Spark image

The discovery of the neutron

In 1932 Bother and Becker bombarded a piece of beryllium with alpha particles using the apparatus shown diagrammatically in Figure 1. They observed a penetrating radiation emerging from the beryllium that they thought to be gamma rays. When a piece of paraffin wax was placed in the path of the beam, however, the reading on the detector actually increased!

It was realised that the increase in reading was due to the emission of protons from the wax and they then tried to work out how this could be happening. If the proton emission had been due to bombardment of the wax by gamma radiation the gamma rays would have to have energy of around 16 MeV very large for such a reaction.

The British physicist James Chadwick realized that the radiation produced by the bombardment of the wax by alpha particles was not gamma radiation at all but particles of about the same mass as the proton. The high penetration was explained by assuming that these particles were uncharged and could therefore pass though material without any electrostatic scattering.

The particle was named the neutron, a name suggested years before by Rutherford as a combination of a proton and an electron. The reaction between alpha particles and beryllium that produces neutrons is:

The neutrons collide elastically with the atoms of the wax and lose much more energy to a light atom than to a heavy one.

(See: Elastic collisions and snooker you will find an explanation here in terms of the momentum of the interacting particles)

It is for this reason that irradiation by neutrons is particularly dangerous to human tissue which contains large numbers of hydrogenous atoms with a mass similar to that of the neutron.

The neutron is now known to have a mass slightly greater than that of a proton, the neutron's mass being 1.008 665 u compared with the protons 1.007 276 u. Free neutrons are unstable decaying into a proton and electron, the process having a half life of about 900 s (15 minutes).

The most common large-scale source of neutrons is the nuclear fission reactor. Materials are often placed in reactors to see how they stand up to neutron bombardment and also to create new isotopes by neutron absorption.

© Keith Gibbs 2011