AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Count-rate is the number of decays recorded each second by a detector (eg…
Full specification wording
Radioactive decay and nuclear radiation: Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube).
Radioactive decay is random, so a detector cannot predict when the next decay occurs.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 4 of 10
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Radioactive decay is random, so a detector cannot predict when the next decay occurs. Instead, we measure the count-rate: the number of decays recorded each second by a detector such as a Geiger-Muller tube. The tube produces a pulse each time ionising radiation enters it, and a counter or data logger totals these pulses over a chosen time. Dividing the total count by the time in seconds gives the count-rate in counts per second (cps). For example, 600 counts in 20 s gives 600 ÷ 20 = 30 cps. Because decay is random, repeat readings vary slightly, so a mean of several measurements is more reliable. Count-rate is not the same as activity: activity is the number of decays per second in the source (measured in becquerels, Bq), while count-rate is what the detector records, which is lower because some radiation is absorbed or misses the tube.
What earns the marks
- Count-rate is defined as the number of decays recorded each second by a detector.
- A Geiger-Muller tube is a detector that produces a pulse for each ionising particle or photon entering it.
- Count-rate is calculated by dividing the total number of counts recorded by the time in seconds.
- The unit of count-rate is counts per second (cps); it is not measured in becquerels (Bq), which is the unit for activity.
- Count-rate is measured rather than predicted because radioactive decay is random.
- Count-rate is lower than the source activity because not all emitted radiation reaches or is detected by the tube.
- Repeating a measurement and calculating a mean improves reliability because random decay causes variation between readings.
Common mistakes and how to put them right
Confusing count-rate with activity.
Put it right by stating that count-rate is a measured quantity that is usually lower than activity.
Forgetting to divide by time.
Put it right by dividing total counts by the time in seconds.
Assuming every emitted particle is detected.
Put it right by explaining that count-rate underestimates activity.
Treating count-rate as fixed.
Put it right by taking repeats and using a mean.
Exam tips
- State the definition precisely: number of decays recorded each second by a detector.
- When calculating, show the division clearly, for example 600 counts ÷ 20 s = 30 cps.
- Use the correct unit, counts per second, and do not write 'counts' alone or 'becquerels' as the unit of count-rate.
- If asked why repeats are taken, link your answer to the random nature of radioactive decay.
- Distinguish clearly between count-rate and activity if both appear in a question.
Try the lesson
The start of the Radioactive decay and nuclear radiation: Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube). lesson, made from checked teaching notes for this specification point.
Have a go first
A Geiger-Muller tube records 240 decays in 60 seconds. What is the count-rate in decays per second?
Listen
Radioactive decay is random, so a detector cannot predict when the next decay occurs. Instead, we measure the count-rate: the number of decays recorded each second by a detector such as a Geiger-Muller tube. The tube produces a pulse each time ionising radiation enters it, and a counter or data logger totals these pulses over a chosen time. Dividing the total count by the time in seconds gives the count-rate in counts per second (cps). For example, 600 counts in 20 s gives 600 ÷ 20 = 30 cps. Because decay is random, repeat readings vary slightly, so a mean of several measurements is more reliable. Count-rate is not the same as activity: activity is the number of decays per second in the source (measured in becquerels, Bq), while count-rate is what the detector records, which is lower because some radiation is absorbed or misses the tube. A strong answer earns marks for these points. Count-rate is defined as the number of decays recorded each second by a detector. A Geiger-Muller tube is a detector that produces a pulse for each ionising particle or photon entering it. Count-rate is calculated by dividing the total number of counts recorded by the time in seconds. The unit of count-rate is counts per second (cps); it is not measured in becquerels (Bq), which is the unit for activity. A common slip: Confusing count-rate with activity: count-rate is what the detector records each second (cps), whereas activity is the number of decays per second in the source (Bq). Instead, stating that count-rate is a measured quantity that is usually lower than activity. Exam tip: State the definition precisely: number of decays recorded each second by a detector.
Key words
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Count-rate
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Geiger-Muller tube
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Random nature of radioactive decay
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Activity
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See every definition
- Count-rate
- The number of decays recorded each second by a detector such as a Geiger-Muller tube.
- Geiger-Muller tube
- A detector that produces a pulse each time ionising radiation enters it.
- Random nature of radioactive decay
- Decay is random, so a detector cannot predict when the next decay occurs and repeat readings vary slightly.
- Activity
- The number of decays per second in the source, measured in becquerels (Bq).
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