Beta Energy & Neutrinos
Beta Energy & Neutrinos
- Alpha-particles have discrete energies. Beta-particles leave with a continuous range of energies because the decay energy is shared with a neutrino or antineutrino.
Why there is a neutrino
- A particular nuclear transition has a fixed total energy available to its products. If the beta-particle were the only moving product, it would always receive one fixed energy.
- But beta-particles come out with a whole range of energies, from zero up to one maximum. Some energy is missing.
- The missing energy is taken away by another particle with no charge: the antineutrino (in beta-minus) or the neutrino (in beta-plus). This is why that extra particle must be in the equation.
- This also lets you read a graph. A continuous range of particle energies means beta decay. A set of discrete energies is produced in alpha decay. (9702/23/M/J/24 Q6) (9702/22/M/J/25 Q7)
- The quantities conserved in any decay are the nucleon number, the proton number (charge), momentum and total energy. (9702/22/F/M/25 Q7(c))
Your turn9702/22/M/J/25 Q7
A graph shows that beta-particles from one source have a continuous range of kinetic energies. Explain why. [2] (9702/22/M/J/25 Q7)
Show worked answer
The fixed decay energy is shared between the beta-particle and an antineutrino or neutrino. Different events divide the energy differently, so the beta-particle has a continuous range of energies.
Common mistake
Get the partner right: beta-minus gives an antineutrino, beta-plus gives a neutrino. And when a question shows a range of energies and asks which decay it is, a range means beta, not alpha. Alpha-particle energies are discrete rather than continuous.