AQA · Physics · 8463
GCSE Physics: Half-lives and the random nature of radioactive decay
Atomic structure · AQA GCSE Physics (8463), specification point 4.4.2.3. Cards are generated from the exam board's own specification for "Half-lives and the random nature of radioactive decay", and every card links back to the page it came from. Specification © AQA — used with attribution.
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Radioactive decay is a ? process.
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random
Define the half-life of a radioactive isotope in terms of nuclei.
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The time it takes for the number of nuclei of the isotope in a sample to halve.
Define half-life in terms of the count rate (activity) of a sample.
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The time it takes for the count rate (or activity) from a sample to fall to half its initial level.
The half-life is the time for the count rate from a sample to fall to ? its initial level.
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half
Why can the decay of a single nucleus not be predicted?
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Because radioactive decay is random — each nucleus has a fixed chance of decaying per unit time, but there is no way to tell when it will actually happen.
Why is half-life constant if radioactive decay is random?
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A sample contains an enormous number of nuclei, so although each decay is unpredictable, the fraction decaying in a given time is statistically constant — hence the time to halve is constant.
How is half-life read from a graph of activity against time?
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Find the time taken for the activity to fall from its initial value to half that value (any starting point on the curve gives the same interval).
What fraction of the original nuclei remains after two half-lives?
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One quarter (1/4) — the number halves, then halves again.
What fraction of the initial activity remains after three half-lives?
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One eighth (1/8).
Express the net decline in activity after n half-lives as a ratio.
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The activity falls to 1/2ⁿ of its initial value, i.e. initial : final = 2ⁿ : 1.
A sample's count rate is 800 Bq and its half-life is 2 hours. What is the count rate after 6 hours?
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100 Bq — 6 hours is 3 half-lives, so 800 → 400 → 200 → 100.
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