The Litho-Density tool (LDT) uses a pad-mounted gamma-ray source and two scintillation detectors to measure the bulk density and the photoelectric absorption cross section (P_e). The P_e measurement is closely related to the lithology of the formation. A spectral analysis of the detected gamma rays is used for the P_e measurement and to improve the response of the density measurement. See also compensated formation density log. Litho-Density is mark of Schlumberger.