To provide a yttrium tantalate X-ray phosphor which reduces the afterglow of an yttrium tantalate X-ray phosphor without increasing the particle size and increases XOF luminance, and a method of preparing the same.

A mixed flux to be used in the synthesis of an yttrium tantalate phosphor contains a mixture of lithium sulfate and sodium sulfate. A phosphor prepared by using this mixed flux exhibits a dramatic increase of XOF luminance, a reduction of afterglow and a reduction of particle size. A method of preparing a phosphor comprises forming an oxide mixture of yttrium oxide, tantalum oxide and, optionally, strontium carbonate, adding a mixed flux of lithium sulfate and sodium sulfate to the oxide mixture and firing the resulting mixture at a sufficient temperature for a sufficient period of time to form a phosphor.