Curie (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]

Curie
A sample of radium, the element which was used in the original definition of the curie.
General information
Unit of	Activity
Symbol	Ci
Named after	Pierre Curie
Conversions
1 Ci in ...	... is equal to ...
rutherfords	37000 Rd
SI derived unit	37 GBq
SI base unit	3.7×10^10 s^−1

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 3.66×10¹⁰ 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 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 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 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 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) × λ (s⁻¹) = 1 Ci = 3.7 × 10¹⁰ Bq,

and so

N = 3.7 × 10¹⁰ Bq / λ,

where λ is the decay constant in s⁻¹.

We can also express activity in moles:

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

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, ordered by half-life:

Isotope	Half-life	Mass of 1 curie	Specific activity (Ci/g)
^232Th	1.405×10^10 years	9.1 tonnes	1.1×10^−7 (110,000 pCi/g, 0.11 µCi/g)
^238U	4.471×10^9 years	2.977 tonnes	3.4×10^−7 (340,000 pCi/g, 0.34 µCi/g)
^40K	1.25×10^9 years	140 kg	7.1×10^−6 (7,100,000 pCi/g, 7.1 µCi/g)
^235U	7.038×10^8 years	463 kg	2.2×10^−6 (2,160,000 pCi/g, 2.2 µCi/g)
^129I	15.7×10^6 years	5.66 kg	0.00018
^99Tc	211×10^3 years	58 g	0.017
^239Pu	24.11×10^3 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 μg	9,621
^228Ra	5.75 years	3.67 mg	273
^60Co	1925 days	883 μg	1,132
^210Po	138 days	223 μg	4,484
^131I	8.02 days	8 μg	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

Quantity	Unit	Symbol	Derivation	Year	SI equivalence
Activity (A)	becquerel	Bq	s^−1	1974	SI unit
	curie	Ci	3.7 × 10^10 s^−1	1953	3.7×10^10 Bq
	rutherford	Rd	10^6 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

See also

• Geiger counter
• Ionizing radiation
• Radiation exposure
• Radiation poisoning
• Radiation burn
• United Nations Scientific Committee on the Effects of Atomic Radiation

References

1. Rutherford, Ernest (6 October 1910). "Radium Standards and Nomenclature". Nature. 84 (2136): 430–431. Bibcode:1910Natur..84..430R. doi:10.1038/084430a0.
2. Frame, Paul (1996). "How the Curie Came to Be". Health Physics Society Newsletter. Retrieved 3 July 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 3 July 2015.
5. "Nist Special Publication 811, paragraph 5.2". NIST. Retrieved 22 March 2016.