Abstract: A radiation detector comprising a scintillator exhibiting luminescence by the action of radioactive rays and a light detector detecting light output of said scintillator, characterized in that said scintillator is a phosphor represented by the formula:(A.sub.1-x-y Ln.sub.x Ce.sub.y).sub.2 O.sub.2 S:Xwherein A is at least one element selected from the group consisting of Y, La and Gd; Ln is at least one element selected from the group consisting of Pr, Tb and Eu; and X is at least one element selected from the group consisting of F, Cl and Br; and the values of x and y are 0.000003.gtoreq.x.gtoreq.0.2, and 0.000001.gtoreq.y.gtoreq.0.005, and the amount of X is 0 to 1000 ppm on weight basis, has a very short after glow time because of having Ce in the phosphor.