# Becquerel

The becquerel (symbol Bq) (pronounced: 'be-kə-rel) is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1. The becquerel is named after Henri Becquerel, who shared a Nobel Prize with Pierre and Marie Curie in 1903 for their work in discovering radioactivity.[1]

## Definition

1 Bq = 1 s−1[2]

A special name was introduced for the reciprocal second (s−1) to represent radioactivity to avoid potentially dangerous mistakes with prefixes. For example, 1 µs−1 could be taken to mean 106 disintegrations per second: 1·(10−6 s)−1 = 106 s−1.[3] Other names considered were hertz (Hz), a special name already in use for the reciprocal second, and fourier (Fr).[3] The hertz is now only used for periodic phenomena.[2]

This SI unit is named after Henri Becquerel. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Bq). However, when an SI unit is spelled out in English, it should always begin with a lower case 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 capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.

## Prefixes

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). For practical application, 1 Bq is a small unit; therefore, the prefixes are common. For example, natural potassium (40K) in a typical human body produces 4,000 disintegrations per second, 4 kBq of activity.[4] The global inventory of carbon-14 is estimated to be 8.5×1018 Bq (8.5 EBq, 8.5 exabecquerel).[5] The nuclear explosion in Hiroshima (14 kt or 59 TJ) is estimated to have produced 8×1024 Bq (8 YBq, 8 yottabecquerel).[6]

## Relationship to the curie

The becquerel succeeded the curie (Ci), an older, non-SI unit of radioactivity based on the activity of 1 gram of radium-226. The curie is defined as 3.7·1010 s−1, or 37 GBq.[3]

Conversion factors:

1 Ci = 3.7×1010 Bq = 37 GBq
1 μCi = 37,000 Bq = 37 kBq
1 Bq = 2.7×10−11 Ci = 2.7×10−5 μCi
1 GBq = 0.027 Ci

For a given mass $m$ (in grams) of an isotope with atomic mass $m_a$ (in g/mol) and a half-life of $t_{1/2}$ (in s), the amount of radioactivity can be calculated using:

radioactivity(in Bq) = $\frac{m}{m_a}N_A\frac{\ln(2)}{t_{1/2}}$

With $N_A$=6.022 141 79(30)×1023 mol−1 the Avogadro constant.

For instance, one gram of potassium contains 0.000117 gram of 40K (all other isotopes are stable) that has a $t_{1/2}$ of 1.248×109years=39.38×1015 seconds, and has an atomic mass of 39.963 g/mol, so the radioactivity is 31 Bq.