The development of buffer-gas trapping and magnetized beam formation has provided positron beams with significantly improved energy resolution (ΔE≤0.025 eV) compared to those available previously. Analysis techniques havebeen developed to take advantage of the fact that the beam is in a magnetic field of ∼0.1 T. This has enabled scattering experiments at lower energies and with significantly improved state selectivity for excitation experiments than had been possible previously. A detailed description of these techniques is presented. Data are presented for a variety of cross sections for scattering of positrons from atomic and molecular targets.

/pdf/positron-scattering-from-atoms-and-molecules-using-a-35qu5jz9d4.pdf

Positron scattering from atoms and molecules using a magnetized beam

Detailed measurements of the total ionization cross-sections for Ne, Ar, Kr and Xe by positron impact are presented in the energy range from threshold up to ~1 keV. From these and other available data, the cross-sections for positronium formation are extracted. Comparison is made with other results where available. Structure in the positronium formation cross-section has been found which appears to be associated with positronium formation in excited states. Upper and lower limits on the contribution from these are estimated.

Total positron-impact ionization and positronium formation from the noble gases

Absolute measurements are presented for the positron-impact cross sections for direct ionization and positronium formation of noble gas atoms in the range of energies from threshold to 90 eV. The experiment uses a cold, trap-based positron beam and the technique of studying positron scattering in a strong magnetic field. The current data show generally good, quantitative agreement with previous measurements taken using a qualitatively different method. However, significant differences in the cross sections for both direct ionization and positronium formation are also observed. An analysis is presented that yields another, independent measurement of the direct ionization and positronium formation cross sections that is in agreement with the present, direct measurements to within $\ifmmode\pm\else\textpm\fi{}10%$ for argon, krypton, and xenon. Comparison with available theoretical predictions yields good quantitative agreement for direct ionization cross sections, and qualitative agreement in the case of positronium formation.

/pdf/ionization-and-positronium-formation-in-noble-gases-2s0fjactsd.pdf

Ionization and positronium formation in noble gases

We reexamine in detail the various processes undergone by positrons in the ISM from their birth to their annihilation using the most recent results of positron interaction cross sections with H, H2 and He. The positrons' lives are divided into two phases: the 'in-flight' phase and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

/pdf/the-lives-and-deaths-of-positrons-in-the-interstellar-medium-5f2y1hrbtz.pdf

The lives and deaths of positrons in the interstellar medium

Much progress in the theory of positron scattering on atoms has been made in the ten years since the review of Surko, Gribakin and Buckman [J. Phys. B 38, R57 (2005)]. We review this progress for few-electron targets with a particular emphasis on the two-centre convergent close-coupling and other theories which explicitly treat positronium (Ps) formation. While substantial progress has been made for Ps formation in positron scattering on few-electron targets, considerable theoretical development is still required for multielectron atomic and molecular targets.

https://iopscience.iop.org/article/10.1088/0953-4075/49/22/222002/pdf

Recent progress in the description of positron scattering from atoms using the convergent close-coupling theory

Recent developments in measurements of total and (or) positronium-formation cross sections for positrons (in the range of 1 to 300 eV) scattered by alkali, magnesium, and hydrogen atoms are reviewed. Measurements of total and positronium (Ps)-formation cross sections for positrons scattered by sodium, potassium, and rubidium have revealed an interesting pattern of differences and similarities between these collision systems. These measurements, together with recent calculations, provide evidence that coupling effects between Ps formation and other scattering channels are very important at low energies for the alkali atoms. The calculations also indicate that formation of Ps in excited states in the cases of potassium and rubidium is more likely than formation in the ground state when the positron energy is greater than about 5 eV. Measurements of total and Ps-formation cross sections for positrons scattered by magnesium are in a preliminary stage, but the differences between them and the results of availa...

Measurements of total and (or) positronium-formation cross sections for positrons scattered by alkali, magnesium, and hydrogen atoms

Measurements of positronium (Ps) -formation cross sections (upper and lower limits) for positrons (1{endash}17 eV) scattered by Rb atoms are reported. These measurements, along with recent measurements and calculations of total cross sections, provide evidence that coupling effects between Ps formation and other scattering channels may be very important for energies below 10 eV for Rb. {copyright} {ital 1996 The American Physical Society.}

Measurements of positronium-formation cross sections for positrons scattered by Rb atoms

The authors are measuring total and positronium (Ps) formation cross sections for positron-Mg scattering. Their experimental approach involves using a beam transmission technique to measure the total cross sections and setting lower and upper limits on Ps formation cross sections using a combination of (1) the detection of the coincidences of 511 keV annihilation gamma rays produced by the decay of para-Ps and by the interaction of ortho-Ps with the walls of the scattering cell in which the Ps is formed, and (2) the determination of scattering cross sections associated with the measured transmission of the positron beam through the vapor in their scattering cell with the angular discrimination of their apparatus deliberately made as poor as possible. Comparing with the theoretical results of Gribakin and King, their measured total cross sections differ in shape and their Ps formation lower limits are lower than the theoretical predictions.

Measurements of total and positronium formation cross sections for positron-Mg scattering

Recent developments in measurements of total and positronium (Ps) formation cross sections for positrons (in the energy range of 1–300 eV) scattered by hydrogen, alkali metal, and other atoms are reviewed Measurements and calculations of total and Ps formation cross section (QPs's) for positrons scattered by hydrogen atoms are in very good agreement, and for the most part there is also good agreement for sodium, potassium, and rubidium atoms, but there is a puzzling discrepancy between measured and recently calculated QPs's for sodium Preliminary measurements of QPs's for Mg show a very rapid rise to a large maximum value less than 2 eV above the Ps formation threshold energy (08 eV) which may be related to the proximity of that threshold to zero energy It appears that structure observed in e+–Ar and Kr QPs measurements may be related to capture of inner-subshell electrons

Measurements of positron scattering by hydrogen, alkali metal, and other atoms

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Measurements of total and (or) positronium-formation cross sections for positrons scattered by alkali, magnesium, and hydrogen atoms

Measurements of positronium-formation cross sections for positrons scattered by Rb atoms

Measurements of total and positronium formation cross sections for positron-Mg scattering

Measurements of positron scattering by hydrogen, alkali metal, and other atoms