A gamma ray detector suitable for use in a positron CT and others is disclosed which comprises a scintillator formed of a single crystal of cerium activated gadolinium silicate which is expressed by a general formula Gd.sub.2(1-x-y) Ln.sub.2x Ce.sub.2y SiO.sub.5 (where Ln indicates at least one element selected from a group consisting of yttrium and lanthanum, and x and y lie in ranges given by formulae 0.ltoreq.x.ltoreq.0.5 and 1.times.10.sup.-3 .ltoreq.y.ltoreq.0.1).