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] who was on the standards committee and perhaps a more likely candidate for the naming honors.

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), equal to one reciprocal 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 have an activity 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:

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

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	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
^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	1132
^210Po	138 days	223 μg	4484
^3H	12.32 years	104 μg	9621
^131I	8.02 days	8 μg	125000
^123I	13 hours	0.5 μg	2000000

Radiation related quantities

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

Quantity	Name	Symbol	Unit	Year
Exposure (X)	roentgen	R	esu / 0.001293 g of air	1928
Absorbed dose (D)			erg•g^−1	1950
	rad	rad	100 erg•g^−1	1953
	gray	Gy	J•kg^−1	1975
Activity (A)	curie	Ci	3.7 × 10^10 s^−1	1953
	becquerel	Bq	s^−1	1975
	rutherford	Rd	10^6 s^−1	1946
Dose equivalent (H)	roentgen equivalent man	rem	100 erg•g^−1	1971
	sievert	Sv	J•kg^−1	1977
Fluence (Φ)	(reciprocal area)		cm^−2 or m^−2	1962

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.
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.