2Π(u) shape resonance which occurs for electrons at an incident velocity of 0.5 a.u. Further features are discussed and theoretical input is sought.

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.

Positronium, Ps, is the atom-like bound state of an electron and its antimatter counterpart, the positron. This work encompasses the experimental techniques required to produce a collimated beam of Ps atoms and recent investigations of Ps collisions on a variety of atomic and molecular targets to determine Ps total and fragmentation cross-sections. For the former experiment, the present work is compared to theory - for which there is now very good agreement for low energy Ps–He collisions - and to other projectiles where the present data shows that total cross-sections for equivelocity Ps and electrons are nearly identical for a wide range of targets and velocities. The Ps fragmentation measurements proceed via the detection of ejected electrons or positrons for Ps collisions on Xe at 18 and 30 eV. Detection of ejected positrons is a unique signature of Ps fragmentation and the recent integrated measurements for this process agree well with theory, as do the shapes of the differential cross-sections with respect to the longitudinal energy of the ejected positrons. Concerning these, the energy dependence of the positron distributions appear similar for the present measurements on Xe and those previously measured for a He target. When detecting ejected electrons, the experiment is sensitive to reactions in which the projectile and/or target are ionised, and the integrated cross-sections indicate that target-ionisation is significant at only ∼3 eV above threshold in Xe.

Cross-sections are presented, both integrated and differential with respect to the longitudinal energy of the ejected positrons, for the fragmentation of Ps in collisions with Xe at 18 and 30eV impact energy and compared with available theory. The shapes of the positron spectra are also compared with those measured in collisions with He at the same impact energy.