PhysicsExam code: 4PH1

Radioactivity, Uses & Dangers

Radioactivity & Particles

PractiseFlashcards

Activity and Decay

What activity means

  • A radioactive source 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

Common exam question

Giving the unit of activity

Question: Give the unit of activity, or pick it out from a list of units (1 mark).

Asked in 4 of the 24 papers, twice as a multiple-choice item. The answer is the becquerel (Bq): one decay per second. Any prefix is accepted (kBq, for example), and so is a recognisable misspelling; one scheme also allowed the curie. The multiple-choice distractors are other SI units (the kilogram, newton, pascal, coulomb, joule and watt), so you only need to recognise the one unit that counts decays.

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 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
Count rate against time measured with a GM tube: the trace fluctuates up and down as it falls, rather than following a perfectly smooth curve, showing that decay is random
Source: Activity & Decay by Save My Exams

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

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