How do X rays and gamma rays cause crystal phosphors to glow?

X-rays and gamma rays cause crystal phosphors to glow through a process of energy transfer involving secondary electrons. Because these high-energy photons are uncharged, they can travel significant distances through matter before interacting with it.1

When an interaction does occur, it is often catastrophic, resulting in the transfer of all or part of the photon's energy to one or more electrons within the phosphor material. These resulting secondary electrons are energetic and charged, allowing them to interact with the crystal lattice in the same manner as primary fast electrons. This energy is then transferred to the electrons responsible for luminescence, causing them to jump from their ground-state energy level to a higher energy level.12

As these excited electrons return to their ground state, they emit the excess energy as visible light. In some cases, this energy may move through the crystal as excitons until it reaches a luminescent centre where the emission process occurs.2