According to an embodiment of the present invention, a radiation detector is provided with a photoelectric conversion substrate, which converts light into electric signals, and a scintillator layer, which is in contact with the photoelectric conversion substrate, and which converts radiation into light, said radiation having been inputted from outside. The scintillator layer is formed of a fluorescent material containing Tl, as an activator, in CsI that is a halide, and concentration of the activator in the fluorescent material is 1.6 mass%+/-0.4 mass%, and activator concentration distributions in the in-plane direction and film thickness direction are within +/-15%.