Activity & Decay (Edexcel IGCSE Physics (Modular))
Revision Note
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
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.
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