van Reeth, Peter George;
(1997)
Theoretical studies of positronium formation in low energy positron-helium collisions.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Theoretical investigations of the collisions of low energy positrons with helium atoms have been made in the energy region where only the elastic scattering and the positronium formation channels are open. We have used the two channel Kohn variational method and new accurate numerical procedures have been developed to perform the many six dimensional integrations. The effect of the inexactness of the target wavefunction on the final results has been investigated and consequently we have used very elaborate helium wavefunctions in our calculations. The s-wave positronium formation cross section has been calculated using trial functions containing 502 short-range terms and is found to be very small. The s-wave elastic scattering cross section is found to be the main contributor to the total elastic cross section and a detailed investigation of the behaviour of the cross section at the positronium formation threshold reveals a 'rounded step' feature which is predicted by Wigner's threshold theory. The positronium formation cross sections for p- and d-wave scattering have been calculated, and we find the d-wave component to be dominant for energies greater than 1eV above threshold. The total positronium formation cross section is evaluated using the s-, p- and d-wave results of this work and the first Born approximation for the higher partial waves. A difference is found between theory and experiment which is attributed to the uncertainty in the absolute values of the experimental data and the convergence of the theoretical results. The p- and d-wave elastic scattering cross sections have been calculated and are found to be ≈30% of the total elastic scattering cross section for energies above the positronium formation threshold. We have also investigated the annihilition of positrons with the electrons in the helium atom at energies corresponding to room temperature. The angular correlation function has been calculated and is found to agree very well with the latest experimental measurements.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Theoretical studies of positronium formation in low energy positron-helium collisions |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Thesis digitised by ProQuest. |
| Keywords: | Pure sciences; Low energy positron-helium collisions; Positronium formation |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10119561 |
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