Activity & Decay (Edexcel GCSE Physics)

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Activity & Decay

  • Objects containing radioactive nuclei are called sources of radiation
  • Sources of radiation decay at different rates which are defined by their activity
  • The activity is defined as

The rate at which the unstable nuclei from a source of radiation decays

  • Activity is measured in Becquerels
    • The symbol for Becquerels is Bq

  • 1 Becquerel is equal to 1 nucleus in the source decaying in 1 second

  • As an isotope decays, the number of nuclei of that isotope that remain will decrease
  • As a consequence of this, the activity of that isotope will also decrease over time
  • This can be shown on a graph of activity against time for a decaying source:

Half-life-background, IGCSE & GCSE Physics revision notes

Activity decreasing with time for a radioactive isotope

Worked example

A source of radiation has an activity of 2000 Bq. How many unstable atoms decay in 2 minutes?

Step 1: Determine the activity

    • The activity of the source is 2000 Bq
    • This means 2000 nuclei decay every second

Step 2: Determine the time period in seconds

    • The time period is 2 minutes
    • Each minute has 60 seconds
    • The time period in seconds is:

2 × 60 = 120 seconds

Step 3: Multiply the activity by the time period

Activity (Bq) × Time period (s) = 2000 × 120 = 240 000

    • Therefore, 240 000 unstable nuclei decay in 2 minutes

Examiner Tip

Do not confuse activity and count rate.Activity is the rate at which unstable nuclei decay, whereas count rate is the rate at which radioactive emissions are detected.Count rate has the units counts per second

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Ashika

Author: Ashika

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.