A method for gain control of a radiation detector includes measuring standard gamma ray spectra for known concentrations of selected radioactive materials found in earth formations. A standards matrix is calculated from the standard gamma ray spectra. A singular value decomposition is computed from a matrix related to measurements of gamma ray spectra, of formations and the standards matrix. A radiation, detector gain is computed from a minimum ratio of null space with respect to data value space of a subsurface formation measurement related matrix and the standards matrix. Gain of the radiation detector is adjusted based on the computed radiation detector gain.