Becquerel
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The '''becquerel''' (symbol '''Bq''') is the [[SI derived unit]] of [[Radioactive decay|radioactivity]], defined as the [[activity (radioactivity)|activity]] of a quantity of radioactive material in which one [[atomic nucleus|nucleus]] decays per [[second]]. It is therefore equivalent to [[second|s]]<sup>-1</sup>. The becquerel is named for [[Henri Becquerel]], who shared a [[Nobel Prize]] with [[Pierre Curie|Pierre]] and [[Maria Sklodowska-Curie|Marie Curie]] for their work in discovering radioactivity.
In a fixed mass of radioactive material, the number of becquerels changes with time. Therefore, a sample radioactive decay rate is always stated with a timestamp for short-lived isotopes, sometimes after adjustment to some specific date of interest (in the past or in the future). For example, one might quote a ten-day adjusted figure, that is, the amount of radioactivity that will still be present ten days in the future. This can de-emphasize short-lived isotopes.
[[International System of Units|SI]] uses the becquerel rather than the second for the unit of activity measure to avoid dangerous mistakes: a measurement in becquerels is proportional to activity, and thus a more dangerous source of radiation gives a higher reading. A measurement in seconds is inversely proportional. As any SI unit, Bq can be [[SI prefix|prefixed]]; commonly used multiples are kBq (kilobecquerel, 10<sup>3</sub> Bq), MBq (megabecquerel, 10<sup>6</sup> Bq), and GBq (gigabecquerel, 10<sup>9</sup> Bq).
When measuring radioactivity of a sample with a detector, a unit of "counts per second" (cps) or "[[counts per minute]]" (cpm) are often used. These units can be converted to the absolute activity of the sample in Bq if one applies a number of significant conversions, e.g., for the radiation background, for the detector efficiency, for the counting geometry, for self-absorption of the radiation in the sample.
The becquerel can be used for the frequency of aperiodic events; for periodic events, the [[hertz]], which is also defined as s<sup>–1</sup>, is used as unit.
== Relationship to the curie ==
The [[curie]] (Ci) is an older, non-SI unit, of radioactivity. It is equivalent to 3.70 × 10<sup>10</sup> disintegrations per second, which is approximately the amount of activity produced by 1 g of radium-226. Thus the curie is defined as 3.7×10<sup>10</sup> becquerels or 37 GBq.
==Definition==
1 Bq = 1 s<sup>–1</sup>
{{SI unit lowercase|Henri Becquerel|becquerel|Bq}}
==External links==
* [http://www1.bipm.org/en/si/derived_units/2-2-2.html Derived units] on the [[BIPM]] web site
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[[Category:SI derived units]]
[[Category:Units of radioactivity]]
[[Category:Units of frequency]]
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