Scientists discovered the neutron after the electron and the proton. Instruments of the 1920s could not detect particles with no charge.
Ernest Rutherford in 1921 postulated the existence of the neutron. One problem — no evidence.
Detection equipment of the 1920s relied on the electrical charges of particles to detect them. Neutrons, having no electrical charge, would leave no trace.
In 1930, the German physicists Walther Bothe and Herbert Becker noticed that when they shot alpha rays at beryllium, the beryllium emitted a high-energy neutral radiation. They assumed the neutral radiation was high-energy gamma rays.
Chadwick thought the beryllium rays were neutrons and set out to measure their mass. He could not do this directly.
Instead, he measured the masses and energies of everything else and calculated the mass of the neutron by difference.
In 1932 Chadwick reported that the neutral particle had 1.0067 times the mass of the proton. The neutral radiation was the long-sought neutron.