Neptunium In the fall of 2002, a group of scientists at Los Alamos National Laboratory determined that the critical mass of neptuniwn-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/cm3. What would be the radius of a sphere of this material that has a critical mass?