Abstract: The present invention presents a method and apparatus for stabilizing signals in radioactive well logging tools including providing in the tool a main scintillating crystal and photomultiplier tube for detecting radiation induced in the borehole by a source of radiation, and a reference crystal, including a source of mono-energetic radiation, for producing continuous reference signals of a predetermined energy level. The reference signals are monitored and the spectrum is stabilized to correct for drift of the data signals introduced by the photomultiplier tube and the data transmission system. The preferred mono-energetic reference signals are selected to appear in the energy spectrum at a level which minimizes error. An electronic circuit at the surface provides a correction signal for adjusting the gain of a data signal amplifier responsive to changes in the reference signal, thereby correcting for drift in the data signal.