Inner-shell ionization cross sections and aperture size in electron energy-loss spectroscopy

Electron-energy-loss spectra have been recorded as a function of incident beam orientation for $K$-shell ionization of C in diamond. The results show a strong orientation dependence. We show that a common approximation that is often used in calculating the (total) cross section for energy-dispersive x-ray analysis cannot be made for electron-energy-loss spectroscopy when the acceptance angle of the detector is sufficiently small.