In 1932, James Chadwick discovered the neutron, but initially the only sources of neutrons were from the radioactive decay of unstable nuclei. It was not until 1942 when Enrico Fermi constructed the first nuclear reactor in the squash courts beneath the University of Chicago’s Stagg Field, that a controlled and sustained nuclear chain reaction was achieved. After World War II, nuclear reactors became available for civilian research, and in 1945 Ernest Wollan set up a double-crystal diffractometer at ORNL’s Graphite Reactor. This marks the beginning of neutron scattering