Past exam of the particle physics course of the University of Oxford / 2015 / 6 / d

An electron gun releases electrons by thermionic emission, field emission or photoelectric effect, accelerates them through an electric potential, and uses electrostatic or magnetic lenses to form a beam in vacuum. A hot unpolarized cathode does not by itself select spin.

For a polarized source, illuminate a suitable semiconductor photocathode, commonly strained GaAs, with circularly polarized light near its band-gap threshold. Optical angular-momentum selection rules preferentially populate one conduction-electron spin; emission from a negative-electron-affinity surface extracts the selected population. Reversing the light's circular polarization reverses the beam polarization. Strain and a suitable excitation wavelength suppress transitions that dilute the selection. Solenoids and spin rotators orient the spin after acceleration, and a scattering polarimeter measures the achieved polarization; it need not be $100\%$.