Chadwick used beryllium because earlier workers had used it in their experiments.

In 1930, Walther Bothe and Herbert Becker shot α rays at beryllium. It emitted a neutral radiation that could penetrate 200 mm of lead. They assumed the radiation was high-energy γ rays.

Irène Curie and her husband then found that a beam of this radiation knocked protons loose from paraffin.

Chadwick felt that the radiation could not be γ rays. The α particles could not provide enough energy to do this. He thought the beryllium rays were neutrons.

He bombarded a piece of beryllium in a vacuum chamber with α rays. The beryllium emitted the mysterious neutral rays.

In the path of the rays, Chadwick put a paraffin target. The rays knocked protons out of the target.

Chadwick deigned a detector to count the protons and estimate their speeds. Gamma rays could not explain the speed of the atoms. The only good explanation for his result was a neutral particle.

Chadwick could not measure the mass of the particle directly. Instead he measured everything else in the collision. He then used that information to calculate that the mass was 1.0067 times the mass of the proton.

In 1935, Chadwick received the Nobel Prize for his accomplishment.

ADDITION:

Here is another video that is spoken in better English and is really comprehensive in regards to the experiment.

Chadwick’s Alpha Particle and Light Element Experiment

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