SACE Physics — Stage 2
Nuclear Decay — Flashcards & Quiz
Nuclear decay is the spontaneous transformation of unstable nuclei into more stable configurations, releasing radiation. SACE Physics Stage 2 expects you to write balanced decay equations for alpha, beta-minus, beta-plus, and gamma emission, apply half-life to exponential decay calculations, and compare the penetrating power and ionising ability of each radiation type.
Key Points
- Alpha (α) decay: emits a helium-4 nucleus. Mass number decreases by 4, atomic number by 2.
- Beta-minus (β⁻) decay: a neutron converts to a proton, emitting an electron and antineutrino. Mass number unchanged, atomic number +1.
- Beta-plus (β⁺) decay: a proton converts to a neutron, emitting a positron and neutrino. Mass number unchanged, atomic number –1.
- Gamma (γ) emission: high-energy photon released from an excited nucleus. Mass and atomic numbers both unchanged.
- Half-life (t½): time for half the radioactive nuclei to decay. N = N₀ · (½)^(t/t½) or N = N₀ e^(–λt) where λ = ln2/t½.
- Penetration: alpha stopped by paper; beta by thin aluminium; gamma needs lead or thick concrete. Ionising ability: alpha > beta > gamma.
Common Mistakes to Avoid
- Writing alpha decay with the wrong mass number change — it decreases by 4 and atomic number by 2.
- Confusing beta-minus (electron + antineutrino) with beta-plus (positron + neutrino).
- Claiming gamma emission changes the element — it doesn't; only energy is released.
- Forgetting that half-life is constant for each isotope — it does not depend on environment or amount.
- Using the wrong form of the decay equation — exponential (N = N₀e⁻⁻ᵗ) vs half-life (N = N₀ · (½)^(t/t½)).
Exam Strategy
SACE Stage 2 nuclear decay questions ask you to (1) write decay equations, (2) calculate remaining nuclei or activity, or (3) compare radiation types. Method: balance mass numbers and atomic numbers on both sides of the equation, then apply half-life formulas for quantitative questions. For penetration/ionisation questions, remember the trade-off: alpha ionises heavily but travels short distances.
Sample Flashcards
Q1: How is beta-minus decay explained in terms of quarks?
In beta-minus decay a neutron in the nucleus changes into a proton, emitting an electron and an electron antineutrino: n → p + e⁻ + ν̄ₑ. At the quark level, one down quark changes into an up quark (udd → uud) by emitting a W⁻ boson, which then decays into the electron and the antineutrino. The weak nuclear force is responsible, mediated by the W⁻ boson.
Q2: How is beta-plus decay explained in terms of quarks, and how was conservation of momentum linked to neutrinos?
In beta-plus decay a proton in the nucleus changes into a neutron, emitting a positron and an electron neutrino: p → n + e⁺ + νₑ. At the quark level, an up quark changes into a down quark (uud → udd) by emitting a W⁺ boson, which decays into the positron and the neutrino. In beta decay the emitted electrons or positrons have a range of energies, and energy and momentum appeared not to be conserved — this led to the prediction of the neutrino, a neutral, nearly massless lepton that carries away the missing energy and momentum.
Sample Quiz Questions
Q1: In beta-minus decay, a down quark changes into an up quark.
Answer: TRUE
A neutron (udd) becomes a proton (uud) when a down quark changes into an up quark, emitting a W⁻ boson that decays into an electron and an antineutrino.
Revision Tip
Decay equations are pattern recognition — drill a Revizi deck with 10+ parent isotopes asking you to write alpha and beta decay products.
Related Concepts
Last updated: 9 October 2026 · 2 sample flashcards · 1 sample quiz questions