Prior to actual use in a downhole application, gamma spectrum shape for a gamma detector can be quantified, and a relationship established between detector gain and spectrum shape. Given this relationship, the shape of a gamma spectrum measured after downhole deployment of the detector within a wellbore, for example as part of a tool in a drill string, can be quantified and compared to the pre-established relationship to determine whether the detector gain has drifted due to temperature or operating time effects. Using this relationship, a gain-affecting voltage across the detector (e.g., the photocathode voltage across the photomultiplier tube) can be adjusted to compensate for such drifts and thus to compensate for variations caused by temperature or operating time. With such compensation applied to the detector, resulting gamma spectra reliably indicate actual differences in radiation levels, thus enabling an inference of the composition of the formation at various times/depths.