View source for Curie (unit)

{{Short description|Non-SI unit of radioactivity}}
{{Infobox Unit
| name = Curie
| image        = Radium226.jpg
| caption      = A sample of radium, the element which was used in the original definition of the curie.
| standard = 
| quantity = [[Specific activity|Activity]]
| symbol = Ci
| namedafter = [[Pierre Curie]] and [[Marie Curie]]
| units1       = [[Rutherford (unit)|rutherfords]]
| inunits1     = {{val|37000|u=[[Rutherford (unit)|Rd]]}}
| units2       = [[SI derived unit]]
| inunits2     = {{val|37|u=[[becquerel|GBq]]}} 
| units3       = [[SI base unit]]  
| inunits3     = {{val|3.7e10|u=[[second|s]]{{sup|−1}}}}
}}
[[File:Cobalt-60.jpg|thumb|Sample of [[cobalt-60]] that emits 1 μCi (microcurie) of radioactivity; i.e. 37,000 decays per second.]]
The '''curie''' (symbol '''Ci''') is a non-[[International System of Units|SI]] unit of [[Radioactive decay|radioactivity]] originally defined in 1910. According to a notice in ''[[Nature (magazine)|Nature]]'' at the time, it was to be named in honour of [[Pierre Curie]],<ref name=Nature1910>{{cite journal|last1=Rutherford |first1=Ernest |title=Radium Standards and Nomenclature |journal=Nature |date=6 October 1910 |volume=84 |issue=2136 |pages=430–431 |url=https://archive.org/stream/nature841910lock/nature841910lock_djvu.txt |doi=10.1038/084430a0 |bibcode=1910Natur..84..430R |doi-access=free}}</ref> but was considered at least by some to be in honour of [[Marie Curie|Marie Skłodowska-Curie]] as well,<ref name="How the Curie Came to Be">{{cite journal|last1=Frame |first1=Paul |title=How the Curie Came to Be |journal=Health Physics Society Newsletter |date=1996 |url=http://www.orau.org/ptp/articlesstories/thecurie.htm |access-date=3 July 2015 |archive-date=20 March 2012 |archive-url=https://web.archive.org/web/20120320124750/http://www.orau.org/ptp/articlesstories/thecurie.htm |url-status=dead}}</ref> and is in later literature considered to be named for both.<ref>{{cite book |url=https://books.google.com/books?id=7fUrAAAAIAAJ&pg=RA5-PA93 |page=93 |title=Semiannual Report of the Atomic Energy Commission, Volume 9 |author=[[United States Atomic Energy Commission]] |year=1951}}</ref>

It was originally defined as "the quantity or mass of [[radon|radium emanation]] in equilibrium with one gram of [[radium]] (element)",<ref name=Nature1910 /> but is currently defined as 1&nbsp;Ci = {{val|3.7|e=10}} [[radioactive decay|decays]] per [[second]]<ref>{{cite web|url=https://www.bipm.org/en/CGPM/db/12/7/ |title=Resolution 7 of the 12th CGPM |url-status=dead |archive-url=https://web.archive.org/web/20210219084448/https://www.bipm.org/en/CGPM/db/12/7 |archive-date=2021-02-19 |date=1964 |publisher=[[International Bureau of Weights and Measures]] (BIPM)}}</ref> after more accurate measurements of the activity of {{sup|226}}Ra (which has a specific activity of {{val|3.66|e=10|u=Bq/g}}<ref>{{cite journal|last1=Delacroix |first1=D. |title=Radionuclide and Radiation Protection Data Handbook 2002 |date=2002 |publisher=Nuclear Technology Publishing |journal=Radiation Protection Dosimetry |volume=98 |number=1 |page=147 |doi=10.1093/oxfordjournals.rpd.a006705 |pmid=11916063 |url=http://rpd.oxfordjournals.org/content/98/1/1|archive-url=https://web.archive.org/web/20160305114238/http://rpd.oxfordjournals.org/content/98/1/1|url-status=dead|archive-date=2016-03-05|url-access=subscription }}</ref>).

In 1975 the [[General Conference on Weights and Measures]] gave the [[becquerel]] (Bq), defined as one nuclear decay per second, official status as the [[International System of Units|SI unit]] of activity.<ref>{{cite journal|title=SI units for ionizing radiation: becquerel |journal=Resolutions of the 15th CGPM |date=1975 |issue=Resolution 8 |access-date=3 July 2015 |url=http://www.bipm.org/en/CGPM/db/15/8/}}</ref> 
Therefore:
: 1 Ci = {{val|3.7|e=10|u=Bq}} = 37 GBq 
and
: 1 Bq ≅ {{val|2.703|e=−11|u=Ci}} ≅ 27 pCi

While its continued use is discouraged by the [[National Institute of Standards and Technology]] (NIST)<ref>{{cite report|url=https://www.nist.gov/pml/pubs/sp811/sec05.cfm#52 |title=NIST Special Publication 811, paragraph 5.2 |date=28 January 2016 |publisher=NIST |access-date=22 March 2016}}</ref> and other bodies, the curie is still widely used throughout government, industry and medicine in the United States and in other countries.

