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:
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
