Calibrating radiation detectors in logging apparatus

The apparatus described includes a gamma radiation source and at least two pairs of radiation detectors carried by a housing so that radiation may be emitted from the source and scattered by the environment of the housing and detected by the detectors. Each detector is calibrated using test environments of different known densities and for each environment, with the detector at different known distances from the surface of the surrounding formation of the environment. The flanges of the functional relationships between the logarithms of the count rates of the detector as a function of known densities is obtained and the logarithm of the slopes is compared as a function of the corresponding distances to obtained calibration constants. A method of calibrating the rotation detector in liquids of known densities is also described. The housing carrying the radiation source and detectors may be positioned in a borehole by means of a wireline, or may be included in a pipe string used in drilling for example. (author)