1930 the German physicists Bothe and Becker bombarded
the light metal beryllium with alpha particles, and
noticed that a very penetrating radiation was emitted.
This radiation was non-ionising, and they assumed it
was gamma rays.
1932 Irène and Frédéric Joliot-Curie
investigated this radiation in France. They let the
radiation hit a block of paraffin wax, and found it
caused the wax to emit protons. They measured the speeds
of these protons and found that the gamma rays would
have to be incredibly energetic to knock them from the
Chadwick reported the Joliot-Curie's experiment to Rutherford,
who did not believe that gamma rays could account for
the protons from the wax. He and Chadwick were convinced
that the beryllium was emitting neutrons. Neutrons have
nearly the same mass as protons, so should knock protons
from a wax block fairly easily.