Possible Existence of a Neutron

James Chadwick, Nature, p. 312 (Feb. 27, 1932)
It has been shown by Bothe and others that beryllium when bombarded by α-particles of polonium emits a radiation of great penetrating power, which has been an absorption coefficient in lead of about 0.3 (cm)-1 ….
Read more at http://web.mit.edu/22.54/resources/Chadwick.pdf