I cant figure out how to get the answer with the given info, pleas help. In the fall of 2002, a group of scientists at Los Alamos National Laboratory determined that the critical mass of neptunium-237 is about . The critical mass of a fissionable material is the minimum amount that must be brought together to start a chain reaction. Neptunium-237 has a density of . What would be the radius of a sphere of neptunium-237 that has a critical mass? Express your answer in centimeters to three significant figures.