It was Enrico Fermi who discovered that when a radioactive substance such as uranium is bombarded by neutrons, it produces by-products that are not uranium, and are lighter than the original sample.
In 1935, he bombarded uranium- which is considered to be element 92- with neutrons, and produced what appeared to be element 93 and element 94.
He won the Nobel prize in 1938 for his work in radioactivity, and this allowed his to escape form ltaly during world war ll, and settle in the united states. He then built the first nuclear reactor, and worked on the manhattan project. Fermi died in Chicago in 1954. Element 100, fermium, is named in his honour.