an x-ray tube is a simple vacuum tube that contains a cathode, which directs a stream of electrons into a vacuum, and an anode, which collects the electrons and is made of tungsten to evacuate the heat generated by the collision. when the electrons collide with the target, about 1% of the resulting energy is emitted as x-rays, with the remaining 99% released as heat. due to the high energy of the electrons that reach relativistic speeds the target is usually made of tungsten even if other material can be used particularly in xrf applications.