In nuclear physics, nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei). The fission process often produces free neutrons and releases a very large amount of energy.
Thus, The Nuclear Fission is caused by the impact of Neutrons.
It was on February 28th, 1928 that CV Raman announced his discovery of New radiation-subsequently named the Raman Effect- that won him the Noble prize.