What holds a nucleus together, and why do some nuclei decay while others are stable?
The strong nuclear force and its range, the balance of forces in the nucleus, alpha, beta-minus and gamma radiation, and how the equation for beta-minus decay reveals the existence of the neutrino.
A focused answer to AQA A-Level Physics 3.2.1.2, covering the strong nuclear force and its range, why nuclei are stable or unstable, alpha, beta-minus and gamma radiation, and how the beta-minus decay equation gave evidence for the neutrino.
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What this dot point is asking
AQA specification point 3.2.1.2 wants you to describe the strong nuclear force and its range, explain why a nucleus can be stable or unstable, recall the nature of alpha, beta-minus and gamma radiation, and write the beta-minus decay equation, showing how it predicted the (anti)neutrino.
The strong nuclear force
The strong force acts equally between any pair of nucleons regardless of charge (it is charge-independent), which is essential to bind protons together despite their mutual repulsion.
Why nuclei are stable or unstable
A nucleus is stable when the strong force and the electrostatic repulsion are balanced. Large nuclei have many protons, so the long-range electrostatic repulsion grows while the short-range strong force can only bind nearest neighbours, so the strong force cannot bind every nucleon, making heavy nuclei unstable. The neutron-to-proton ratio also matters: an excess of either neutrons or protons leads to beta decay that adjusts the balance towards stability.
Types of radiation
Beta-minus decay and the neutrino
In beta-minus decay a neutron changes into a proton, emitting an electron and an electron antineutrino:
Measured beta particles have a continuous range of energies up to a maximum, not a single value. If only an electron were emitted it would always carry a fixed energy. Pauli proposed a third particle, the antineutrino, to carry the missing energy and momentum, conserving both. This was the evidence that predicted the neutrino long before it was directly detected.
Try this
Q1. State the range over which the strong nuclear force is attractive and where it becomes repulsive. [2 marks]
- Cue. Attractive from about to about , repulsive below about .
Q2. Explain how the continuous energy spectrum of beta particles provided evidence for the neutrino. [3 marks]
- Cue. Beta particles share energy with another particle (the antineutrino), so they have a range of energies up to a maximum, conserving energy and momentum.
Q3. State the composition of an alpha particle. [1 mark]
- Cue. Two protons and two neutrons (a helium nucleus).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20184 marksExplain how the observation that beta particles are emitted with a continuous range of energies provided evidence for the existence of the neutrino.Show worked answer →
If beta-minus decay produced only an electron alongside the daughter nucleus, conservation of energy and momentum would require the electron to be emitted with a single, fixed energy each time.
Instead, beta particles are observed with a continuous range of energies up to a maximum. This implies the decay energy is shared between the electron and another, undetected particle, which can carry varying amounts of the energy and momentum.
Pauli proposed this particle, the (anti)neutrino, to conserve energy and momentum. It must be neutral and almost massless to have escaped detection.
Markers reward the expected fixed energy if only an electron were emitted, the observed continuous spectrum, and a shared-energy third particle (the antineutrino) restoring conservation.
AQA 20214 marksUranium-238 () decays by alpha emission. Write the balanced decay equation, and explain why the strong nuclear force cannot hold the largest nuclei together.Show worked answer →
An alpha particle removes 2 protons and 2 neutrons, so the nucleon number falls by 4 and the proton number by 2: .
The strong nuclear force is very short range (effective only up to about ), so each nucleon only attracts its nearest neighbours. The electrostatic repulsion between protons is long range, so in a very large nucleus the total repulsion grows faster than the strong-force binding, making the nucleus unstable.
Markers reward a correctly balanced alpha equation and explaining instability through the short-range strong force versus the long-range Coulomb repulsion.
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Sources & how we know this
- AQA A-level Physics (7408) specification — AQA (2017)