A well-logging instrument package in which a source of gamma radiation, usually 2760Co or 137Cs, bombards the wall of the borehole, and the backscatter of gamma rays, together with natural radiation, is recorded some 45 cm above the source. The record is known as a density log, as the backscatter is an exponential function of electron density in the rocks. Mostly the instrument responds to the nearest 10 cm of rock wall. Sometimes two detectors are used at different locations to distinguish between mud cake and rock.