Gain stabilization of a natural gamma ray tool

An apparatus comprising:

• a gamma ray detector to detect gamma rays reflected from materials in a wellbore;
• and
• a processor to
• receive gamma ray measurements from the gamma ray detector;
• generate a spectrum of the gamma ray measurements, the spectrum including a plurality of channels corresponding to energy values of the received gamma rays and count rates for the plurality of channels;
• fit a curve to a portion of the spectrum;
• determine a location of the maximum of the first derivative of the curve;
• and
• adjust a gain of the gamma ray detector based on the location of the maximum of the first derivative of the curve.

A system comprising:

• a logging tool, including a housing to house a gamma ray measurement tool, the gamma ray measurement tool including
• a gamma ray detector for detecting gamma radiation at a plurality of energy levels and for generating detector output signals each representing a detected count of gamma radiation;
• and
• a processor to
• receive gamma ray measurements from the gamma ray detector;
• generate a spectrum of the gamma ray measurements, the spectrum including a plurality of channels corresponding to energy values of the received gamma rays and count rates for the plurality of channels;
• fit a curve to a portion of the spectrum;
• determine a location of the maximum of the first derivative of the curve;
• and
• adjust a gain of the gamma ray detector based on the location of the maximum of the first derivative of the curve.