# Becquerel

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"Bq" redirects here. For other uses, see BQ and Becquerel (disambiguation).

The becquerel (symbol Bq) (pronounced: BEK-ə-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] Whereas 1 Hz is 1 cycle per second, 1 Bq is 1 aperiodic event per second.

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 applications, 1 Bq is a small unit; therefore, the prefixes are common. For example, the roughly 0.0169 g of potassium-40 present in a typical human body produces approximately 266,000 disintegrations per minute, which equates to about 4,400 disintegrations per second or 4.4 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),[7] 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

## Calculation of radioactivity

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:

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

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

Since m/ma is the number of mols (n), the amount of radioactivity $A$ can be calculated by:

$A_{Bq} = nN_A\frac{\ln(2)}{t_{1/2}}$

For instance, one gram of potassium contains 0.000117 gram of 40K (all other naturally occurring isotopes are stable) that has a $t_{1/2}$ of 1.277×109 years = 4.030×1016 s,[8] and has an atomic mass of 39.964 g/mol,[9] so the radioactivity is 30 Bq.

Graphic showing relationships between radioactivity and detected ionizing radiation

The following table shows radiation quantities in SI and non-SI units.

Quantity Name Symbol Unit Year
Exposure (X) roentgen R esu / 0.001293 g of air 1928
Absorbed dose (D) erg•g−1 1950
gray Gy J•kg−1 1974
Activity (A) curie Ci 3.7 × 1010 s−1 1953
becquerel Bq s−1 1974
Dose equivalent (H) roentgen equivalent man rem 100 erg•g−1 1971
sievert Sv J•kg−1 1977
Fluence (Φ) (reciprocal area) cm−2 or m−2 1962