Positronium emission from MgO smoke nanocrystals

We report experiments in which positronium (Ps) atoms were created in a thick layer of MgO smoke powder deposited on a thin silicon nitride substrate. The experimental arrangement was such that a positron beam could be implanted directly into the top of the MgO layer or be transmitted through the substrate, allowing Ps to be produced within ≈100 nm or 30 μm of the powder-vacuum interface. The transverse kinetic energy of Ps atoms emitted into vacuum was measured via the Doppler broadening of 13S${}_{1}\,\to {2}^{3}$P J transitions, and found to be E x ≈ 350 meV, regardless of how far Ps atoms had traveled through the powder layer. Our data are not consistent with the model in which energetic Ps atoms emitted into the internal free volume of a porous material are cooled via multiple surface collisions, and instead indicate that in nanocrystals lower energy Ps is generated, with negligible subsequent cooling in the large open volumes of the powder. Our experiments also demonstrate that SiN substrates coated with MgO smoke can provide a simple and inexpensive method for producing Ps transmission targets.This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) under grants EP/R006474/1, EP/K01739X/1, and EP/P013503/1

Positron scattering from pyridine

We present a range of cross section measurements for the low-energy scattering of positrons from pyridine, for incident positron energies of less than 20 eV, as well as the independent atom model with the screening corrected additivity rule including interference effects calculation, of positron scattering from pyridine, with dipole rotational excitations accounted for using the Born approximation. Comparisons are made between the experimental measurements and theoretical calculations. For the positronium formation cross section, we also compare with results from a recent empirical model. In general, quite good agreement is seen between the calculations and measurements although some discrepancies remain which may require further investigation. It is hoped that the present study will stimulate development of ab initio level theoretical methods to be applied to this important scattering system.The authors would like to acknowledge the Australian Research Council (ARC) Discovery Programmes for funding support (No. DP140102854) and Ross Tranter for technical support for the experimental apparatus. L.E.G., F.B., and G.G. also acknowledge partial financial support from the Spanish Ministry MINECO (No. FIS2016-80440) and the European Union ITN-Marie Curie programme (No. ARGENT-608163)

Fundamental studies of positron scattering processes from simple targets

Positron scattering is useful in the investigation of positron interactions with atoms and molecules, which in turn helps with the classification of fundamental quantum processes. Methods for measuring differential cross sections as well as ionisation cross sections on the Australian National University (ANU) positron beamline are discussed, as well as the measurement of the positronium formation differential cross section (DCS) at the University College London (UCL) positronium formation beamline. This thesis reports the first absolute elastic differential cross section results for positron scattering on neon. DCS measurements were made for the energies 1,2,3 and 5 eV and were compared to three theoretical methods; relativistic optical potential (ROP), convergent close coupling (CCC), and many body theory (MBT). CCC and ROP were found to be a better match to the data over most angles for the lowest energies measured. The higher energy DCS deviate significantly from theory despite agreement in the total cross section (TCS), and possible reasons for this are also discussed. Also presented are near threshold ionisation measurements, which are the first results produced within 2 eV above the ionisation energy, with high energy resolution. These measurements test the Wannier threshold law, where the behaviour of the cross section is dependent on the interactions of the escaping particles. This leads to a semiclassical approximation for the cross section in which a strict escape geometry is predicted for ionisation near threshold. It was found that using either positrons or electrons, and hence changing the projectile sign, did not appear to have a significant effect on the measured exponent. This is despite theoretical predictions the exponent would be much higher for positron impact than the corresponding electron scattering process. The progress towards a positron reaction microscope is also reported, with the eventual goal to measure fully differential cross sections from positron ionisation of the noble gases. Scattered positrons, ejected electrons, and residual ions are all detected in coincidence, with the ionisation event coincidence detection rate increased by a factor of 30 compared to previous work, and an overall $\approx 1$ million triple candidates collected. This suggests viability for a difficult experiment, given the low yields of positrons compared to electrons. Positronium formation in the forward direction was measured using the University College London positronium beamline. The results presented are forward formation differential cross sections for positrons on neon, and further the investigation into viable targets for efficient positronium beam formation. The measurements taken for neon were compared to measurements made for helium and argon previously on the UCL beamline, with the forward collimation of positronium found to have similar trends for neon and argon, with the greatest yield from helium

A new technique for measuring positron impact direct ionisation

We present a method for measuring the electron yield in positron impact direct ionisation of atomic or molecular targets using a magnetically confined positron beam. This method can be used to determine the (relative) direct ionisation cross section. The use of this technique and a Surko trap based positron beam allows the electron yield to be measured at impact energies as low as 100 meV above the direct ionisation threshold. Application of this technique to the measurement of the energy dependence of the direct ionisation cross section near threshold is also discussed.This work was supported by the Australian Research Council’s Discovery and Discovery Early Career Researcher Program

Near-Threshold Ionization of Argon by Positron Impact

The direct single-ionization cross section for Ar by positron impact has been measured in the region above the first ionization threshold. These measurements are compared to semiclassical calculations which give rise to a power law variation of the cross section in the threshold region. The experimental results appear to be in disagreement with extensions to the Wannier theory applied to positron impact ionization, with a smaller exponent than that calculated by most previous works. In fact, in this work, we see no difference in threshold behavior between the positron and electron cases. Possible reasons for this discrepancy are discussed

Low-energy elastic and inelastic scattering of positrons from argon

Absolute measurements of elastic differential cross sections for positron scattering from argon at energies from 2–50 eV are presented as well as total elastic and total inelastic cross sections from 2–20 eV. Comparisons of the current data, previous experimental data, and theoretical models using the convergent close-coupling and relativistic optical potential methods are made

A new technique for measuring positron impact direct ionisation

We present a method for measuring the electron yield in positron impact direct ionisation of atomic or molecular targets using a magnetically confined positron beam. This method can be used to determine the (relative) direct ionisation cross section. The use of this technique and a Surko trap based positron beam allows the electron yield to be measured at impact energies as low as 100 meV above the direct ionisation threshold. Application of this technique to the measurement of the energy dependence of the direct ionisation cross section near threshold is also discussed

Differential positronium-formation cross sections for Ne, Ar, Kr, and Xe

Experimental determinations of the absolute differential positronium-formation cross sections near 0∘ 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.The Engineering and Physical Sciences Research Council is gratefully acknowledged for supporting this work under Grant No. EP/P009395/1 and for providing S.E.F., M.S., and D.M.N. with research studentships. T.J.B. is grateful for The Robert and Helen Crompton Award