# Becquerel

Short description: SI derived unit of radioactivity
becquerel
Unit systemSI
Unit ofActivity
SymbolBq
Named afterHenri Becquerel
Conversions
1 Bq in ...... is equal to ...
rutherford   10−6 Rd
curie   2.703×10−11 Ci27 pCi
SI base unit   s−1

The becquerel (English: /bɛkəˈrɛl/; symbol: Bq) is the unit of radioactivity in the International System of Units (SI). 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,[1] and SI multiples of the unit are commonly used.[2]

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.[3]

## Definition

1 Bq = 1 s−1

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 would mean 106 disintegrations per second: 1·(10−6 s)−1 = 106 s−1,[4] whereas 1 µBq would mean 1 disintegration per 1 million seconds. Other names considered were hertz (Hz), a special name already in use for the reciprocal second, and Fourier (Fr).[4] The hertz is now only used for periodic phenomena.[5] Whereas 1 Hz is 1 cycle per second, 1 Bq is 1 aperiodic radioactivity event per second.

The gray (Gy) and the becquerel (Bq) were introduced in 1975.[6] Between 1953 and 1975, absorbed dose was often measured in rads. Decay activity was measured in curies before 1946 and often in rutherfords between 1946[7] and 1975.

## Unit capitalization and prefixes

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.[8]

Like any SI unit, Bq can be prefixed; commonly used multiples are kBq (kilobecquerel, 103 Bq), MBq (megabecquerel, 106 Bq, equivalent to 1 rutherford), GBq (gigabecquerel, 109 Bq), TBq (terabecquerel, 1012 Bq), and PBq (petabecquerel, 1015 Bq). Large prefixes are common for practical uses of the unit.

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

$\displaystyle{ A_\text{Bq} = \frac{m} {m_\text{a}} N_\text{A} \frac{\ln 2} {t_{1/2}} }$

With $\displaystyle{ N_\text{A} }$ = 6.02214076×1023 mol−1, the Avogadro constant.

Since $\displaystyle{ m/m_\text{a} }$ is the number of moles ($\displaystyle{ n }$), the amount of radioactivity $\displaystyle{ A }$ can be calculated by:

$\displaystyle{ A_\text{Bq} = nN_\text{A} \frac{\ln 2} {t_{1/2}} }$

For instance, on average each gram of potassium contains 117 micrograms of 40K (all other naturally occurring isotopes are stable) that has a $\displaystyle{ t_{1/2} }$ of 1.277×109 years = 4.030×1016 s,[9] and has an atomic mass of 39.964 g/mol,[10] so the amount of radioactivity associated with a gram of potassium is 30 Bq.

## Examples

For practical applications, 1 Bq is a small unit. For example, the roughly 0.0169 g of potassium-40 present in a typical human body produces approximately 4,400 disintegrations per second or 4.4 kBq of activity.[11]

The global inventory of carbon-14 is estimated to be 8.5×1018 Bq (8.5 EBq, 8.5 exabecquerel).[12] The nuclear explosion in Hiroshima (an explosion of 16 kt or 67 TJ) is estimated to have injected 8×1024 Bq (8 YBq, 8 yottabecquerel) of radioactive fission products into the atmosphere.[13]

These examples are useful for comparing the amount of activity of these radioactive materials but should not be confused with the amount of exposure to ionizing radiation that these materials represent. The level of exposure and thus the absorbed dose received are what should be considered when assessing the effects of ionizing radiation on humans.

## Relation to the curie

The becquerel succeeded the curie (Ci),[14] 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.[4][15]

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 MBq = 0.027 mCi

## Relation to other radiation-related quantities

The following table shows radiation quantities in SI and non-SI units. WR (formerly 'Q' factor) is a factor that scales the biological effect for different types of radiation, relative to x-rays. (e.g. 1 for beta radiation, 20 for alpha radiation, and a complicated function of energy for neutrons) In general conversion between rates of emission, the density of radiation, the fraction absorbed, and the biological effects, requires knowledge of the geometry between source and target, the energy and the type of the radiation emitted, among other factors.[16]

Quantity Unit Symbol Derivation Year SI equivalence
Activity (A) becquerel Bq s−1 1974 SI unit
curie Ci 3.7 × 1010 s−1 1953 3.7×1010 Bq
rutherford Rd 106 s−1 1946 1,000,000 Bq
Exposure (X) coulomb per kilogram C/kg C⋅kg−1 of air 1974 SI unit
röntgen R esu / 0.001293 g of air 1928 2.58 × 10−4 C/kg
Absorbed dose (D) gray Gy J⋅kg−1 1974 SI unit
erg per gram erg/g erg⋅g−1 1950 1.0 × 10−4 Gy
Dose equivalent (H) sievert Sv J⋅kg−1 × WR 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1 1971 0.010 Sv