Non-sphericity parameter
The ratio of surface area of a sphere to that of a non-spherical body of equal mass, i.e. η = A0/A. (with regard to ellipsoids, such non-spherical shapes are defined by the body mass and the relative lengths of the minor and major axes). (ICRP Publication 108, 2008, para 96)
ICRP Glossary entry - May 2019