AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
The existence of the neutrino was hypothesised to account for conservation…
Full specification wording
Stable and unstable nuclei: The existence of the neutrino was hypothesised to account for conservation of energy in beta decay.
In beta-minus decay, a neutron becomes a proton and an electron is emitted.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In beta-minus decay, a neutron becomes a proton and an electron is emitted. If only the electron were emitted, its kinetic energy would have a single fixed value because the energy released is fixed. In practice, beta particles have a continuous range of kinetic energies up to a maximum. This apparent loss of energy suggested a third, unseen particle was emitted. Pauli hypothesised the neutrino to carry away the missing energy and momentum. Later the antineutrino was detected, confirming the idea. The same reasoning applies to beta-plus decay, where a neutrino is emitted. This shows how conservation laws drive the discovery of new particles.
What earns the marks
- Beta decay emits electrons with a continuous range of kinetic energies, not a single fixed value.
- Conservation of energy requires the missing energy to be carried by another particle.
- The neutrino was hypothesised as a neutral, nearly massless particle to account for this missing energy.
- Conservation of momentum also requires a third particle in beta decay.
- The antineutrino is emitted in beta-minus decay and the neutrino in beta-plus decay.
- Experimental detection of the neutrino later supported the hypothesis.
Common mistakes and how to put them right
Thinking the neutrino is charged.
Put it right: It is neutral, which is why it is hard to detect.
Believing energy is not conserved in beta decay.
Put it right: The neutrino carries away the missing energy.
Confusing neutrino and antineutrino.
Put it right: Beta-minus decay emits an antineutrino while beta-plus decay emits a neutrino.
Exam tips
- Link the continuous beta energy spectrum directly to the need for the neutrino.
- Mention both energy and momentum conservation when explaining the hypothesis.
- Use the correct particle names and symbols for neutrino and antineutrino in your answer.
Try the lesson
The start of the Stable and unstable nuclei: The existence of the neutrino was hypothesised to account for conservation of energy in beta decay. lesson, made from checked teaching notes for this specification point.
Have a go first
Why was the existence of the neutrino first hypothesised?
Listen
In beta-minus decay, a neutron becomes a proton and an electron is emitted. If only the electron were emitted, its kinetic energy would have a single fixed value because the energy released is fixed. In practice, beta particles have a continuous range of kinetic energies up to a maximum. This apparent loss of energy suggested a third, unseen particle was emitted. Pauli hypothesised the neutrino to carry away the missing energy and momentum. Later the antineutrino was detected, confirming the idea. The same reasoning applies to beta-plus decay, where a neutrino is emitted. This shows how conservation laws drive the discovery of new particles. A strong answer earns marks for these points. Beta decay emits electrons with a continuous range of kinetic energies, not a single fixed value. Conservation of energy requires the missing energy to be carried by another particle. The neutrino was hypothesised as a neutral, nearly massless particle to account for this missing energy. Conservation of momentum also requires a third particle in beta decay. A common slip: Thinking the neutrino is charged. Instead, it is neutral, which is why it is hard to detect. Exam tip: Link the continuous beta energy spectrum directly to the need for the neutrino.
Key words
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Conservation of energy
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Conservation of momentum
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Hypothesis testing in physics
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Particle physics and the neutrino
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See every definition
- Conservation of energy
- Requires the missing energy in beta decay to be carried by another particle.
- Conservation of momentum
- Also requires a third particle in beta decay.
- Hypothesis testing in physics
- Conservation laws drive the discovery of new particles, and experimental detection of the neutrino later supported the hypothesis.
- Particle physics and the neutrino
- A neutral, nearly massless particle hypothesised to account for the missing energy, with the antineutrino emitted in beta-minus decay and the neutrino in beta-plus decay.
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