Ionizing, electromagnetic radiation which is created in an x-ray tube. It is polychromatic radiation. This means that a continuous spectrum of different energies (or wavelengths) is present. This spectrum can be changed with pre-filters.
When highly energetic electrons are fired, inner electrons become ionized. During the subsequent filling of the hole which has occurred with electrons from higher shells, the energy difference becomes noticeable as x-ray quanta (characteristic x-ray) or as an Auger electron.
The continuous x-ray spectrum is formed as the electrons are slowed down in the Coulomb field of the atom nucleus and lose their kinetic energies continuously in the form of bremsstrahlung. The bremsspectrum contains only a little analytical information about the sample and is therefore to be seen as background interference.