Discovery of Neutrons by James Chadwick

Question Variations

How did James Chadwick discover neutrons?

Who discovered neutrons and how?

What leads to the discovery of neutrons?

In 1932, an English physicist James Chadwick found that when alpha particles were bombarded with beryllium, unknown radiation was detected which turned out to be particles and not electromagnetic radiation. He experimented further to find that electric and magnetic fields didn't impact these particles which showed that these particles were neutral.  

Chadwick measured the mass of the neutron by bombarding beryllium with alpha particles. Then the proposed neutrons were directed towards the paraffin wax. Paraffin wax contains a good amount of hydrogen atoms which means they are rich in protons. So when the proposed neutrons would hit the paraffin wax, they should kick out these protons. So, by using a detector, the number of ejected protons could be calculated. 

Then, Chadwick applied the law of conservation of energy and law of conservation of momentum and calculated backward to find the mass of the neutron by using the mass of the proton. He found the mass to be very near to that proton. His experiment showed that neutrons also exist.

mn = mass of a neutron in a.m.u 1.008665 amu

Why was it hard to directly assess neutrons?

It is not easy to assess neutrons directly because they cannot be easily manipulated. They don't carry any charge, thus they remain unaffected by electrical or magnetic fields.