When high-energy electrons collide with the target atoms in the anode, they interact with both the atomic nuclei and the orbital electrons of the target material. Most interactions simply transfer energy to the atoms as heat through excitation and ionisation. Only a small fraction of interactions produce X-ray photons.

The efficiency of X-ray production increases with both atomic number and electron energy. This relationship can be approximated by:

Efficiency ∝ Z × kVp

where Z is the atomic number of the target material.

This occurs because atoms with higher atomic numbers have stronger nuclear electric fields, which increases the likelihood that incoming electrons will undergo significant deceleration and produce Bremsstrahlung radiation. Higher tube voltage also increases the kinetic energy of electrons, making high-energy photon production more probable.

In practical terms, the efficiency of X-ray production can be estimated using:

Efficiency ≈ 9 × 10^-10 × Z × kVp

For a tungsten target with Z = 74 operating at 100 kVp, the efficiency is approximately:

9 × 10^-10 × 74 × 100 ≈ 0.0079

or roughly 0.7%. This means that less than 1% of the electron energy becomes X-rays, while the rest becomes heat.

This low efficiency explains why X-ray tubes must dissipate large amounts of heat and why rotating anodes and cooling systems are essential.