Theoretical Physics of Mott Polarimetry

Mott polarimetry is a means of measuring the spin polarization of an electron beam, and finds its most frequent application in photo-emission spectroscopy [67] and the study of surfaces, thin films and multi-layers [68]. The latter is the application with which this thesis is primarily concerned. Mott polarimeters are based on the left-right asymmetry in the scattering of polarized electrons by atoms, i.e. the relative difference in scattering cross-section between scattering events through the same angle in opposite senses (equation 3.2,) and its use as an indicator of the incident beam's spin polarization [34]. Common choices of scatterer for Mott polarimeters are gold, thorium and uranium foils [5,34]. All of these have high proton numbers $Z$, leading to a large spin-dependent potential correction (equation 3.1.) Thorium and uranium provide a value of total scattering cross-section $\sigma _0$ (equation 3.2) which is around $15\%{}$ greater than that for gold [5,34] and an analysing power $S$ (equation 3.2) which is improved by between $10\%{}$ and $20\%{}$ [34] or even as much as $30\%{}$ [5]. On the other hand, thorium and uranium have the disadvantage that their chemical reactivity renders them more sensitive than gold to atmospheric contaminants, which may reduce both overall cross-section and Sherman function (section 3.1.2) [5,34]. For the polarimeter used in this project, a thorium foil has been chosen (figure 3.1.)