= Solution
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 $100\%$.
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