AQA A-Level Physics Paper 2, June 2023: Question 28
1 mark · Easy difficulty · Multiple Choice
Identify what cannot be predicted due to the random nature of radioactive decay.
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Question text
28 The random nature of radioactive decay means that it is never possible to predict
[1 mark]
A when a particular nucleus will decay.
B whether a β− particle or a β+ particle is emitted.
C the approximate time taken for the activity to decrease to a specified value.
the approximate thickness of an absorber needed to reduce the count rate to
D
a specified value.
Mark scheme
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28 A when a particular nucleus will decay.
How to answer it
Radioactive Decay: The Nature of Randomness
What this question tests
This question evaluates your foundational understanding of the core characteristics of radioactive decay:
- Random Nature: Recognising that at a microscopic, individual level, decay events cannot be predicted in advance.
- Microscopic vs Macroscopic: Distinguishing between the unpredictable nature of an individual nucleus and the statistically predictable behaviour of a bulk sample containing a huge ensemble of nuclei.
- Decay Modes: Understanding that the mode of decay (e.g. β⁻ vs β⁺) is strictly governed by nuclear stability (neutron-proton ratio) and not by chance.
Question 28 Walkthrough
Total: 1 MarkThe random nature of radioactive decay means that it is never possible to predict...
✅ Correct Answer: A
A: when a particular nucleus will decay.
Each unstable nucleus has a constant probability of decay per unit time (the decay constant, λ). However, whether that specific nucleus decays in the next second, next hour, or in a million years is entirely unpredictable.
💡 Key Knowledge
- Random: We cannot predict which nucleus will decay next, nor when any given nucleus will decay. All nuclei of the same isotope share an identical decay probability per unit time (λ).
- Spontaneous: Decay is unaffected by external physical or chemical factors (e.g. temperature, pressure, chemical bonding, electric or magnetic fields).
- Decay Constant (λ): Defined as the probability of decay of an individual nucleus per unit time: ΔN / Δt = -λN .
❌ Analysis of Incorrect Options
- B is incorrect: The mode of decay (emitting β⁻ vs β⁺) is determined by whether the nucleus is neutron-rich (β⁻) or proton-rich (β⁺) relative to the stability line. This is completely predictable from the nuclide's composition.
- C is incorrect: Macroscopic activity follows the exponential relationship A = A₀ e⁻ᵝᵗ . Because macroscopic samples contain vast numbers of nuclei (~10²⁰), statistical fluctuations smooth out, allowing the decay time to be predicted with high precision.
- D is incorrect: The attenuation of radiation through a medium follows well-defined absorption laws (e.g. exponential attenuation for γ-rays, definite range for α-particles). This is fully calculable.
🧠 Exam Technique & Examiner Tips
- Look for the word "particular": When AQA asks about the random nature of decay, the answer almost invariably links to an individual or particular nucleus.
- Beware macroscopic vs microscopic traps: Students often conflate the impossibility of predicting a single event with an inability to predict bulk behaviour. Remember: statistical laws become extremely reliable when N is large!
- Eliminate deterministic properties: Nuclear decay modes (α, β⁻, β⁺, γ) and radiation absorption are deterministic properties governed by fundamental conservation laws and cross-sections.
Topics
Physics · 3.8 Nuclear physics (A-level only)
Question and mark scheme from the AQA A-Level Physics examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.