At the 1910 meeting, which originally defined the curie, it was proposed to make it equivalent to 10&nbsp;[[nanogram]]s of radium (a practical amount). But Marie Curie, after initially accepting this, changed her mind and insisted on one gram of radium. According to Bertram Boltwood, Marie Curie thought that "the use of the name 'curie' for so infinitesimally small [a] quantity of anything was altogether inappropriate".<ref name="How the Curie Came to Be"/>

The power emitted in radioactive decay corresponding to one curie can be calculated by multiplying the [[decay energy]] by approximately 5.93&nbsp;[[milliwatt|mW]]&nbsp;/&nbsp;[[MeV]].

A [[radiotherapy]] machine may have roughly 1000 Ci of a radioisotope such as [[caesium-137]] or [[cobalt-60]]. This quantity of radioactivity can produce serious health effects with only a few minutes of close-range, unshielded exposure.

Radioactive decay can lead to the emission of particulate radiation or electromagnetic radiation. Ingesting even small quantities of some particulate emitting radionuclides may be fatal. For example, the [[median lethal dose]] (LD-50) for ingested [[polonium]]-210 is 240 μCi; about 53.5 nanograms.

The typical human body contains roughly 0.1 μCi (14 mg) of naturally occurring [[potassium-40]]. A human body containing {{cvt|16|kg}} of carbon (see ''[[Composition of the human body]]'') would also have about 24 nanograms or 0.1&nbsp;μCi of [[carbon-14]]. Together, these would result in a total of approximately 0.2&nbsp;μCi or 7400 decays per second inside the person's body (mostly from beta decay but some from gamma decay).

==As a measure of quantity==
{{radiation_units.svg}}
Units of activity (the curie and the becquerel) also refer to a quantity of radioactive atoms. Because the probability of decay is a fixed physical quantity, for a known number of atoms of a particular [[radionuclide]], a predictable number will decay in a given time.  The number of decays that will occur in one second in one gram of atoms of a particular radionuclide is known as the [[specific activity]] of that radionuclide.

The activity of a sample decreases with time because of decay.

The rules of [[radioactive decay]] may be used to convert activity to an actual number of atoms. They state that 1&nbsp;Ci of radioactive atoms would follow the expression
: ''N'' (atoms) × ''λ'' (s{{sup|−1}}) = 1 Ci = 3.7 × 10{{sup|10}} Bq,
and so
: ''N'' = 3.7 × 10{{sup|10}} Bq / ''λ'',
where ''λ'' is the [[exponential decay|decay constant]] in s<sup>−1</sup>.

