A method for adjusting a gain of a gamma detector comprises detecting gamma radiation using the detector, recording the detected radiation as count rates in channels, wherein the last channel accumulates all counts above the maximum recorded energy; comparing the last channel count rate (LCCR) to a threshold X and, if LCCR>X, decreasing the gain by a preset amount Y. If LCCR≤X, establishing a first estimate of a needed voltage HV1 using tool temperature and a temperature lookup table, and establishing a second estimate of a needed voltage HV2 using a backscatter peak value and a backscatter lookup table; comparing |HV1−HV2| to a threshold Z; if |HV1−HV2|