Scientists first created the artificial element neptunium in 1940, around the time they made the first atoms of plutonium. But because plutonium turned out to be so much better for nuclear weapons, researchers have paid scant attention to neptunium.
Now a team at Los Alamos (N.M.) National Laboratory has measured how much neptunium it would take to make a bomb–the element's critical mass–with far greater accuracy than ever before. From a preliminary data analysis, the investigators find that neptunium's critical mass is around 60 kilograms. The critical mass of plutonium-239 is about 10 kg, and for uranium-235, the most widely used nuclear explosive after plutonium, it's about 50 kg.