X-rays – everyone has heard of them, but how do they work?

Mobile C-arm for X-ray diagnostics during ongoing surgery

X-radiation has a high frequency (10^18 Hz), wavelengths between 0.01 and 1 nanometer, and, typically, quantum energy in a range above 100 electron volts (eV). It is created when accelerated electrons encounter solid bodies and are braked and deflected by their atoms in an electric field (coulomb field). On the one hand, this creates bremsstrahlung. On the other hand, characteristic X-rays are emitted through high-energy transitions in the electron shells of atoms or molecules.

Basically, classic X-ray tubes have not changed since they were invented by Wilhelm Conrad Röntgen. A cathode emits electrons, which are accelerated by high voltage and directed to an opposing anode. There, they interact with the electric field of the anode atoms—and emit X-rays. A collimator directs the beam, which easily penetrates a solid body. However, soft tissue and bones absorb parts of the radiation more or less depending on their density. The differences are made visible on photographic plates or, with today’s digital X-ray methods, with high resolution, full-field image sensors.