Radioactive decay
Cambridge A-Level Physics (9702) · Unit 23: Nuclear physics · 9 flashcards
Radioactive decay is topic 23.2 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 23 — Nuclear physics , alongside Mass defect and nuclear binding energy. In one line: 'Spontaneous' means decay is not influenced by external factors (temperature, pressure). 'Random' means that it is impossible to predict when a specific nucleus will decay; only probabilities can be stated.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 9 flashcards — 4 definitions, 1 key concept and 4 calculations — covering the precise wording mark schemes reward. Use the 4 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Distinguish between 'spontaneous' and 'random' in the context of radioactive decay
'Spontaneous' means decay is not influenced by external factors (temperature, pressure). 'Random' means that it is impossible to predict when a specific nucleus will decay; only probabilities can be stated.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand that the fluctuations seen in count rate are evidence that radioactive decay is random.
- Understand the random and spontaneous nature of radioactive decay.
- Define the decay constant and activity; recall A = λN and use it.
- Define half-life.
- Use λ = 0.693/t½.
- Understand that radioactive decay is exponential, and sketch and use x = x₀e^(−λt), with x standing for the received count rate, the activity or the number of undecayed nuclei.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Radioactive decay
- › Define the random nature of decay as: it is impossible to predict when a specific nucleus will decay, or which nucleus will decay next.
What experimental evidence supports the random nature of radioactive decay?
Fluctuations in count rate over short periods provide evidence. The count rate varies even when measuring the same radioactive source due to the unpredictable nature of individual decay events.
Distinguish between 'spontaneous' and 'random' in the context of radioactive decay.
'Spontaneous' means decay is not influenced by external factors (temperature, pressure). 'Random' means that it is impossible to predict when a specific nucleus will decay; only probabilities can be stated.
Define 'activity' (A) of a radioactive source.
Activity (A) is the rate at which nuclei decay in a radioactive source. It is measured in Becquerels (Bq), where 1 Bq = 1 decay per second.
Define the 'decay constant' (λ).
The decay constant (λ) is the probability of decay of a nucleus per unit time. It is related to the activity and number of undecayed nuclei by A = λN.
State the relationship between activity (A), decay constant (λ), and number of undecayed nuclei (N).
The relationship is given by the equation: A = λN, where A is activity, λ is the decay constant, and N is the number of undecayed nuclei.
Define 'half-life' (t₁/₂).
Half-life (t₁/₂) is the time taken for half of the radioactive nuclei in a sample to decay, or the time taken for the activity of a sample to decrease to half its initial value.
State the relationship between half-life (t₁/₂) and decay constant (λ).
The relationship is given by the equation: λ = 0.693 / t₁/₂, where λ is the decay constant and t₁/₂ is the half-life.
Describe the exponential nature of radioactive decay using an equation.
The number of undecayed nuclei (N) at time t is given by: N = N₀e^(-λt), where N₀ is the initial number of nuclei and λ is the decay constant. Similar equations apply for activity and count rate.
A radioactive sample has an initial activity of 400 Bq and a half-life of 10 days. What is its activity after 30 days?
After 30 days (3 half-lives), the activity is reduced by half three times: 400 Bq -> 200 Bq -> 100 Bq -> 50 Bq. Therefore, the activity after 30 days is 50 Bq.
Review the material
Read full revision notes on Radioactive decay — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 23 — Nuclear physics
Radioactive decay sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Radioactive decay deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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