Neutron sources, which could be classified as catalyst, provided extra neutrons that could sustain the chain reaction even when Uranium-235 was under its critical mass.
Technically, both polonium and radium separated from natural uranium could form compounds with beryllium to serve as neutron sources. Roland did not foresee any technical difficulties, for polonium could release a large number of neutrons when colliding with alpha particles. All of the three elements were existing in nature, which saved them a lot of time to create elements artificially.
Roland preferred polonium to radium purely out of safety concerns. Although polonium-210 had a short half-life, it released very few photons upon a decay, which meant it was no more dangerous than inactive uranium. However, radium, which released radon and photons upon activation, posed a potential safety hazard. As such, polonium was a better candidate to provide neutrons.
