Abstract: A radiation detector operating at high temperatures is shown comprising a LuAP scintillating material for producing light when excited by incident radiation, a photocathode, and an electron multiplier. The photocathode is deposited directly onto the surface of the scintillating material that is oriented toward the electron multiplier. In a preferred embodiment, a metal flange is hermetically sealed to the scintillating material and this is fusion welded to the electron multiplier to create a vacuum envelope. This invention is particularly useful in high temperature noisy environments such as downhole operations to detect radiation within a well hole.