Explanation:
Chadwick concluded that neutrons are neutral particles because
1) Firstly when he kept a charge detector right after the Be sheet there was no deflection when the alpha particles struck the Be sheet. (When alpha particles struck the Be sheet A particles would knock out the particles.)
2) After that when he placed a paraffin after Be sheet and then placed a charge detector after the paraffin Chadwick noticed that the neutron could knock out a proton. He noticed this because of the deflection at charge detector at positive side.
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