N the fall of 2002, a group of scientists at Los Alamos National Observatory determined that the critical mass of neptunium - 237 is ab out 60.0 kg. (The critical mass of a fissionable material is the minimum amount that must be brought together to start a nuclear chain reaction.) Neptunium has a density of 19.5 g/cm 3 . What would be the radius of a sphere made of this material that has a critical mass