This invention relates to transparent polycrystalline ceramic scintillators and methods of preparing the same. The invention also relates to gadolinium-yttrium oxide ceramic scintillators with europium added as an activator, which are optically transparent and whose cubic structures are maintained even when gadolinium oxide constitutes more than 50% of the scintillator. The scintillators of the present invention also have high light yield due to excellent luminescent characteristics in the visible light region.