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 of a radioactive source is defined as:
The rate at which the unstable nuclei decay
- 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
How does activity vary with time?
- The activity of a radioactive source decreases with time
- This is because each decay event reduces the overall number of radioactive particles in the source
- Radioactive decay is a random process
- The randomness of radioactive decay can be observed by measuring the count rate of a source using a Geiger-Muller (GM) tube
- When the count rate is plotted against time, fluctuations can be seen
- These fluctuations provide evidence for the randomness of radioactive decay
The decreasing activity of a source can be shown on a graph against time. The fluctuations show the randomness of radioactive decay
Worked example
A source of radiation has an activity of 2000 Bq. How many unstable atoms decay in 2 minutes?
Answer:
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.
