Bonner, T. W., Kraus, A. A., Marion, J. B., and Schiffer, J. P., Phys. Rev., 102, 1348 (1956).

In the center of the bomb, he said, was a beryllium sphere that provided a source of neutrons to make the plutonium explode more suddenly. He may have had in mind a mixture of beryllium and radium ...

They did this by first firing another isotopic element, calcium-48, at a ball of beryllium. This produced lighter atoms, ...

For bullets they used neutrons. The neutrons were knocked out of beryllium by alpha particles from radium. The beryllium and 200 milligrams of radium sulphate, worth $4,000, were in a metal tube.