The diagram below represents the apparatus used by Chadwick (in 1932) which led to his discovery of the neutron.

......

Alpha particles from the polonium hit the beryllium and caused it to emit a very penetrating radiation. The "penetrating power" was measured by finding the thickness of metal needed to absorb the radiation.

When the radiation hit paraffin wax (or any other substance containing hydrogen), protons were emitted. The maximum energy of the protons was estimated using the absorber sheets and found to correspond to a speed, u = 3·3×10⁷ms^-1.

Chadwick proposed the following reaction:

where    represents the "new" particle, the neutron.

It was then found that when the radiation collided with nitrogen atoms they moved with a maximum speed of V = 4·7×10⁶ms^-1.

When a particle of mass m moving at speed u has a "head-on" elastic collision with a stationary particle of mass M, it can be shown that the particle of mass M has a speed V, after the collision, given by

Verify this for yourself or click here

Chadwick assumed that the protons emitted by the paraffin wax were the nuclei of hydrogen atoms which had been hit by neutrons. A nitrogen atom has a mass of about 14 times the mass of a proton.

From the collision between neutrons and protons

From the collision between neutrons and nitrogen nuclei

This gives the mass of a neutron, m » 1·15 times the mass of a proton.

For this explanation of the experimental results, Chadwick was considered the discoverer of the neutron.

• Atomic and Nuclear Physics Homepage
• Return to top of page