Activity is measured in becquerel (Bq) Count-rate is the number of decays…

The idea

The unit of activity is the becquerel: 1 Bq is one decay per second, so a source of 500 Bq has 500 nuclei decaying every second. Count-rate is what a detector such as a Geiger-Muller tube actually records each second, and it is always smaller than the activity. Radiation is emitted in every direction, so the tube intercepts only a fraction of it, some radiation is absorbed by the air or by the tube window, and the detector itself is not perfectly efficient. The reading also includes background radiation, which must be measured with the source removed and subtracted before the count-rate from the source is used.

What earns the marks

• Naming the becquerel, Bq, as the unit of activity.
• Stating that 1 Bq is one decay per second.
• Count-rate being the number of decays recorded each second by a detector such as a Geiger-Muller tube.
• Stating that the count-rate is lower than the activity, and that the background count must be subtracted.

Common mistakes and how to put them right

• Writing decays per second when the question asks for the name of the unit.

• Treating count-rate and activity as identical quantities.

• Forgetting to subtract the background count-rate before using a reading.

• Quoting a count rather than a count-rate, so leaving out the per second.

Exam tips

• If a question says give the unit, write becquerel or Bq; a description of the unit may not be credited.
• Always check whether a count-rate in a table has already had background subtracted.
• Explain a low count-rate by the detector catching only part of the radiation emitted in all directions.