Half-life

    OCR
    GCSE

    Radioactive decay is a random, stochastic process where half-life is defined as the time taken for the number of radioactive nuclei in an isotope to halve. Equivalently, it is the time required for the activity or count rate from a sample to decrease to half its initial value. This concept underpins the exponential decay model, allowing for the prediction of activity over time despite the unpredictability of individual decay events. Mastery requires the interpretation of decay curves, correction for background radiation, and calculations involving net decline ratios to determine sample age or safety.

    0
    Objectives
    3
    Exam Tips
    3
    Pitfalls
    5
    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining half-life as the time taken for the number of radioactive nuclei (or activity/count rate) in a sample to halve
    • Credit determination of half-life from a graph where candidates select a starting y-value, halve it, and read the time difference on the x-axis
    • Award 1 mark for showing the iterative halving sequence in calculations (e.g., 800 → 400 → 200) rather than just the final answer
    • In medical contexts, credit responses that link a short half-life to minimizing patient radiation dose/risk while allowing sufficient time for detection

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly read the graph, but you didn't show your working lines — examiners need to see how you derived the value"
    • "Remember that half-life applies to the *rate* of decay, not just the mass; try to use the term 'activity' or 'nuclei count' in your definition"
    • "You calculated the number of half-lives correctly, but the question asked for the *fraction remaining*, not the amount decayed"
    • "Excellent link between half-life and safety; to improve, explicitly mention the reduction of 'exposure time' or 'tissue damage'"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining half-life as the time taken for the number of radioactive nuclei (or activity/count rate) in a sample to halve
    • Credit determination of half-life from a graph where candidates select a starting y-value, halve it, and read the time difference on the x-axis
    • Award 1 mark for showing the iterative halving sequence in calculations (e.g., 800 → 400 → 200) rather than just the final answer
    • In medical contexts, credit responses that link a short half-life to minimizing patient radiation dose/risk while allowing sufficient time for detection

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When determining half-life from a graph, always draw clear construction lines (dashed lines) on the grid to show exactly where you read the values; this is often required for the method mark
    • 💡If a question provides a 'background count', you must subtract this from all reading values *before* calculating the half-life
    • 💡For 'how much remains' questions, write out the decay chain (e.g., 1/2, 1/4, 1/8) explicitly to avoid miscounting the number of half-lives passed

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Dividing the total time period by 2 to find the half-life, rather than finding the time taken for the activity to drop to half its initial value
    • Failing to subtract background radiation from the total count rate before plotting data or calculating half-life, leading to an incorrect curve tail
    • Assuming the radioactive source completely disappears or reaches zero activity after a few half-lives, rather than approaching zero asymptotically

    Study Guide Available

    Comprehensive revision notes & examples

    Read Study Guide

    Key Terminology

    Essential terms to know

    Stochastic nature of radioactive decay
    Definitions of half-life (nuclei count vs. activity)
    Exponential decay curves and graphical analysis
    Calculation of net decline and remaining mass
    Applications in radiometric dating and medical tracers

    Likely Command Words

    How questions on this topic are typically asked

    State
    Calculate
    Determine
    Explain
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"P7 (Suggested)","title":"Modelling radioactive decay","relevance":"Using dice or coins to simulate the random nature of decay and generate decay curves"}

    Ready to test yourself?

    Practice questions tailored to this topic