 Perrin defined a critical mass of uranium to be the smallest amount that could sustain a chain reaction. The neutrons used to cause fission in uranium are considered slow neutrons, but when neutrons are released during a fission reaction they are released as fast neutrons which have much more speed and energy. Thus, in order to create a sustained chain reaction, there existed a need for a neutron moderator to contain and slow the fast neutrons until they reached a usable energy level. The College de France found that both water and graphite could be used as acceptable moderators.
