Measurement of the Mott Asymmetry in Double Scattering of Electrons

Abstract

The Mott asymmetry was observed at 121 kev for gold targets and scattering angles of ${\ensuremath{\theta}}_{1}=90\ifmmode^\circ\else\textdegree\fi{}$ and ${\ensuremath{\theta}}_{2}=80\ifmmode^\circ\else\textdegree\fi{} \mathrm{to} 140\ifmmode^\circ\else\textdegree\fi{}$. The cosine dependence of the asymmetry on the azimuthal angle was shown in all cases. A weak magnetic lens (maximum field: 12.5 gauss) was used between the two scatterers, and a low-resolution (8%) electrostatic energy analyzer was used after second scattering. The measured asymmetry amplitudes are compared with the calculations of Sherman (pure Coulomb field) and of Mohr and Tassie (screened Coulomb field). The experimental values for ${\ensuremath{\theta}}_{2}=90\ifmmode^\circ\else\textdegree\fi{}, 100\ifmmode^\circ\else\textdegree\fi{}, 110\ifmmode^\circ\else\textdegree\fi{}$ agree well with either theoretical curve but are not accurate enough to distinguish between them. The measured value at ${\ensuremath{\theta}}_{2}=80\ifmmode^\circ\else\textdegree\fi{}$ is enough higher (17%) than the pure Coulomb field curve to indicate a screening effect. The experimental values for ${\ensuremath{\theta}}_{2}=120\ifmmode^\circ\else\textdegree\fi{}, 130\ifmmode^\circ\else\textdegree\fi{}, \mathrm{and} 140\ifmmode^\circ\else\textdegree\fi{}$ fall from 15% to 20% below the theoretical curves. This discrepancy is attributed to plural scattering. Measurements made using the magnetic lens but not the energy analyzer yield asymmetries reduced in amplitude about 33% and shifted in phase by 26\ifmmode^\circ\else\textdegree\fi{} when compared with the measurements discussed above. This is believed due to the presence of an appreciable number of polarized electrons at a considerably lower energy than 121 kev in the first scattered beam.