Slide 1
Half-life describes an expected change in a large population
A nuclide is an atom type defined by its proton and neutron counts.
Its radioactive half-life is the time in which half a large population of that unstable type is expected to decay.
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It does not assign a scheduled lifetime to each nucleus. Individual decay events are unpredictable, while the aggregate behaviour becomes increasingly reliable when many nuclei are present.
Slide 2
Each half-life halves the population still undecayed
After two half-lives, one quarter of the original parent population remains on average. After three, one eighth remains.
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The same fraction disappears per interval, not the same number of nuclei. Starting with 800 expected parent nuclei gives 400, 200 and 100 after successive half-lives; a real small sample fluctuates around expectations.
Slide 3
One becquerel is one decay per second
Activity is the rate of radioactive decay, measured in becquerels. One becquerel corresponds to one decay per second.
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For a single nuclide, activity depends on both its decay constant and the number of undecayed nuclei. Equal masses of different nuclides can therefore have very different activities.
Slide 4
Radioactive daughters create changing decay chains
A decay product can itself be radioactive. A chain then contains several populations changing at different rates.
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Measuring total activity in such a mixture need not produce the simple curve of one isolated parent. Interpreting it requires the daughter half-lives and the sample’s earlier history.
Slide 5
Timing alone does not determine the hazard
Half-life describes timing of decay. Radiation type, energy, amount and exposure pathway also matter when assessing a radioactive material.
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A longer half-life does not automatically mean greater activity for the same number of nuclei. Nor does lower activity alone determine lower harm in every context, because biological interaction and location differ.
Sources
- Half-life
US Nuclear Regulatory Commission
- Reactor Concepts Training Course
US Nuclear Regulatory Commission