Disclosed is a luminescent material for scintillators, which comprises a single crystal of an Yb-containing mixed-crystal oxide having a base crystal consisting of a garnet single crystal or a borate single crystal. The oxide single crystal has a composition represented by either one selected from the group consisting of R.sub.3Al.sub.5O.sub.12, R.sub.3Ga.sub.5O.sub.12, Li.sub.6R(BO.sub.3).sub.3, LaR.sub.2Ga.sub.3O.sub.12 and Gd.sub.3R.sub.2Ga.sub.3O.sub.12, wherein R is a mixture of Yb and either one of Y, Gd and Lu. The Yb as an element capable of forming an optically active state called CTS together with a neighboring negative ion (oxygen ion).