Résumé dans la langue de publication Novel phosphors represented by the general formula: (Gd1-x Cex )3 Al5-y Gay O12 (wherein x is a value satisfying the relationship: 0.0005<=x<=0.02; and y is a value satisfying the relationship: 0<y<5). These phosphors exhibit high fluorescence outputs by virtue of the Al and Ga coexistent with the Gd phosphors. In particular, when the Ga concentration (y) lies within the range of 2 to 3, the phosphor exhibits a fluorescence output as high as twice that of the phosphor of the prior art. Radiation detectors wherein the phosphors are used as the ceramic scintillator exhibit extremely low afterglow and high scintillation outputs.