In 1930 Walther Bothe and Herbert Becker in Germany found that alpha particles striking light elements such as beryllium, boron, or lithium would release a highly penetrating radiation, at first believed to be gamma radiation, although it was more penetrating than any gamma rays known.
The next important contribution was reported in 1932 by Irène Joliot-Curie and Frédéric Joliot in Paris, who showed that if this unknown radiation fell on paraffin wax or any other hydrogen-containing compound it ejected protons of very high energy.
Finally, in 1932 the physicist James Chadwick in England performed a series of experiments showing that the gamma ray hypothesis was untenable, and suggested that the new radiation consisted of uncharged particles of approximately the mass of the proton.
He performed a series of experiments to verify this, these uncharged particles were eventually called "neutrons", and Chadwick is credited with this discovery.
[1] The alpha emitter and the beryllium are pulverized and mixed together in close intimate contact to ensure a high percentage of alpha-emitter and beryllium nuclei in close contact, since the alpha particle has a very short range through material, and would lose energy preventing reaction if sufficiently far away.