Curie (unit)
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Curie Unit
Curie
Radium226.jpg
A sample of radium, the element which was used in the original definition of the curie.
General information
Unit ofActivity
SymbolCi 
Named afterPierre Curie
Conversions
rutherfords
SI derived unit
SI base unit

The curie (symbol Ci) is a non-SI unit of radioactivity originally defined in 1910. According to a notice in Nature at the time, it was named in honour of Pierre Curie,[1] but was considered at least by some to be in honour of Marie Curie as well.[2]

It was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium (element)",[1] but is currently defined as 1 Ci = decays per second after more accurate measurements of the activity of 226Ra (which has a specific activity of [3]).

In 1975 the General Conference on Weights and Measures gave the becquerel (Bq), defined as one nuclear decay per second, official status as the SI unit of activity.[4] Therefore:

1 Ci = = 37 GBq

and

1 Bq ? ? 27 pCi

While its continued use is discouraged by National Institute of Standards and Technology (NIST)[5] 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 nanograms 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".[2]

The power emitted in radioactive decay corresponding to one curie can be calculated by multiplying the decay energy by approximately 5.93 mW/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.

Ingesting even a millicurie is usually fatal (unless it is a very short-lived isotope). For example, the median lethal dose (LD-50) for ingested polonium-210 is 240 ?Ci; about 53.5 nanograms.

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


As a measure of quantity

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 Ci of radioactive atoms would follow the expression

N (atoms) × ? (s-1) = 1 Ci = 3.7 × 1010 Bq,

and so

N = 3.7 × 1010 Bq / ?,

where ? is the decay constant in s-1.

We can also express activity in moles:

where NA is Avogadro's number, and t1/2 is the half-life. The number of moles may be converted to grams by multiplying by the atomic mass.

Here are some examples, ordered by half-life:

Isotope Half-life Mass of 1 curie Specific activity (Ci/g)
232Th years 9.1 tonnes (110,000 pCi/g, 0.11 ?Ci/g)
238U years 2.977 tonnes (340,000 pCi/g, 0.34 ?Ci/g)
40K years 140 kg (7,100,000 pCi/g, 7.1 ?Ci/g)
235U years 463 kg (2,160,000 pCi/g, 2.2 ?Ci/g)
129I years 5.66 kg 0.00018
99Tc years 58 g 0.017
239Pu years 16 g 0.063
240Pu 6563 years 4.4 g 0.23
14C 5730 years 0.22 g 4.5
226Ra 1601 years 1.01 g 0.99
241Am 432.6 years 0.29 g 3.43
238Pu 88 years 59 mg 17
137Cs 30.17 years 12 mg 83
90Sr 28.8 years 7.2 mg 139
241Pu 14 years 9.4 mg 106
3H 12.32 years 104 ?g 9,621
228Ra 5.75 years 3.67 mg 273
60Co 1925 days 883 ?g 1,132
210Po 138 days 223 ?g 4,484
131I 8.02 days 8 ?g 125,000
123I 13 hours 518 ng 1,930,000
212Pb 10.64 hours 719 ng 1,390,000

Radiation related quantities

The following table shows radiation quantities in SI and non-SI units:

Ionizing radiation related quantities view ? talk ? edit
Quantity Unit Symbol Derivation Year SI equivalence
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 1,000,000 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 x WR 1971 0.010 Sv
Effective dose (E) sievert Sv J?kg-1 × WR x WT 1977 SI unit
röntgen equivalent man rem 100 erg?g-1 x WR x WT 1971 0.010 Sv

See also

References

  1. ^ a b Rutherford, Ernest (6 October 1910). "Radium Standards and Nomenclature". Nature. 84 (2136): 430-431. Bibcode:1910Natur..84..430R. doi:10.1038/084430a0.
  2. ^ a b Frame, Paul (1996). "How the Curie Came to Be". Health Physics Society Newsletter. Retrieved 2015.
  3. ^ Delacroix, D. (2002). Radionuclide and Radiation Protection Data Handbook 2002. Radiation Protection Dosimetry, Vol. 98 No. 1: Nuclear Technology Publishing. p. 147.CS1 maint: location (link)
  4. ^ "SI units for ionizing radiation: becquerel". Resolutions of the 15th CGPM (Resolution 8). 1975. Retrieved 2015.
  5. ^ "Nist Special Publication 811, paragraph 5.2". NIST. Retrieved 2016.

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Curie_(unit)
 



 



 
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