Theoretical Research On Positron Collisions With Atom At Low And Medium Energies

The study of positron scattering with atoms and molecules has always been afundamental area of physics. Especially, the capability of producing strong positronbeams in the laboratory from the80’s of last century has made the researchinvolving the positron physics become one of the most active scientific subjects.The scattering process between positrons and atoms is the interactions involvingantimatter, which can offer us a unique way to explore the structure of microscopicparticles, the correlation effect between the charged particles, and the precisemechanism of the scattering process. Due to the particular property of positron, thestudy of positron-atom scattering can offer a new means to investigate themicrocosmic structure and basic interaction in many-body systems.The scattering process of positron with atoms show different features atdifferent energy regions. The scattering cross sections of positron-atom system tendto merge with electron case at high energies. At the intermediate energy range, thepositronium (Ps) formation channels and the ionization continuum are of significantimportance for the whole scattering process, and when the energies are below thefirst ionization threshold of the target atom, the positronium formation channelalways plays a dominate role. In the present thesis, we foucs on the scatteringprocess of positrons with atomic lithium and helium at low and intermediateenergies.Lithium is the simpliest alkali atom, and the positronium formation channel isopen even at zero energy in positron-lithium scattering. It is thus of much specialinterest to study the process of positron scattering with atomic lithium. Themomentum-space coupled-channel optical (CCO) method has been applied in ourworks to explore the positron-lithium scattering at low and intermediate energies.The total scattering cross sections and the cross sections of the reaction channels ofpositronium formation and direct ionization have been produced. A peak structurearound the Ps (n=2) threshold has also been observed in our calculations. Thepresent calculations indicate that the cross section of the direct ionization keepsincreasing as the energy increases and arrives at the maximum at about15eV. Thepositronium formation channels are open at the whole energy region, and the crosssection decreases quickly as the energy increases. At the energies below the firstionization threshold of atomic lithium, the positronium formation channel plays adominant role. The partial-wave and differential cross sections of positron-lithiumscattering have also been given at low energies. We have found some resonances and threshold behaviour, and given our explanation.In order to explore the detailed dynamic mechanism of the single ionizationprocess, a positron impact distorted wave Born approximation (P-DWBA) method isdeveloped in present work. The single ionization process is a typically three-bodyColumb interaction, and the positronium formation channels may have an importantinfluence. However, the positronium formation channels have not been included inother theoretical calculations to our knowledge. In the present P-DWBA method, acomplex polarization in momentum-space has been derived to describe thepositronium formation channels. This potential is transformed to the coordinatespace and added in the distorted potential to solve the distorted wave differentialequations.Atomic helium is simple and stable, and it is an ideal target for experimentalmeasurements. Positron-helium system is thus a natural choice for the theoreticalcalculation to check the validity of the theoretical model by the comparison withexperimental measurements. We have chosen the coplanar asymmetric kinematicsituation and calculated the triple differential ionization cross sections ofpositron-helium system at59.2eV with the scattered positron rangle of6°and theejected electron rangle of10°. We can found that the present calculation results arein better agreement with the experiment measurements except that the presentresults are a little higher than the experimental values when the the energies of thescattered positron and electron are close to each other. This maybe due to the factthat the post-collision interaction (PCI) is not included in the present P-DWBAcalculations. Then, we have chosen the coplanar asymmetric kinematic situation andcalculated the triple differential ionization cross sections of positron-helium systemat59.2eV with the scattered positron rangle of10°and the ejected electron energiesof12eV,13.5eV,15eV and16.5eV respectively. The triple differential ionizationcross sections have also been calculated at the energies of99.6eV and499.6eVusing the present P-DWBA method. It not only gives the positions of binary electronpeak and recoil electron peak correctly, but also the ratio of the maximum of twopeaks.In our calculations, the present P-DWBA method has been proved effective tobe applied in the study of triple differential cross sections of ionization process ofpositron-atom system. In the future, we would include the effect of post-collisioninteraction and extend the present theoretical model to researches of the singleionization process of positron scattering with more complex atoms and also in otherdynamic conditions.