An illuminant has a short fluorescence lifetime, high transparency, and high light yield and a radiation detector uses the illuminant. The illuminant is appropriate for a radiation detector for detecting gamma-rays, X-rays, α-rays, and neutron rays, and has high radiation resistance, a short fluorescence decay time and high emission intensity. The illuminant has a garnet structure using emission from the 4f5d level of Ce3+, and includes a garnet illuminant prepared by co-doping of at least one type of monovalent or divalent cation at a molar ratio of 7000 ppm or less with respect to all cations, to an illuminant having a garnet structure represented by general formula CexRE3−xM5+yO12+3y/2 (where 0.0001≤x≤0.3, 0≤y≤0.5 or 0≤y≤−0.5, M is one type or two or more types selected from Al, Lu, Ga, and Sc, and RE is one type or two or more types selected from La, Pr, Gd, Tb, Yb, Y, and Lu).