Beta-particles

From measurements of their charge-to-mass ratio Bec¬querel showed that beta particles were in fact elec¬trons, but relativistic effects have to be taken into account in the measurement, as some beta particles are emitted with 90 % of the velocity of light. It can be shown from the special theory of relativity that at this velocity their mass is about 2.4 times their rest mass. Beta particles are emitted from the nuclei of atoms where no electrons exist! How this is possible is now explained.

If we record an energy spectrum of the beta particles emitted from a radioactive source, it is found that they have a range of energies between about zero and a maximum (Figure 1). The reason is that the beta particles come from the decay of a neutron into a proton and an electron and also a very light particle called an anti-neutrino:

The variation in beta-particle energy arises from the way in which the energy is shared out between these three products: sometimes more energy goes to the electron and sometimes more to the neutrino. The mass of the neutron is slightly greater than that of the proton and this additional mass makes the decay possible.

As well as energy the existence of neutrinos is required to conserve a property called LEPTON NUMBER. The lepton number of electrons and neutrinos is +1 while that of positrons and antineutrinos is -1.

Example of beta decay:

This example shows the decay of strontium 90 nucleus, forming a nucleus of yttrium, an electron and an antineutrino.



In the above decay the energy of the emitted electron is about 0.5 MeV.

The decay of a proton

This decay will only occur by the addition of energy since the mass of the neutron is greater than that of the proton.