Enrico Fermi won the 1938 Nobel Prize in Physics for his demonstrations of the existence of new radioactive elements produced by neutron irradiation and his related discovery of nuclear reactions brought about by slow neutrons.
Fermi and his collaborators bombarded many elements with neutrons, creating radioactive isotopes. He discovered that slowing neutrons with materials such as paraffin could make them much more effective at inducing certain nuclear reactions. This insight became important in later nuclear-reactor research.
The Nobel citation reflects the scientific understanding of the late 1930s. Fermi believed some products of neutron bombardment were transuranium elements, but later work showed that several results had actually involved nuclear fission. Otto Hahn and Fritz Strassmann identified barium among the products of uranium bombardment, while Lise Meitner and Otto Frisch explained the fission process.
Fermi later led important work on the first controlled nuclear chain reaction. Chicago Pile-1 achieved that milestone in 1942, four years after his Nobel award.