After the discovery of neutrons, scientists did experiments on first little neutron sources. Otto Hahn used the combination of a radioactive alpha particle emitter and beryllium to obtain neutrons. Hahn let the free neutrons collide with the heavy element uranium. Hahn hoped to create an even heavier nucleus by the neutron capture. Instead, he found nuclei with smaller masses in 1939.

Lise Meitner, nuclear physist and colleague of Hahn, found the only logical explanation: The uranium nuclei did not gain weight by capturing neutrons, but were split in smaller nuclei caused by the neutron capture. The fission by neutrons was discovered, and Otto Hahn received the nobel prize in chemistry in 1944.

Fissuring uranium with the mass number 235 yields two medium weight nuclei. At the same time two to three fast, high-energy neutrons are released, who are moderated by e.g. cooling water. These moderated neutrons themselves fissure more uranium nuclei in a chain reaction.

Free neutrons can also be generated by spallation. Accelerated particles (usually protons) are shot towards atoms and knock out neutrons.