This topic accounts for approximately 7% of your exam marks.
stable
Low
Stable7%
Half-life calculations and uses/dangers of radioactive sources appear in most series.
What activity means
A is any object containing unstable nuclei that emit ionising radiation as they decay
The 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 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 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