The cross section of a material to photoelectric absorption, in barns/cm³. The volumetric cross section, U, is from the product of the photoelectric factor, PEF or P_e , and the electron density. In practice, U is usually calculated using the bulk density instead of the electron density. U is a volumetric quantity, whereas P_e is not. U is more useful in log interpretation since it can be used in a linear mixing law in terms of the volumes of the formation components.