Variables
| Symbol | Quantity | Unit |
|---|---|---|
| A | Activity | Bq (decays/s) |
| λ | Decay constant | 1/s |
| N | Number of radioactive atoms |
What it means
Activity (A) is the number of nuclear disintegrations (decays) per unit time in a radioactive sample. It is given by A = λ N, where λ is the decay constant and N is the number of radioactive nuclei. The SI unit is the becquerel (Bq), defined as one decay per second. The older unit is the curie (Ci), where 1 Ci = 3.7×10¹⁰ Bq. Activity is directly proportional to the amount of radioactive material and is used to characterise sources for medical, industrial, and research applications. It also determines the radiation dose rate and is used in radiation protection to limit exposure. Understanding activity is fundamental for nuclear medicine (dosing), waste management, and environmental monitoring.
Worked example
Radioactive Activity – Two Examples
Real‑World| Parameter | Value |
|---|---|
| λ | 1.2×10⁻⁴ s⁻¹ |
| N | 6×10²⁰ atoms |
| Parameter | Value |
|---|---|
| λ | 1.2×10⁻⁴ s⁻¹ |
| N | 1×10¹⁹ atoms |
Common mistakes
- Radioactive activity A: The number of decays per unit time – units: becquerel (Bq) = 1 decay/s.
- Decay constant λ: In 1/time – must be consistent with time units.
- Number of nuclei N: The number of radioactive atoms present.
- Activity is proportional to N: As N decreases, activity decreases exponentially.
- Specific activity: Activity per unit mass – see ID 847.
Applications
Radioactive activity (A) is the rate of decay of a radioactive substance, given by A = λ·N, where λ is the decay constant and N is the number of radioactive nuclei. It is measured in becquerels (Bq) or curies (Ci). Activity is the fundamental quantity for radiation safety, medical applications, and environmental monitoring. Health physicists use activity to assess the potential radiation dose to workers and the public, to design handling protocols, and to determine the required shielding. In nuclear medicine, the activity of a radiopharmaceutical determines the therapeutic or diagnostic dose. By measuring and calculating activity, professionals can ensure safe and effective use of radioactive materials in all applications.
- Radiation safety and dose assessment in nuclear facilities
- Calibration and use of radiation detectors (Geiger counters, scintillators)
- Design of radiopharmaceutical doses for therapy and imaging
- Environmental monitoring of radioactive contamination
- Inventory management of radioactive sources and waste
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