A scintillator detector package includes a housing, with a scintillator in the housing. There is a radioactive reflective material between at least a portion of the scintillator and the housing. The radioactive reflective material may be a naturally occurring material, such as Lu2O3, and may be in powdered form. A photodetector may be optically coupled to the scintillator package, and gain stabilization circuitry may perform gain stabilization based upon detecting scintillations of the scintillator caused by radiation emitted by the radioactive reflective material striking the scintillator.