X-ray tubes
X-rays are produced in vacuum tubes that are either sealed or capable of being disassembled. The electrons are produced by a tungsten filament to which an electric current is applied. They are accelerated by a difference of potential between the filament and the anticathode that is connected to ground. A concentration cup (or Wehnelt) surrounds the filament; its potential, which is several hundred volts lower than that of the filament, repels the electrons and thus concentrates the beam. The x-rays produced exit the tube via beryllium windows, chosen because it seals a vacuum but allows x-rays to pass.
