sodium iodide

Alkali halide crystals such as sodium iodide combined with about 0.1 percent thallium have been found to emit light when X-rays are absorbed in the material. These devices are known as scintillators, and when used in conjunction with a photomultiplier tube they can easily detect the burst of light

…most common inorganic scintillator is sodium iodide activated with a trace amount of thallium [NaI(Tl)], which has an unusually large light yield corresponding to a scintillation efficiency of about 13 percent. Its decay time is 0.23 microsecond, acceptable for many applications but uncomfortably long when extremely high counting rates or…