The becquerel (; symbol: Bq) is the unit of

radioactivity Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

in the

International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...

(SI). One becquerel is defined as an activity of one per

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

, on average, for aperiodic activity events referred to a radionuclide. For applications relating to human health this is a small quantity, and

SI multiple The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official st ...

s of the unit are commonly used. The becquerel is named after

Henri Becquerel Antoine Henri Becquerel ( ; ; 15 December 1852 – 25 August 1908) was a French nuclear physicist who shared the 1903 Nobel Prize in Physics with Marie and Pierre Curie for his discovery of radioactivity. Biography Family and education Becq ...

, who shared a

Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...

with

Pierre Pierre is a masculine given name. It is a French form of the name Peter. Pierre originally meant "rock" or "stone" in French (derived from the Greek word πέτρος (''petros'') meaning "stone, rock", via Latin "petra"). It is a translation ...

and Marie Curie in 1903 for their work in discovering radioactivity.

Definition

1 Bq = 1 s⁻¹ A special name was introduced for the

reciprocal second The inverse second or reciprocal second (s−1), also called ''per second'', is a unit defined as the multiplicative inverse of the second (a unit of time). It is applicable for physical quantities of dimension reciprocal time, such as frequency ...

(s) to represent radioactivity to avoid potentially dangerous mistakes with prefixes. For example, 1 μs would mean 10 disintegrations per second: , whereas 1 μBq would mean 1 disintegration per 1 million seconds. Other names considered were

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...

(Hz), a special name already in use for the reciprocal second (for ''periodic'' events of any kind), and fourier (Fr; after

Joseph Fourier Jean-Baptiste Joseph Fourier (; ; 21 March 1768 – 16 May 1830) was a French mathematician and physicist born in Auxerre, Burgundy and best known for initiating the investigation of Fourier series, which eventually developed into Fourier analys ...

). The hertz is now only used for periodic phenomena. While 1 Hz replaces the deprecated term

cycle per second The cycle per second is a once-common English name for the unit of frequency now known as the ''hertz'' (Hz). Cycles per second may be denoted by c.p.s., c/s, or, ambiguously, just "cycles" (Cyc., Cy., C, or c). The term comes from repetitive p ...

, 1 Bq refers to one event per second on average for

aperiodic A periodic function, also called a periodic waveform (or simply periodic wave), is a function that repeats its values at regular intervals or periods. The repeatable part of the function or waveform is called a ''cycle''. For example, the tr ...

radioactive decays. The

gray Grey (more frequent in British English) or gray (more frequent in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning that it has no chroma. It is the color of a cloud-covered s ...

(Gy) and the becquerel (Bq) were introduced in 1975. Between 1953 and 1975, absorbed dose was often measured with the rad. Decay activity was given with the

curie Curie may refer to: *Curie family, a family of distinguished scientists: :* Jacques Curie (1856–1941), French physicist, Pierre's brother :* Pierre Curie (1859–1906), French physicist and Nobel Prize winner, Marie's husband :* Marie Curi ...

before 1946 and often with the rutherford between 1946 and 1975.

Unit capitalization and prefixes

As with every International System of Units (SI) unit named after 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 Title case or headline case is a style of capitalization used for rendering the titles of published works or works of art in English. When using title case, all words are capitalized, except for minor words (typically articles, short prepositio ...

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

Examples

For practical applications, 1 Bq is a small unit. For example, there is roughly 0.017 g of

potassium-40 Potassium-40 (K) is a long lived and the main naturally occurring radioactive isotope of potassium. Its half-life is 1.25 billion years. It makes up about 0.012% (120 parts-per notation, ppm) of natural potassium. Potassium-40 undergoes four dif ...

in a typical human body, producing about 4,400 decays per second (Bq). The activity of radioactive

americium Americium is a synthetic element, synthetic chemical element; it has Chemical symbol, symbol Am and atomic number 95. It is radioactive and a transuranic member of the actinide series in the periodic table, located under the lanthanide element e ...

in a home

smoke detector A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors/alarms are usually housed in plastic enclosures, typically shaped like a disk about in diameter and thick, but shape and size vary. Smoke can be ...

is about 37 kBq (1 μCi). The global inventory of

carbon-14 Carbon-14, C-14, C or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic matter is the basis of the radiocarbon dating method pioneered by Willard Libby and coll ...

is estimated to be (8.5 EBq, 8.5

exa A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pre ...

becquerel). 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 Absorbed dose is a dose quantity which represents the specific energy (energy per unit mass) deposited by ionizing radiation in living matter. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protecti ...

received are what should be considered when assessing the effects of ionizing radiation on humans.

Relation to the curie

The becquerel succeeded the

(Ci), an older, non-SI unit of radioactivity based on the activity of 1 gram of

radium-226 Radium-226 () is the longest-lived isotope of radium, with a half-life of 1600 years. It is an intermediate product in the decay chain of uranium-238; as such, it can be found naturally in uranium-containing minerals. Occurrence and decay occ ...

. The curie is defined as , or 37 GBq. Conversion factors: * 1 Ci = = 37 GBq * 1 μCi = = 37 kBq * 1 Bq = = * 1 MBq = 0.027 mCi

Relation to other radiation-related quantities

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

References

External links

Derived units
on the

International Bureau of Weights and Measures The International Bureau of Weights and Measures (, BIPM) is an List of intergovernmental organizations, intergovernmental organisation, through which its 64 member-states act on measurement standards in areas including chemistry, ionising radi ...

(BIPM) web site {{Ionising radiation related quantities SI derived units Units of radioactivity Units of frequency