where Z is the atomic number of the nucleus, A is the mass number of the nucleus, e is the charge of an electron, R 0 is the radius of the undistorted spherical nucleus, S is the surface tension per unit area of the nucleus, a c = 3 e 2 / 5 R 0 and a s = 4 π π R 0 2 S . The equation for the fissionability parameter then becomes: