Radioactivity, Uses & Dangers

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