Neptunium. In the fall of 2002, a group of scientists at Los Alamos National Laboratory determined the critical mass of neptunium- 237 is about 60 kg . The critical mass of a fissionable material is the minimum amount that must be brought together to start a chain reaction. This element has a density of 19.5 g / cm^3 . What would be the radius of a sphere of this material that has a critical mass? | Numerade