X-rays and their production

X-rays are commonly produced in an X-ray tube: electrons are accelerated through a large voltage and slammed into a solid metal target. The resulting spectrum of emitted radiation has two distinct components, produced by two different physical mechanisms, and telling them apart is the key to understanding X-ray production.

The first component is a smooth, continuous spectrum called bremsstrahlung, German for "braking radiation." As an incoming electron passes close to a target atom's nucleus, the nucleus's electric field deflects and decelerates it, and an accelerating (or decelerating) charge radiates electromagnetic energy. Because each electron can lose anywhere from a small fraction to almost all of its kinetic energy in a single such deflection, depending on how closely it passes the nucleus, the resulting photon energies span a continuous range up to a sharp maximum set by the electron's full kinetic energy — a photon carrying away all of an electron's energy in one deceleration event sets the shortest possible wavelength in the spectrum.

The second component consists of sharp, narrow peaks called characteristic X-rays, superimposed on the continuous background. These arise from a different process entirely: a sufficiently energetic incoming electron can knock an electron out of an inner shell of a target atom, leaving a vacancy. An electron from a higher shell then drops down to fill that vacancy, emitting a photon whose energy equals the difference between the two atomic energy levels involved — exactly analogous to the discrete-line emission that produces atomic spectra generally, but here the transition is between the innermost, most tightly bound shells. Because these energy-level spacings are set entirely by the target element's own atomic structure, characteristic X-ray line energies are a fingerprint of the target material, independent of the bombarding electrons' exact energy (so long as it exceeds the threshold needed to eject the inner-shell electron in the first place) — this fingerprint is the basis of X-ray fluorescence techniques used to identify unknown materials.