Activity is measured in becquerel (Bq)

The idea

The becquerel (Bq) is the unit of activity. One becquerel is one nuclear decay per second, so a source of 3000 Bq has 3000 unstable nuclei decaying each second on average. Because activity is a rate, values are written with the unit Bq, and larger activities may be quoted in kBq (10³ Bq) or MBq (10⁶ Bq). For example, if a detector records 40 counts per second from a source whose activity is 400 Bq, the count rate is much lower because most emissions miss the detector or are absorbed before reaching it. Activity is not measured in counts per second; counts per second is a count rate.

What earns the marks

• The becquerel (Bq) is the unit of activity, defined as one decay per second.
• Activity values may be expressed in Bq, kBq or MBq, where 1 kBq = 1000 Bq and 1 MBq = 1 000 000 Bq.
• A source with activity 250 Bq has 250 decays each second on average.
• Count rate is measured in counts per second and is not the same as activity in Bq.
• When calculating or comparing activities, keep the unit Bq with the numerical value and convert prefixes correctly.

Common mistakes and how to put them right

• Writing activity in counts per second instead of Bq.

  Put it right: Counts per second is a count rate, while Bq is the unit of activity.

• Treating kBq and MBq as different physical quantities.

  Put it right: They are simply larger multiples of the becquerel.

• Forgetting to convert kBq to Bq before comparing or calculating, which leads to answers that are out by factors of 1000 or 1 000 000.

Exam tips

• Write the unit Bq after every activity value, and convert kBq or MBq to Bq before doing arithmetic.
• If a question asks for activity and you are given count rate, state any assumption about detector efficiency or absorption before using the value.
• Check that your final answer has the correct power of ten when converting between Bq, kBq and MBq.