The determination of atomic and molecular total scattering cross-sections for positrons of low energy

• Main Author: Mansour, H. L.
• Published: Swansea University 1983
• Subjects: 531.16
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637998

The present work describes measurements of the total scattering cross-sections of low-energy positrons in N2, C02 and Ar gases, performed using a Ramsauer type magnetic spectrometer with a retarding field analyser in front of the target. A review is given of the basic properties of positrons, their interaction with solid surfaces leading to the production of moderated beams, and previous experimental measurements of total cross-sections made by various research groups. Theories are outlined explaining the scattering of positrons by atoms and molecules. The experimental measurements from 20 eV to 1 keV are described, including improvements made to the system (notably a new positron gun assembly). The results agree well with those of other groups in the region of overlap. The emission energy of the slow positrons is estimated from the lower energy measurements. Available theories are not applicable over the present energy range (apart from the optical theory in Ar over the range 100-300 eV), and the energy is not high enough to apply the well-established Bethe-Born theory. The necessary modifications of the apparatus, and its testing up to 3 keV, are described, notably a new target assembly (for which improved background rejection and magnetic shielding are also achieved). The results of measurements in N2 in the energy range 1.25-3 keV are presented. The error due to forward scattering in these high energy measurements is shown to be comparable with other experimental errors. The application of the Bethe-Born theory to these results yields a value of 5.67 for N1tot2 (the sum of dipole moments squared) which is in excellent agreement with other theoretical and experimental estimates, and is the first published value derived from positron scattering data in N2. The first application of the Sum rule to the N2 results is reported. It yields a value of -2.3 ao for the zero energy scattering length, in good agreement with the existing theoretical range of this quantity. It is concluded that the first Born approximation is valid in N2 for energy i 1 keV.