The neutron was discovered as the product of an early radiochemical fusion reaction in 1932. Following a decade of mainly scientific use, the WWII nuclear bombs exploded over Japan each included a neutron generator to ignite the critical mass of fissionable material at the correct time. This event neatly split development of neutron generators between the secret and the open worlds.
The neutron sources available to science and industry included particle accelerators (at that time these filled large rooms), nuclear reactors (filling large buildings), and radioactive materials the size of your little finger. As most researchers and manufacturing companies did not have easy access to reactors and accelerators, a good deal of work toward developing practical applications for neutron sources was carried out with radioactive neutron generators.