Recall that the unit of activity of a radioactive isotope is the…

The idea

The activity of a radioactive isotope measures how many nuclear decays occur each second. Its unit is the becquerel, symbol Bq, where one becquerel equals one decay per second. A sample with an activity of 500 Bq therefore produces 500 decays every second. Activity is not the same as count rate: a detector records only a fraction of the decays because of distance, absorption and geometry, and background radiation must be subtracted. The becquerel is named after Henri Becquerel. When answering, state the unit as Bq and remember that it measures decays per second, not the total number of decays or the radiation dose received.

What earns the marks

• States that the unit of activity is the becquerel, symbol Bq.
• Defines one becquerel as one decay per second.
• Distinguishes activity from count rate, noting that a detector records only a fraction of decays.
• Applies the unit correctly, for example describing a source of activity 200 Bq as producing 200 decays each second.

Common mistakes and how to put them right

• Writes the unit as 'becquerels per second'.

  Put it right by stating that the becquerel already means decays per second.

• Confuses activity with radiation dose.

  Put it right by stating that dose is measured in sieverts, while activity is measured in becquerels.

• Treats count rate as equal to activity.

  Put it right by noting that a detector records only a fraction of the decays.

Exam tips

• Write the unit as Bq, not 'bq' or 'BQ', and give the full name becquerel when asked.
• Remember 1 Bq = 1 decay per second, so an activity of 300 Bq means 300 decays each second.
• Check whether the question asks for activity (Bq) or dose (Sv) before choosing a unit.