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7.2.1Radioactivity and Particles

Activity and Decay

5 min read 5 parts Lesson 9 of 18 1 quiz

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Section 1 of 5Defining Activity

7.2.1 Activity and Decay

Activity describes how quickly the unstable nuclei in a source are decaying.

Activity
  • The rate at which the unstable nuclei in a source decay, measured in becquerels (Bq).
Becquerel
  • One becquerel is one decay per second, so 1 Bq = 1 decay each second.
Measured count rate
  • A detector's count rate is used to estimate activity, but it is only an estimate because not every decay is detected.
Random decay
  • Each nucleus decays unpredictably, so the activity of a large sample can be predicted only on average.

Section 2 of 5Activity Over Time

As a source decays, the number of unstable nuclei left falls, and so does its activity.

1
As nuclei decay, the number of unstable nuclei remaining in the source falls steadily.
2
With fewer unstable nuclei left, each second contains fewer decays, so the activity decreases.
3
A graph of activity against time therefore falls steeply at first and then flattens out as it approaches zero.
4
A GM tube reading of count rate fluctuates from second to second, which reflects the random nature of decay.
5
Only the overall trend, not each individual reading, shows the steady decrease in activity over time.

Section 3 of 5Key Points

Random fluctuations are evidence about decay, not a fault in the apparatus.

Key Points
Source
  • An object containing radioactive nuclei, whose activity depends on the number of unstable nuclei it contains.
Randomness
  • The decay of a single nucleus cannot be predicted, but the overall rate for a large number of nuclei is predictable.
Fluctuation
  • Random fluctuations in count rate show that individual decays occur unpredictably.
Units
  • Activity is measured in becquerels (Bq), whereas count rate is usually measured in counts per second or per minute.

Section 4 of 5Worked Example

Question

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

  1. An activity of 2000 Bq means 2000 nuclei decay every second.
  2. Convert the time to seconds2 × 60 = 120 s.
  3. Number of decays = activity × time = 2000 × 120 = 240 000.

Answer 2.4 × 10⁵ nuclei decay in 2 minutes.

Section 5 of 5Exam Checklist & Tips

7.2.1 – Key Points for the Exam

  • Activity The rate at which unstable nuclei in a source decay.
  • Becquerel (Bq) The unit of activity, equal to one decay per second.
  • Unstable nucleus A nucleus that is likely to decay by emitting radiation.
  • Random decay Decay in which the moment of each nucleus's decay cannot be predicted.
  • Count rate The number of radiation counts recorded by a detector per unit time.

Common Mistakes

  • Saying activity is the number of atoms in a sample; activity is the rate at which they decay.
  • Using minutes in the calculation when the activity is in becquerels, which are per second.
  • Claiming the activity of a source stays constant over time.
  • Treating random fluctuations in readings as a fault in the apparatus.
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