Half-Life

This lesson explains the concept of half-life, how to calculate it from graphs and data, and why radioactive decay is a random process. Half-life is a key concept in the Radioactivity section of the Edexcel GCSE Combined Science specification (1SC0).

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What Is Half-Life?

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

• half of the radioactive nuclei in a sample to decay, or
• the count rate (activity) from a sample to fall to half its original value.

Both definitions are equivalent and either can be used in an exam answer.

Term	Definition
Activity	The rate at which a radioactive source decays, measured in becquerels (Bq). 1 Bq = 1 decay per second.
Count rate	The number of decays detected per second (or per minute) by a detector such as a Geiger-Müller tube.
Half-life ($t_{1/2}$t1/2​)	The time for the activity (or number of undecayed nuclei) to halve.

Exam Tip: Make sure you can state both definitions of half-life — some mark schemes require the definition in terms of undecayed nuclei, others in terms of count rate.

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