PURPOSE: To obtain a scintillator compsn. which exhibits reduced afterglow and improved capability for resisting X-rays without being substantially damaged by incorporating cerium as a hole-trapping species into the same.

CONSTITUTION: This scintillator compsn. mainly comprises gadolinium gallium garnet of the formula: Gd3Ga5O12 and contains chromium in a concn. equivalent to 0.05-1.0 wt.% chromium oxide and also contains cerium as a hole-trapping species present in the gadolinium gallium garnet in a concn. of 0.2-0.255 wt.%. The hole-trapping species is effective in lowering the level of afterglow of a scintillator compsn. not contg. it and improves the capability for resisting X-rays without substantially damaging the compsn. Chromium susbstitutes for Ga in the formula, and the hole-trapping species substitutes for Gd in the formula.