Here are some examples, ordered by half-life:
{| class="wikitable"
! Nuclide !! [[Atomic mass|Isotopic mass]] ([[Dalton (unit)|Da]]){{AME2020 II|ref}} !! Number of atoms in 1 gram !! Half-life{{NUBASE2020|ref}}{{efn|Note that NUBASE2020 uses the <em>tropical</em> year to convert between years and other units of time, not the [[Gregorian year]]. The relationship between years and other time units in NUBASE2020 is as follows: {{nowrap|1=1 y = 365.2422 d = 31 556 926 s}} }} !! Specific activity (Ci/g) !! Mass of 1 curie
|-
| [[bismuth-209|<sup>209</sup>Bi]] || 208.9803986 || 2.8816773×10<sup>21</sup> || 2.01×10<sup>19</sup> years || 8.51×10<sup>−17</sup> || 11.7 billion tonnes
|-
| [[platinum-190|<sup>190</sup>Pt]]|| 189.9599498 || 3.1702160×10<sup>21</sup> || 4.83×10<sup>11</sup> years || 3.90×10<sup>−9</sup> || 257 tonnes
|-
| [[samarium-147|<sup>147</sup>Sm]]|| 146.9149044 || 4.0990673×10<sup>21</sup> || 1.066×10<sup>11</sup> years || 2.28×10<sup>−8</sup> || 43.8 tonnes
|-
| [[thorium-232|<sup>232</sup>Th]] || 232.0380536 || 2.5953246×10<sup>21</sup> || 1.405×10<sup>10</sup> years || 1.10×10<sup>−7</sup> (0.110 μCi/g) || 9.12 tonnes
|-
| [[uranium-238|<sup>238</sup>U]] || 238.0507876 || 2.5297714×10<sup>21</sup> || 4.468×10<sup>9</sup> years || 3.36×10<sup>−7</sup> (0.336 μCi/g) || 2.98 tonnes
|-
| [[potassium-40|<sup>40</sup>K]] || 39.96399817 || 1.50689146×10<sup>22</sup> || 1.248×10<sup>9</sup> years || 7.18×10<sup>−6</sup> (7.17 μCi/g) || 140 kg
|-
| [[uranium-235|<sup>235</sup>U]] || 235.0439281 || 2.5621342×10<sup>21</sup> || 7.038×10<sup>8</sup> years || 2.16×10<sup>−6</sup> (2.16 μCi/g) || 463 kg
|-
| [[iodine-129|<sup>129</sup>I]] || 128.9049836 || 4.6717672×10<sup>21</sup> || 1.614×10<sup>7</sup> years || 1.72×10<sup>−4</sup> (172 μCi/g) || 5.82 kg
|-
| [[technetium-99|<sup>99</sup>Tc]] || 98.90624968 || 6.0887363×10<sup>21</sup> || 2.111×10<sup>5</sup> years || 1.71×10<sup>−4</sup> || 58.4 g
|-
| [[plutonium-239|<sup>239</sup>Pu]] || 239.0521616 || 2.5191744×10<sup>21</sup> || 2.411×10<sup>4</sup> years || 6.20×10<sup>−2</sup> || 16.1 g
|-
| [[plutonium-240|<sup>240</sup>Pu]] || 240.0538117 || 2.5086628×10<sup>21</sup> || 6561 years || 0.227 || 4.41 g
|-
| [[carbon-14|<sup>14</sup>C]] || 14.00324199 || 4.30053323×10<sup>22</sup> || 5700 years || 4.48 || 223 mg
|-
| [[radium-226|<sup>226</sup>Ra]] || 226.0254082 || 2.6643645×10<sup>21</sup> || 1600 years || 0.989 || 1.01 g
|-
| [[americium-241|<sup>241</sup>Am]] || 241.0568273 || 2.4982245×10<sup>21</sup> || 432.6 years || 3.43 || 292 mg
|-
| [[plutonium-238|<sup>238</sup>Pu]] || 238.0495582 || 2.5297845×10<sup>21</sup> || 87.7 years || 17.1 || 58.4 mg
|-
| [[caesium-137|<sup>137</sup>Cs]] || 136.9070893 || 4.3987063×10<sup>21</sup> || 30.04 years || 86.9 || 11.5 mg
|-
| [[strontium-90|<sup>90</sup>Sr]] || 89.9077279 || 6.6981347×10<sup>21</sup> || 28.91 years || 138 || 7.27 mg
|-
| [[plutonium-241|<sup>241</sup>Pu]] || 241.0568497 || 2.4982243×10<sup>21</sup> || 14.329 years || 104 || 9.66 mg
|-
| [[tritium|<sup>3</sup>H]] || 3.016049281320 || 1.996698393×10<sup>23</sup> || 12.32 years || 9.62×10<sup>3</sup> || 104 μg
|-
| [[radium-228|<sup>228</sup>Ra]] || 228.0310686 || 2.6409299×10<sup>21</sup> || 5.75 years || 273 || 3.67 mg
|-
| [[cobalt-60|<sup>60</sup>Co]] || 59.93381554 || 1.00479849×10<sup>22</sup> || 5.2714 years || 1.13×10<sup>3</sup> || 884 μg
|-
| [[polonium-210|<sup>210</sup>Po]] || 209.9828737 || 2.8679200×10<sup>21</sup> || 138.376 days || 4.49×10<sup>3</sup> || 223 μg
|-
| [[iodine-131|<sup>131</sup>I]] || 130.9061264 || 4.6003506×10<sup>21</sup> || 8.0249 days || 1.24×10<sup>5</sup> || 8.05 μg
|-
| [[iodine-123|<sup>123</sup>I]] || 122.9055898 || 4.8998103×10<sup>21</sup> || 13.2232 hours || 1.93×10<sup>6</sup> || 519 ng
|-
| [[lead-212|<sup>212</sup>Pb]] || 211.9918959 || 2.8407410×10<sup>21</sup> || 10.627 hours || 1.39×10<sup>6</sup> || 719 ng
|-
| [[francium-223|<sup>223</sup>Fr]] || 223.0197342 || 2.7002726×10<sup>21</sup> || 22.00 minutes || 3.83×10<sup>7</sup> || 26.1 ng
|-
| [[polonium-212|<sup>212</sup>Po]] || 211.9888680 || 2.8407816×10<sup>21</sup> || 294.4 nanoseconds || 1.81×10<sup>17</sup> || 5.53 ag
|}

==Radiation related quantities==
The following table shows radiation quantities in SI and non-SI units:
{{Radiation related quantities}}

==See also==
*[[Geiger counter]]
*[[Ionizing radiation]]
*[[Radiation burn]]
*[[Radiation exposure (disambiguation)|Radiation exposure]]
*[[Radiation poisoning]]
*[[United Nations Scientific Committee on the Effects of Atomic Radiation]]

==References==
{{reflist}}
{{reflist|group=lower-alpha}}

{{Marie & Pierre Curie}}
{{Scientists whose names are used as units}}
{{Ionising radiation related quantities}}

[[Category:Non-SI metric units]]
[[Category:Radioactivity]]
[[Category:Units of radioactivity]]
[[Category:Pierre Curie]]
[[Category:Radium]]