Radioactive decay (A-level only) — AQA A-Level Physics

Radioactive decay is a fundamental topic in AQA A-Level Physics that explores the spontaneous and random disintegration of unstable atomic nuclei. You will study the three main types of radiation (alpha, beta and gamma), their properties, and how they interact with matter and magnetic fields. The topic also introduces the exponential decay law, activity, decay constant and half-life, which are essential for understanding nuclear stability and dating techniques.

This topic matters because it underpins many real-world applications, from medical imaging and cancer treatment to carbon dating and nuclear power. It also develops your ability to handle exponential functions and logarithms in a physical context, skills that are examined in both written papers and practical assessments. A strong grasp of radioactive decay is crucial for later topics such as nuclear energy and particle physics.

• →Radioactive decay is spontaneous and random: it is unaffected by external conditions and you cannot predict which nucleus decays or when.
• →Alpha decay emits a helium nucleus (2 protons, 2 neutrons), beta-minus decay emits an electron and an antineutrino, and gamma decay emits a high-energy photon. Each changes the nucleus in different ways.
• →Activity (A) is the rate of decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second. It decreases exponentially over time.
• →The decay constant (lambda) is the probability of decay per nucleus per second, with units s^-1. It is related to half-life by t_half = ln(2) / lambda.
• →The exponential decay equation A = A0 e^(-lambda t) (or N = N0 e^(-lambda t)) describes how activity or number of undecayed nuclei changes with time.

• State that the hazard of radioactive waste depends on half-life, activity and type of radiation emitted.
• Explain that long half-life waste remains radioactive for a very long time, so isolation from the environment is needed.
• Describe storage methods such as vitrification, sealing in canisters and deep geological disposal.
• Explain that short half-life waste can be stored until its activity has fallen to a safe level.
• Describe radioactive dating as using the ratio of parent to daughter nuclei and the known half-life.
• Apply the decay equation or half-life relationship to calculate an age or a remaining fraction.
• Compare carbon-14 dating with uranium-lead dating in terms of useful range and half-life.

• 💡Link each application to the half-life value and the type of radiation emitted, not just to the word radioactive.
• 💡Show the decay calculation clearly, including the fraction remaining and the number of half-lives.
• 💡Use precise terms such as parent nuclei, daughter nuclei, activity, half-life and isolation when describing waste storage.
• 💡Always define half-life in terms of 'number of undecayed nuclei' or 'activity', not 'mass' or 'count rate', unless the question specifically asks for count rate.
• 💡When using the exponential decay equation, ensure all units are consistent (time in seconds, activity in Bq) and show your rearrangement clearly to gain method marks.
• 💡For 'explain' questions on randomness, use the precise wording: 'spontaneous' means not affected by external factors, and 'random' means you cannot predict which nucleus decays or when.

• Thinking that a long half-life means the waste is safe because activity is low; correction: a long half-life means it stays radioactive for a very long time, so isolation is needed for thousands of years.
• Confusing half-life with the time for all activity to stop; correction: activity halves in each half-life and never reaches exactly zero in a finite time.
• Using carbon-14 dating for samples older than about 50000 years or for non-living material; correction: carbon-14 has a relatively short half-life and only dates once-living material over a limited range.
• Students often think that after one half-life, all the radioactive material has gone. In fact, half of the undecayed nuclei remain, and the activity halves, but the sample never reaches zero activity in theory.
• Many believe that heating or applying pressure can speed up radioactive decay. Radioactive decay is spontaneous and unaffected by external factors such as temperature, pressure or chemical state.
• Some confuse the penetrating power of radiation: alpha is the least penetrating (stopped by paper), beta is stopped by a few mm of aluminium, and gamma is the most penetrating (requires thick lead). Remember that alpha is the most ionising, not the least.

1. 1Day 1-2: Review the three types of radiation (alpha, beta, gamma) and their properties, including penetration, ionisation and behaviour in electric/magnetic fields. Make a comparison table.
2. 2Day 3-4: Learn the definitions of activity, decay constant and half-life. Practice converting between units (Bq, kBq, MBq) and using t_half = ln(2) / lambda.
3. 3Day 5-6: Master the exponential decay equation A = A0 e^(-lambda t) and N = N0 e^(-lambda t). Solve problems involving finding activity, time, or decay constant, ensuring unit consistency.
4. 4Day 7-8: Work through exam-style questions, especially those requiring explanations of randomness and spontaneity. Mark your answers against the mark scheme and note key phrases.
5. 5Day 9-10: Create flashcards for key definitions and formulas. Test yourself on active recall prompts and past paper questions under timed conditions.

• 📋Definition and explanation questions: e.g. 'Define half-life' or 'Explain why radioactive decay is random and spontaneous.' Use precise terminology and avoid vague language.
• 📋Calculation questions: e.g. 'Calculate the activity after a given time' or 'Determine the half-life from a graph of activity against time.' Show all steps and units.
• 📋Graph analysis: e.g. 'Sketch a graph of activity against time and explain its shape.' Remember it is an exponential decay curve, never touching the time axis.
• 📋Practical application: e.g. 'Describe an experiment to determine the half-life of a radioactive source.' Include details of background radiation correction and repeated readings.