Q.Explain the physical origin of characteristic X-ray lines, describing what happens inside a target atom when an inner-shell electron is knocked out by a bombarding electron.

If a bombarding electron in the Coolidge tube carries enough kinetic energy, it can knock an electron completely out of one of the target atom's INNERMOST shells -- most importantly the K-shell, nearest the nucleus -- ionising that atom and leaving a vacancy there. This vacancy does not persist: almost instantaneously, an electron from one of the atom's OUTER shells (the L-shell, M-shell, etc.) falls inward to occupy it, and the energy released in doing so -- exactly equal to the difference between the binding energies of the two shells involved -- is emitted as a single photon. …

Trace the two-step process in order: ejection (creates the vacancy) then filling (releases the characteristic photon …

• Confusing this with the continuous-spectrum mechanism (Exercise 5) -- deceleration produces a RANGE of energies; vacancy-filling produces one FIXED energy per transition. …