Electron polarimetry with bremsstrahlung

Due to the spin-orbit interaction, the electron scattering from the nucleus is sensitive to the spin orientation of that electron. This is used for polarimetry of electron beams in the Mott method. The spin-orbit interaction was also observed in bremsstrahlung. In this article we analyze its potential for polarimetry as an alternative to the Mott method. It can simultaneously measure all three electron polarization components. It should work in the energy range of 50 keV up to several MeV and can be applied at beam intensities higher than 100 nA. It needs a thin heavy element target, two or four x-ray detectors and one x-ray linear polarimeter. Polarized electron beams are frequently used in solid state, atomic, nuclear and particle physics. Their typical applications are studies of the spin-effects in atomic collisions, parity violation effects in electron scattering and precision nucleon structure, see [1] and references therein. Photo-ionized polarized electrons can yield information on the magnetic properties of surfaces [2]. Polarization of the Compton-scattered electrons contains information on the circular polarization of the incoming gamma-rays; this can be used for gamma-ray polarimetry [3]. In the energy region of 50 keV up to several MeV electron polarization is typically measured with the Mott technique. It uses the influence of the spin-orbit interaction on the Coulomb collision with an atomic nucleus. More specifically it uses the part that affects the electrons spin-oriented perpendicular to their momentum. The spin-orbit component of the scattering potential is VSO ∝ L·S, where L is the orbital momentum which is perpendicular to the scattering plane, see figure 1. The spin-up electrons predominantly scatter to the right of the plane defined by the momentum and the spin; the spin-down electrons predominantly scatter to the left: σ(θ) ∝ (1 + S(θ)S · L/L). Here S(θ) is the Sherman function [4], see figure 2. This asymmetry depends on the electron energy, the charge of the nucleus and the polar scattering angle θ. At its maximum, which is tens of percents, it is easily observed. By now Mott polarimeters achieved a high level of maturity. Most of their implementations use a thin gold scattering foil and several detectors which sample the azimuthal angular distribution of the scattered electrons. Excellent reviews are available in [1, 5] and particularly in the book Polarized Electrons by J. Kessler [6]. The energy range where Mott polarimetry can be used is defined by the following factors. XXVIII International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC 2013) IOP Publishing Journal of Physics: Conference Series 488 (2014) 012057 doi:10.1088/1742-6596/488/1/012057 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1