The detection of nuclear fission and the discovery that, during the fission of heavy nuclei such as uranium, more than one neutron is released by neutron bombardment, form the basis for the generation of chain reactions and neutron beams. It took a few more years until they could be routinely used in science. The optimization process is still not finished. Machines which utilize the controlled chain reaction to produce nuclear fission are referred to as nuclear reactors.
In addition to the self-sustaining chain reaction, it is also possible to bombard suitable atoms (targets, such as lead or bismuth) with secondary particles, e.g. protons, thereby producing free neutrons. However, this reaction is not self-sustaining and comes to an end when the bombardment stops. This type of neutron source is known as a "spallation" source.
The controlled generation of intense, well-defined neutron beams requires elaborate nuclear installations, even though matter is composed predominantly of neutrons.