In $1932$, James Chadwick verified the hypothesis that the atomic nucleus which contains theproton other than neutral mass is in the integral multiple of the fundamental unit.

Chadwick observed emission of neutral radiation when beryllium nuclei were bombarded with$\alpha$-particles. ( $\alpha$-particles are bombarded).

In addition he observed that this neutral radiation could knock out protons from light nuclei suchas those of helium, carbon and nitrogen.

The only neutral radiation known at that time was photons (electromagnetic radiation).

Application of the principles of conservation of energy and momentum showed that if the neutralradiation consisted of photons, the energy of photons would have to be much higher than isAvailable from the bombardment of beryllium nuclei with $\alpha$-particles.

Chadwick satisfactorily solved, was to assume that the neutral radiation consists of a new type ofneutral particle called neutrons.

From conservation of energy and momentum he was able to determine the mass of new particle(neutrons) 'as very nearly the same as mass of proton'.Currently, the mass of neutron precisely,

$m_{n}=1.00866 \mathrm{u}=1.6749 \times 10^{-27} \mathrm{~kg}$

Chadwick was awarded the $1935$ Nobel prize in Physics for his discovery of the neutron.