Becquerel

Becquerel
Unit system	SI derived unit
Unit of	Radioactivity
Symbol	Bq
Named after	Henri Becquerel
In SI base units	s^-1

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

Capitalization

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.^[2]

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 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.^[4] 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.^[5] 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^[6] and 1975.

Prefixes

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). 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.^[7] The global inventory of carbon-14 is estimated to be 8.5×10¹⁸ Bq (8.5 EBq, 8.5 exabecquerel).^[8] The nuclear explosion in Hiroshima (An explosion of 16 kt or 67 TJ) is estimated to have produced 8×10²⁴ Bq (8 YBq, 8 yottabecquerel).^[9]

Relationship to the curie

The becquerel succeeded the curie (Ci),^[10] 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 MBq = 0.027 mCi

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 moles (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,^[11] and has an atomic mass of 39.964 g/mol,^[12] so the radioactivity is 30 Bq.

Radiation-related quantities

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
	rad	rad	100 erg·g^−1	1953
	gray	Gy	J·kg^−1	1974
Activity (A)	curie	Ci	3.7 × 10^10 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

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)

References

1. "BIPM - Becquerel". BIPM. Retrieved 2012-10-24.
2. "SI Brochure: The International System of Units (SI)". SI Brochure (8 ed.). BIPM. 2014.
3. Allisy, A. (1995), "From the curie to the becquerel", Metrologia 32 (6): 467–479, Bibcode:1995Metro..31..467A, doi:10.1088/0026-1394/31/6/006
4. "BIPM - Table 3". BIPM. Retrieved 2015-07-19. (d) The hertz is used only for periodic phenomena, and the becquerel is used only for stochastic processes in activity referred to a radionuclide.
5. Harder, D (1976), "[The new radiologic units of measurement gray and becquerel (author's translation from the German original)]", Röntgen-Blätter 29 (1): 49–52, PMID 1251122.
6. Lind, SC (1946), "New units for the measurement of radioactivity", Science 103 (2687): 761–762, Bibcode:1946Sci...103..761L, doi:10.1126/science.103.2687.761-a, PMID 17836457.
7. Radioactive human body — Harvard University Natural Science Lecture Demonstrations - Accessed October 2013
8. G.R. Choppin, J.O.Liljenzin, J. Rydberg, "Radiochemistry and Nuclear Chemistry", 3rd edition, Butterworth-Heinemann, 2002. ISBN 978-0-7506-7463-8.
9. Michael J. Kennish, Pollution Impacts on Marine Biotic Communities , CRC Press, 1998, p. 74. ISBN 978-0-8493-8428-8.
10. It was adopted by the BIPM in 1975, see resolution 8 of the 15th CGPM meeting
11. "Table of Isotopes decay data". Lund University. 1990-06-01. Retrieved 2014-01-12.
12. "Atomic Weights and Isotopic Compositions for All Elements". NIST. Retrieved 2014-01-12.

External links

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