Electrons are emitted by the glowing cathode and accelerated by several tens of kVs across the vacuum towards the anode, which consists of a metal target made of a characteristic material, usually copper or chromium, for protein crystallography. As the electron beam impacts the anode, the high kinetic energy of the electrons is converted during deceleration into X-rays producing a) a continuous spectrum consisting of bremsstrahlung ("braking radiation") and b) emission lines characteristic for electronic transitions caused in the anode material.