OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(d) activity of a source; decay constant m of an isotope; A = m N
Full specification wording
Radioactivity: (d) activity of a source; decay constant m of an isotope; A = m N
Activity A is the number of nuclear decays per second, measured in becquerels (Bq).
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Activity A is the number of nuclear decays per second, measured in becquerels (Bq). The decay constant λ (often written as m in some texts) is the probability of decay per nucleus per unit time, with unit s⁻¹. For a sample containing N undecayed nuclei, activity is given by A = λN. This means a larger decay constant or more nuclei gives a higher activity. For example, if a source has N = 2.0 × 10²⁰ nuclei and λ = 1.0 × 10⁻⁶ s⁻¹, then A = 2.0 × 10¹⁴ Bq. Activity decreases over time as N falls, so A is not constant. The equation applies to any radionuclide and is fundamental to radioactive decay calculations.
What earns the marks
- Activity A is the rate of decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second.
- Decay constant λ (or m) is the probability of decay per nucleus per unit time, with unit s⁻¹.
- The relationship between activity, decay constant and number of undecayed nuclei is A = λN.
- Activity is proportional to the number of undecayed nuclei present at that instant.
- Using A = λN to calculate one quantity given the other two, with correct units.
Common mistakes and how to put them right
Confusing activity with count rate.
Put it right: Use activity for A = λN and count rate only when corrected for background and efficiency.
Using the wrong unit for decay constant.
Put it right: Bq is for activity, s⁻¹ for decay constant.
Assuming activity is constant.
Put it right: Use A = λN at a specific instant, and combine with exponential decay for changes over time.
Mixing up N (number of undecayed nuclei) with mass or number of moles.
Put it right: N is the actual count of nuclei, which may need to be calculated from mass and Avogadro constant.
Exam tips
- Check that N is the number of undecayed nuclei at the time of interest, not the initial number unless specified.
- In multiple-choice questions, ensure the unit of λ matches the time unit used for activity (e.g., s⁻¹ with Bq).
- Practise rearranging A = λN to find λ or N, and keep track of powers of ten.
Try the lesson
The start of the Radioactivity: (d) activity of a source; decay constant m of an isotope; A = m N lesson, made from checked teaching notes for this specification point.
Listen
Activity A is the number of nuclear decays per second, measured in becquerels (Bq). The decay constant λ (often written as m in some texts) is the probability of decay per nucleus per unit time, with unit s⁻¹. For a sample containing N undecayed nuclei, activity is given by A = λN. This means a larger decay constant or more nuclei gives a higher activity. For example, if a source has N = 2.0 × 10²⁰ nuclei and λ = 1.0 × 10⁻⁶ s⁻¹, then A = 2.0 × 10¹⁴ Bq. Activity decreases over time as N falls, so A is not constant. The equation applies to any radionuclide and is fundamental to radioactive decay calculations. A strong answer earns marks for these points. Activity A is the rate of decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second. Decay constant λ (or m) is the probability of decay per nucleus per unit time, with unit s⁻¹. The relationship between activity, decay constant and number of undecayed nuclei is A = λN. Activity is proportional to the number of undecayed nuclei present at that instant. A common slip: Confusing activity with count rate: activity is the actual number of decays per second, while count rate is what a detector records and may be lower due to efficiency. Instead, use activity for A = λN and count rate only when corrected for background and efficiency. Exam tip: Check that N is the number of undecayed nuclei at the time of interest, not the initial number unless specified.
See it
Key words
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Activity
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Decay constant
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A = λN
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Number of undecayed nuclei
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See every definition
- Activity
- The number of nuclear decays per second, measured in becquerels (Bq).
- Decay constant
- The probability of decay per nucleus per unit time, with unit s⁻¹.
- A = λN
- The relationship between activity, decay constant and number of undecayed nuclei.
- Number of undecayed nuclei
- The actual count of nuclei present at that instant, to which activity is proportional.
Label the diagram
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1Box 1
2Box 2
3Box 3
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