What would be the radius r of a sphere of neptunium-237 that has a critical mass?

In the fall of 2002, scientists at Los Alamos National Laboratory determined that 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 nuclear chain reaction. Neptunium-237 has a density of 19.5 g/cm. What would be the radius of a sphere of this material that has a critical mass?