The process begins at the cathode, where electrons are released by thermionic emission. When a high potential difference (kVp) is applied between cathode and anode, these electrons are accelerated across the vacuum of the X-ray tube.

Each electron gains kinetic energy equal to:

E = e x kVp

where e is the elementary charge. In practical units, the maximum photon energy (in keV) is numerically equal to the applied kVp. For example, at 100 kVp, the maximum possible photon energy is 100 keV.

When these high-speed electrons collide with the tungsten anode, they interact with target atoms in two principal ways.

In Bremsstrahlung radiation, the electron passes close to the positively charged nucleus. The nuclear electric field decelerates the electron, causing it to lose kinetic energy. That lost energy is emitted as an X-ray photon. Because the amount of energy lost varies continuously depending on how closely the electron approaches the nucleus, Bremsstrahlung produces a continuous spectrum of photon energies.

In characteristic radiation, the incoming electron ejects an inner-shell electron from the target atom. When an outer-shell electron falls into the vacancy, the energy difference between the shells is released as an X-ray photon with a discrete energy specific to the target material.

Despite these interactions, X-ray production is inefficient. Approximately 99% of the kinetic energy of incident electrons is converted into heat, while only about 1% becomes X-ray photons. This inefficiency explains the need for rotating anodes and careful heat management.