Scintillator compositions having a garnet crystal structure useful for the detection of high-energy radiation, such as X, beta, and gamma radiation, contain (1) at least one of terbium and lutetium; (2) at least one rare earth metal; and (3) at least one of Al, Ga, and In. Terbium or lutetium may be partially substituted with Y, La, Gd, and Yb. In particular, the scintillator composition contains both terbium and lutetium. The scintillators are characterized by high light output, reduced afterglow, short decay time, and high X-ray stopping power.