I think of cancer, and radiation therapy for it, as modern phenomena. It was surprising, then, to learn that radon ‘seeds’ were being used for radiation therapy of cancer in 1929 – before the neutron had even been discovered. Indeed, polonium extracted from depleted seeds from Baltimore’s Kelly Hospital provided James Chadwick with the neutron source for the experiment that confirmed their existence. Neutrons are critical for inducing the fission of atoms, whether in bombs or power plants, because they are not repelled by the electric fields around atoms.
A detailed account of the experiment is in chapter 6 of Richard Rhodes’ The Making of the Atomic Bomb.