Activity and 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
Decreasing Activity
- The activity of a source decreases with time
- This is because each decay event reduces the overall number of radioactive particles in the source
- The nature of radioactive decay can be demonstrated by observing the count rate of a source using a Geiger-Muller (GM) tube, and then plotting these results against time
- These fluctuations in count rate on the GM tube also 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?
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
Exam 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.