What activity means
- A radioactive source is any object containing unstable nuclei that emit ionising radiation as they decay
- The activity of a source is defined as:
the rate at which the unstable nuclei in the source decay
- Activity is measured in becquerels (Bq), where:
1 Bq = 1 decay per second (one unstable nucleus in the source breaks down each second)
- A source with an activity of 1000 Bq has 1000 nuclei decaying per second; a source of 1 MBq has a million decays per second
Activity decreases over time
- Every time a nucleus decays, it disappears from the pool of unstable nuclei
- So the number of unstable nuclei left in the sample is always falling
- Fewer unstable nuclei means fewer decays per second, so the activity falls with time as well
- The fall is not linear; it is faster at the start (when many unstable nuclei are present) and slower as time goes on. The shape is an exponential decay curve
Decay is random
- Radioactive decay is a random process: it is impossible to predict which nucleus will decay next, or exactly when any one nucleus will break down
- All you can say is the probability that a given nucleus will decay in the next second
- The randomness can be seen by measuring the count rate of a source using a Geiger–Müller (GM) tube
- The count rate is the number of decays the detector registers per second. When count rate is plotted against time the trace fluctuates up and down rather than following a perfectly smooth curve
- These fluctuations are direct evidence that decay is random. If decay were a steady, clockwork process the count rate would lie exactly on a smooth curve
Activity vs count rate (don't mix them up)
- Activity = rate at which unstable nuclei in the source actually decay (Bq)
- Count rate = rate at which the detector registers radiation
- Count rate is usually smaller than activity, because:
- The detector does not surround the source completely, so much of the radiation flies off and misses it
- Some radiation is absorbed by air, the GM tube wall, or any object in between
- Count rate is the practical, measurable quantity; activity is the theoretical quantity
Example — A radioactive source has an activity of 1500 Bq. How many unstable nuclei decay in 4 minutes?
- Step 1 — Read the activity. 1500 Bq means 1500 nuclei decay every second
- Step 2 — Convert the time to seconds. 4 minutes = 4 × 60 = 240 s
- Step 3 — Multiply: number of decays = activity × time = 1500 × 240 = 360 000 decays
- So 360 000 unstable nuclei break down during the 4-minute window