Abstract: Characterization, calibration and gain stabilization of gamma ray detectors are carried out using one or more spectral features that are present in the background radiation. Such a spectral feature may be the spectral peak associated with Potassium-40 nuclide. The disclosed methods, apparatus and computer program products enable uninterrupted operation of gamma ray detection systems, while eliminating the costs associated with the procurement and integration of external reference sources into these systems, and without compromising the sensitivity of the detection system.