Plasma-screening effects on positronium formation

Plasma-screening effects on positronium （Ps） formation for positron-hydrogen collisions in a Debye plasma environ- ment is further investigated using the screening approximation model with the inclusion of the modified structure of Ps. More accurate Ps formation cross sections （n = 1,2） are obtained for various Debye lengths from the Ps formation thresh- olds to 50 eV. The influence of considering modified bound-state wave functions and eigenenergies for the Ps is found to result in the reduction of the Ps formation cross sections at low energies, whereas it cannot counteract the enhancement of the Ps formation by the Debye screening.

Positronium formation for positron scattering from metastable hydrogen

Positronium （Ps） formation for positron impact on metastable hydrogen in 2s state has been studied by using the two- channel, two-center eikonal final state-continuum initial distorted wave （EFS-CDW） method. The differential, integrated, and total cross sections for Ps formation in different states have been calculated from each channel opening thresholds to high energy region. The results are compared with other theoretical calculations available in the literature. For Ps formation in s-state at intermediate and high energies, our results are in good agreement with the prediction of distorted wave theory. Those formed in p-states and the total Ps formation cross sections are reported for the first time. It is shown that the total Ps formation cross sections for positron scattering from H（2s） state are significantly larger at relatively low energies, while smaller at high energies, compared with those obtained from hydrogen in ground state.

Optical potential approach for positron scattering by metastable 2~3S state of helium

The momentum space coupled channels optical（CCO） method for positron scattering has been extended to study the scattering of positrons by metastable helium for impact energies in the range from the positronium threshold up to high energies. Both the positronium formation and ionization continuum channels are included in the calculations via a complex equivalent local potential. The positronium formation, ionization, elastic and 2~3S–2~3P excitation, and total scattering cross sections are all presented and compared with the available information.

Positronium annihilation in silica aerogel studied by a positron age-momentum correlation technique

A high-performance positron age-momentum correlation （AMOC） spectrometer was newly developed. The counting rate is increased up to 200 cps much larger than the value 20 cps reported by other international groups. And at the same time, the time resolution still keeps at the international level of 220 ps. Furthermore, positronium （Ps） annihilation in silica aerogel was investigated by AMOC, which indicates： （1） Ps annihilation between the grains dominantly undergoes pick-off process and spin conversion from o-Ps to p-Ps; （2） Annealing below 400 ℃ changes the grain surface conditions, i. e. the desorption of hydrogen and the decrease of the defect centers concentration.