Half-Life and Radioactive Contamination

This lesson covers the concept of half-life, how to interpret decay curves, the randomness of radioactive decay, and the crucial distinction between contamination and irradiation. This is part of the AQA GCSE Combined Science Trilogy specification (8464, sections 6.4.2 and 6.4.3).

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What Is Half-Life?

The half-life of a radioactive isotope is the time it takes for:

• The number of unstable nuclei in a sample to halve, or
• The activity (count rate) of a source to fall to half its original value.

Key Points

• Half-life is constant for any given isotope — it does not change over time.
• After one half-life, half the original unstable nuclei remain. After two half-lives, one quarter remain. After three half-lives, one eighth remain, and so on.
• Radioactive decay is random — you cannot predict which individual nucleus will decay next, but you can predict how many will decay in a given time (statistically).

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Activity and Count Rate