Thanks for your question regarding neutrons.
As you know, an atom contains protons, neutrons, and electrons. Protons and neutrons have an atomic mass of 1 and electrons have very little mass (about 1/1800 amu)
Ernest Rutherford first floated the idea of a neutral particle in the nucleus of the atom because atomic number and atomic mass did not match.
James Chadwick in 1932 at the University of Chicago proved the existence of neutrons through his experiments.
One proof of neutrons is using particle accelerators (used to be called atom smashers in my younger days). Particles are accelerated using magnetic fields to faster and faster speeds. These fast-moving particles are then "smashed" into a target with some atoms. The atoms then come apart.
Plates track the movement of the subatomic particles. One side of the plate is positive and the other side is negative. Negatively charged subatomic particles will be attracted to the positive side and positively charged subatomic particles will be attracted to the negative side (as opposites attract).
When the atom was smashed into its smaller pieces, the travel at rates inversely proportional to their masses. Lighter particles (like electrons) travel very fast; heavier particles (like protons and neutrons) travel slower. These plates showed that there were heavy particles in the atom that were not attracted by the positive or the negative plates.
Scientists deduced that the neutron is equal to the mass of the proton (almost) and the neutron has no charge.
Hope this helps!