Curie

The curie (symbol Ci) is a non-SI unit of radioactivity, named 'in honour of' Pierre Curie,^[1] according to his widow, the famed radiation researcher Marie Curie.^[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 = 3.7 × 10¹⁰ decays per second after more accurate measurements of the activity of ²²⁶Ra (which has a specific activity of 3.66 x 10¹⁰ Bq/g.^[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 = 3.7 × 10¹⁰ Bq = 37 GBq

and

1 Bq ≅ 2.703 × 10⁻¹¹ Ci ≅ 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 the government, industry and medicine in the United States and in other countries.

The curie is a large amount of activity, and was intentionally so. 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 typical human body contains roughly 0.1 μCi (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 μCi of Carbon-14. Together, these would result in a total of approximately 0.2 μCi or 7400 Bq inside the person's body.

Curie 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) × λ (s⁻¹) = 1 Ci = 3.7 × 10¹⁰ (Bq)

and so,

N = 3.7 × 10¹⁰ / λ,

where λ is the decay constant in (s⁻¹).

We can also express activity in moles:

{\begin{aligned}{\text{1 Ci}}&={\frac {3.7\times 10^{10}}{(\ -\ln 1/2)N_{\rm {A}}}}{\text{ moles}}\times t_{1/2}{\text{ in seconds}}\\&\approx 8.8639\times 10^{-14}{\text{ moles}}\times t_{1/2}{\text{ in seconds}}\\&\approx 5.3183\times 10^{-12}{\text{ moles}}\times t_{1/2}{\text{ in minutes}}\\&\approx 3.1910\times 10^{-10}{\text{ moles}}\times t_{1/2}{\text{ in hours}}\\&\approx 7.6584\times 10^{-9}{\text{ moles}}\times t_{1/2}{\text{ in days}}\\&\approx 2.7972\times 10^{-6}{\text{ moles}}\times t_{1/2}{\text{ in years}}\end{aligned}}

where N_A is Avogadro's number and t_1/2 is the half life. The number of moles may be converted to grams by multiplying by the atomic mass.

Here are some examples:

Isotope	Half life	Mass of 1 curie	Specific activity (Ci/g)
²³²Th	7010140500000000000♠1.405×10¹⁰ years	9.1 tonnes	6993110000000000000♠1.1×10⁻⁷ (110,000 pCi/g, 0.11 µCi/g)
²³⁸U	7009447100000000000♠4.471×10⁹ years	2.977 tonnes	6993340000000000000♠3.4×10⁻⁷ (340,000 pCi/g, 0.34 µCi/g)
⁴⁰K	7009125000000000000♠1.25×10⁹ years	140 kg	6994709999999999999♠7.1×10⁻⁶ (7,100,000 pCi/g, 7.1 µCi/g)
²³⁵U	7008703800000000000♠7.038×10⁸ years	463 kg	6994220000000000000♠2.2×10⁻⁶ (2,160,000 pCi/g, 2.2 µCi/g)
¹²⁹I	7007157000000000000♠15.7×10⁶ years	5.66 kg	0.00018
⁹⁹Tc	7005211000000000000♠211×10³ years	58 g	0.017
²³⁹Pu	7004241100000000000♠24.11×10³ years	16 g	0.063
²⁴⁰Pu	6563 years	4.4 g	0.23
²²⁶Ra	1601 years	1.01 g	0.99
²⁴¹Am	432.6 years	0.29 g	3.43
¹⁴C	5730 years	0.22 g	4.5
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
⁶⁰Co	1925 days	883 μg	1,132
²¹⁰Po	138 days	223 μg	4,484
³H	12.32 years	104 μg	9,621
¹³¹I	8.02 days	8 μg	125,000
¹²³I	13 hours	0.5 μg	2,000,000

Radiation related quantities

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

Radiation related quantities
Quantity	Name	Symbol	Unit	Year	SI Quantity
Activity (A)	curie	Ci	3.7×10¹⁰ s⁻¹	1953	3.7×10¹⁰ Bq
	becquerel	Bq	s⁻¹	1974	SI
	rutherford	Rd	10⁶s⁻¹	1946	1,000,000 Bq
Exposure (X)	röntgen	R	esu / 0.001293g of air	1928	2.58×10⁻⁴ C/kg
Fluence (Φ)	(reciprocal area)		m⁻²	1962	SI
Absorbed dose (D)			erg·g⁻¹	1950	1.0×10⁻⁴ Gy
	rad	rad	100 erg·g⁻¹	1953	0.010 Gy
	gray	Gy	J·kg⁻¹	1974	SI
Dose equivalent (H)	röntgen equivalent man	rem	100 erg·g⁻¹	1971	0.010 Sv
Dose equivalent (H)	sievert	Sv	J·kg⁻¹×W_R	1977	SI

References

1 2 Rutherford, Ernest (6 October 1910). "Radium Standards and Nomenclature". Nature. 84 (2136): 430–431. Bibcode:1910Natur..84..430R. doi:10.1038/084430a0.
1 2 Frame, Paul (1996). "How the Curie Came to Be". Health Physics Society newsletter. Retrieved 3 July 2015.
↑ Delacroix, D (2002). Radionuclide and Radiation Protection Data Handbook 2002. RADIATION PROTECTION DOSIMETRY Vol. 98 No 1: Nuclear Technology Publishing. p. 147.
↑ "SI units for ionizing radiation: becquerel". Resolutions of the 15th CGPM (Resolution 8). 1975. Retrieved 3 July 2015.
↑ "Nist Special Publication 811, paragraph 5.2". NIST. Retrieved 22 March 2016.

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Curie

Curie as a measure of quantity

Radiation related quantities

See also

References