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Well-logging tools based on gamma-ray scattering consist of a source of gamma rays and multiple detectors, mounted in an assembly which is kept in mechanical contact with the borehole wall during the moving measurement. Compton scattering is exploited for formation density information while analysis of the photoelectric absorption of multiply-scattered gamma rays provides the average atomic number of the scattering formation. The parameters that control the device performance, constrained by operation in the borehole, are examined. A technique of combining measurements from several detectors allows estimation of bulk density over a range of environmental conditions. The photoelectric absorption measurement is reviewed to indicate its relationship to formation lithology type. Borehole factors that may compromise both measurements are related to tool design parameters and fundamental physical limitations.