AQA A-Level Physics Paper 1, June 2025: Question 10

AQA A-Level Physics • Particles and Radiation

Beta-Minus (β⁻) Decay and Fundamental Conservation Laws

What this question tests

This multiple-choice question assesses recall and fundamental understanding of nuclear decay processes, specifically β⁻ decay, the weak interaction at the quark level, and the universal conservation laws (energy, momentum, charge, and lepton number) that govern all particle interactions.

Question 10 (Multiple Choice)

Identifying the true statement regarding β⁻ decay

✅ Correct Answer

A: energy is conserved.

Awarded [1 mark] (Assessment Objective AO1)

In any closed physical interaction, including radioactive decays, total energy (mass-energy) is strictly conserved.

💡 Key Knowledge: Inside β⁻ Decay

Nucleon level: A neutron decays into a proton, an electron, and an electron antineutrino:
n → p + e⁻ + ν̅ₑ
Quark level: Since n = udd and p = uud , a down quark changes into an up quark:
d → u + e⁻ + ν̅ₑ
Exchange particle: Mediated by the W⁻ boson via the weak interaction.
Location: Occurs within an unstable, neutron-rich nucleus.

📐 Analysis of Each Option

A: "energy is conserved."
TRUE: Total mass-energy is always conserved. The release of an electron antineutrino ( ν̅ₑ ) accounts for the continuous kinetic energy spectrum of emitted β⁻ particles.
B: "an up quark changes into a down quark."
FALSE: That occurs in β⁺ decay ( u → d ). In β⁻ decay, a down quark changes into an up quark ( d → u ).
C: "the interaction occurs outside the nucleus of an atom."
FALSE: Radioactive β⁻ decay originates from within the atomic nucleus (caused by an unstable neutron-to-proton ratio).
D: "momentum is not conserved."
FALSE: Linear momentum is always conserved in all particle interactions.

❌ Common Errors & Misconceptions

Quark confusion: Mixing up β⁻ ( d → u ) with β⁺ ( u → d ). A helpful memory trick: "β Minus has More down quarks initially: udd → uud".
Neutrino history trap: Historically, β particles were observed with a continuous spread of kinetic energy, which led early physicists to wonder if energy was conserved. However, Wolfgang Pauli postulated the neutrino precisely to ensure that both energy and momentum are conserved.
Atomic location: Confusing the emission of an electron (which ends up flying out of the atom) with the origin of the decay event (which occurs deep inside the nucleus).

🧠 Exam Technique: The Universal Conservation Rule

Whenever an AQA question asks about conservation in particle physics:

Energy, Momentum, Charge, Baryon number, and Lepton number are conserved in every single interaction.
If an option suggests that energy or momentum is not conserved in a valid physical reaction, you can immediately rule it out.
Conversely, if an option states that energy, momentum, or charge is conserved, it is almost certainly the correct statement!