where the ratio of the constant ( a c / 2 a s ) − − 1 is referred to as ( Z 2 / A ) c r i t i c a l . The fissionability of a given nucleus can then be categorized relative to ( Z 2 / A ) . As an example, plutonium-239 has a ( Z 2 / A ) value of 36.97, while less fissionable nuclei like bismuth-209 have a ( Z 2 / A ) value of 32.96.