Becquerel

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

becquerel
Unit system	SI
Unit of	activity
Symbol	Bq
Named after	Henri Becquerel
Conversions
1 Bq in ...	... is equal to ...
rutherford	10−6 Rd
curie	2.703×10−11 Ci ≅ 27 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, 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.

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, 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). The hertz is now only used for periodic phenomena. 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. 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 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.

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.

Calculation of radioactivity

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

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

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

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

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

For instance, on average each gram of potassium contains 117 micrograms of ⁴⁰K (all other naturally occurring isotopes are stable) that has a $t_{1/2}$ of 1.277×10⁹ years = 4.030×10¹⁶ s, and has an atomic mass of 39.964 g/mol, 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, there is roughly 0.0169 g of potassium-40 present in a typical human body, decaying at a rate of approximately 4,430 decays per second.

The global inventory of carbon-14 is estimated to be 8.5×10¹⁸ Bq (8.5 EBq, 8.5 exabecquerel). 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.

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), 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.

Conversion factors:

1 Ci = 3.7×10¹⁰ Bq = 37 GBq

1 μCi = 37,000 Bq = 37 kBq

1 Bq = 3.7×10⁻¹¹ Ci = 3.7×10⁻⁵ μCi

1 MBq = 0.037 mCi

Relation to other radiation-related quantities

Graphic showing relationships between radioactivity and detected ionizing radiation

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

Ionizing radiation related quantities

Quantity	Unit	Symbol	Derivation	Year	SI equivalent
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	1000000 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
	rad	rad	100 erg⋅g−1	1953	0.010 Gy
Equivalent dose (H)	sievert	Sv	J⋅kg−1 × WR	1977	SI unit
	röntgen equivalent man	rem	100 erg⋅g−1 × WR	1971	0.010 Sv
Effective dose (E)	sievert	Sv	J⋅kg−1 × WR × WT	1977	SI unit
	röntgen equivalent man	rem	100 erg⋅g−1 × WR × WT	1971	0.010 Sv

See also

• Background radiation
• Banana equivalent dose
• Counts per minute
• Ionizing radiation
• Orders of magnitude (radiation)
• Radiation poisoning
• Relative biological effectiveness
• Rem (unit)
• Rutherford (unit)
• Sievert (biological dose equivalent of radiation)

External links

• Derived units on the International Bureau of Weights and Measures (BIPM) web site