There are several materials which are suitable for use as phosphors. The type of radiation to be detected to some extent determines which is chosen. One of the most commonly used scintillating materials for detecting gamma-rays is activated sodium iodide . `Activated' in this context means that it is doped with about 1% thallium to increase its efficiency as a scintillator.
The iodine provides most of the stopping power in the sodium iodide (NaI) and it is the resettling of the orbital electrons of iodine after ionisation which produces the blue, violet, and ultraviolet light of the scintillation. A scintillation is really a very brief shower of minute light flashes, each flash resulting from the resettlement of the electrons in an ionized atom. In a sodium iodide crystal the whole process is completed in less than a microsecond, so that the shower appears as a single flash.
The more energetic the absorbed gamma photon is, the greater will be the number of atoms ionized in the crystal, and therefore the more intense the scintillation. In other words we can measure the energy of the gamma photon by measuring the intensity of the scintillation.
The scintillator is optically bonded to a photomultiplier and the assembly is encased in a thin aluminum can which allows gamma rays in, but excludes light from any source other than the scintillator. This also prevents moisture from damaging the NaI crystal which is hydroscopic.