The Becquerel: a very small unit…:
The becquerel, the commonly-used unit of activity, corresponds to a decay rate of one nucleus per second. This unit on the scale of the atom is so small (and therefore ill-adapted to describe the activities of most radioactive substances we deal with) that multiples of becquerels are frequently used: kilo (thousands), mega (millions), giga (billions) and terabecquerels (thousands of billions), etc. The diagram above shows the range of activities different substances – from a litre of water up to the spent fuel extracted from the core of nuclear reactors – can have. Historic units such as curies and millicures have also been indicated.