What does a Becquerel unit measure?

The becquerel (symbol: Bq) is the SI derived unit of radioactivity. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. For applications relating to human health this is a small quantity, and SI multiples of the unit are commonly used.

The becquerel is named after Henri Becquerel, who shared a Nobel Prize in Physics with Pierre and Marie Skłodowska Curie in 1903 for their work in discovering radioactivity.

As with every International System of Units (SI) unit named for a person, the first letter of its symbol is uppercase (Bq). However, when an SI unit is spelled out in English, it should always begin with a lowercase letter (becquerel) except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case.

Like any SI unit, Bq can be prefixed; commonly used multiples are kBq (kilobecquerel, 103 Bq), MBq (megabecquerel, 106 Bq), GBq (gigabecquerel, 109 Bq), TBq (terabecquerel, 1012 Bq), and PBq (petabecquerel, 1015 Bq). Large prefixes are common for practical uses of the unit.