[en] A method is described for controlling and linearizing the gain of a nuclear well logging system comprising the steps of: 1) measuring the energy spectrum of gamma rays produced by materials of the earth formation surrounding a well borehole at a resolution sufficient to discern at least one recognizable energy peak of known energy in the gamma ray energy spectrum; 2) comparing the apparent location of at least one energy peak in the spectrum with its known location and generating an error signal proportional to the difference between its known location and its apparent location; and 3) controlling the gain of the energy measuring step in response to the error signal in such a manner as to minimize the error signal. (author)