Abstract: An afterglow property of cesium iodide:thallium (CsI:Tl), in which CsI is a host material and doped with thallium, is improved. It is possible to improve the afterglow property of a scintillator by doping a crystal material including CsI (cesium iodide), as a host material, and thallium (Tl), as a luminescent center, with bismuth (Bi).