Steve Marsden’s

In general neutrons make good projectiles for penetrating the electron cloud and entering the nucleus because there are no electrostatic repulsive forces to deal with. Unfortunately those properties also make them difficult to accelerate and direct.

The first evidence for transmutation of elements came from Rutherford's observation that some alpha particles released by the decay of radium interacted with the nitrogen in the air to produce O-17:

Further evidence of transmutation led to the discovery of the neutron in 1932 by James Chadwick after observers noted that when some light elements were bombarded with alpha particles they emitted highly penetrating, neutral particles.