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how did James Chadwick discovered neutron?

Discovery of neutrons: In 1932, James Chadwick bombarded beryllium (Be) with alpha aprticles. He allowed the radiation emitted by beryllium to incident on a paraffin wax. It was found that protons were shot out form the paraffin wax. People began to look for what was in the "beryllium radiations". Some people suggested that the radiations may be gamma radiation. However, Chadwick found that the radiation could not be gamma radiation since energy and momentum were not conserved in its production. He showed that all the observations could be explained if the radiation consisted of neutral particles of mass approximately equal to that of proton. This neutral particle was named neutron.

Diagramatic representation of experiment: 

 

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theneutronwas not discovered until 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle. Since the time ofRutherfordit had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus. As of about 1930 it was presumed that the fundamental particles were protons and electrons, but that required that somehow a number of electrons were bound in the nucleus to partially cancel the charge of A protons. But by this time it was known from theuncertainty principleand from "particle-in-a-box" type confinement calculations that there just wasn't enough energy available to contain electrons in the nucleus.
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