X-rays are produced by the deceleration of charged particles, usually electrons. We generate X-rays by bombarding a target material with high energy electrons. The decelerating electrons produce a continuous spectrum of energies when they hit the target called the ‘bremsstrahlung’ which is German for ‘braking radiation’ as it is produced by stopping electrons.

If the incident electrons have sufficient kinetic energy, it can knock an electron out of its shell, leaving the atom in a high energy state. One of the outer electrons immediately falls into the lower energy empty shell. Since the falling electron moved from a higher energy shell to a lower energy shell, this difference in energy is emitted as a phonon which has an energy characteristic of the electron shell. This creates what we refer to as the “characteristic spectrum” of the target material.

In XRD we usually use the K alpha peak, shown below. Due to the quantum mechanics of electron energies and their shell arrangement, we find a typical shape to the characteristic peaks where the most intense peak is the K alpha 1, and this is accompanied by a smaller K alpha 2 peak which is approximately half the intensity of the K alpha 1.