AQA AS Level Physics Paper 2, June 2023: Question 16
1 mark · Easy difficulty · Multiple Choice
Identify the decay products of a free neutron in beta-minus decay.
Practise this questionQuestion
Question text
16 A free neutron decays to produce a proton and
[1 mark]
A an electron and an antineutrino.
B an electron and a neutrino.
C a positron and an antineutrino.
D a positron and a neutrino.
Mark scheme
Show the mark scheme
16 A an electron and an antineutrino.
How to answer it
Free Neutron Decay
What this question tests
This question assesses your core knowledge of particle physics, specifically weak nuclear interactions, beta-minus decay equations, and the conservation laws governing particle reactions (charge, baryon number, and lepton number).
Part 16: Decay Products of a Free Neutron
✅ Correct Answer
A: an electron and an antineutrino.
💡 Key Knowledge
- Beta-minus decay equation: n → p + e⁻ + ν_bar (antineutrino).
- Quark transformation: A down quark changes into an up quark (udd → uud).
- Lepton number conservation: Lepton numbers must balance on both sides (neutron = 0; proton = 0, electron = +1, electron antineutrino = -1). Total = 0.
🧠 Exam Technique
Eliminate options quickly by checking charge first, then lepton numbers. A neutron has a charge of 0. A proton has a charge of +1, so the other decay product(s) must combine to give a net charge of -1. This immediately points to an electron (e⁻). To conserve lepton number, an electron antineutrino is required.
❌ Common Errors
Students sometimes confuse neutrinos with antineutrinos, or mistakenly choose a positron (which occurs in beta-plus decay, transforming a proton into a neutron, not a free neutron decay).
Topics
Physics · 3.2 Particles and radiation
Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.