Polarization transfer in relativistic electron-nucleus bremsstrahlung

The emission of circularly polarized photons during the scattering of fast spin-polarized electrons from heavy nuclei is studied within the Dirac–Sommerfeld-Maue approach. Predictions are made for the dependence of the polarization correlations C32 and C12 on collision energy, photon energy and nuclear charge. A comparison with pilot measurements of the transmission asymmetry, sensitive to C32, for 3.5 MeV e + Pb verifies that the polarization transfer increases with photon energy for small emission angles. The precise knowledge of the polarization transfer from the beam particles to the photons emitted in a bremsstrahlung process, combined with an accurate measurement of the photon polarization via recently developed Compton polarimetry [1]-[4], provides a promising method to determine the degree of beam polarization and its change with time during an experiment [5]. Such knowledge is important for nuclear structure investigations. Bremsstrahlung may also be used as a source of radiation for a variety of applications, among those for the study of parity conservation in photofission experiments which requires a high degree of circular polarization of the photons [6]. Early theoretical predictions [7, 8], based on the Sommerfeld-Maue (SM) approximation introduced by Bethe and Maximon [9], indicated that the polarization transfer is particularly large at the short-wavelength limit (SWL). There, the electron transfers all its kinetic energy to the photon such that relativistic (spin) effects become highly important. These SM results for the polarization correlations were topped by accurate relativistic partial-wave calculation for a selection of collision systems [10]-[12], and only recently a systematic partial-wave study up to collision energies of 3 MeV has become available [13]. At even higher energies the partial-wave theory (which then suffers from convergence problems) may be supplemented at the SWL by the Dirac–Sommerfeld-Maue (DSM) theory [14, 15] where the slow scattered electron is described by an exact Dirac state and the fast incoming electron by an SM function. We consider the case where the target is a bare nucleus with charge number Z and where the scattered electron is not observed. The doubly differential cross section dσ for the emission of a photon with momentum k, energy ω = kc and polarization eλ into the solid angle dΩk is given (in atomic units, ~ = m = e = 1) by [16, 17] dσ ≡ d σ dω dΩk = 4πωkfEf c3 v ∑ σf ∫ dΩf |eλ Wrad(σf , σi)| 2 , eλWrad(σf , σi) = ∫ dr ψ (σf )+ f (r) (αe ∗ λ) e −ikr ψ (σi) i (r). (1) In (1), ψi i and ψ (σf ) f describe, respectively, the initial and final electronic states with total energy Ei and Ef = Ei−ω, momentum ki and kf and spin projection σi and σf . The vector α comprises the Dirac