The curie 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, but was considered at least by some to be in honour of Marie Curie as well. It was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium ", but is currently defined as 1 Ci = decaysper second after more accurate measurements of the activity of 226Ra. In 1975 the General Conference on Weights and Measures gave the becquerel, defined as one nuclear decay per second, official status as the SI unit of activity. Therefore: and While its continued use is discouraged by National Institute of Standards and Technology 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. 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 quantity of anything was altogether inappropriate". 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. For example, the median lethal dose for ingested polonium-210 is 240 μCi; about 53.5 nanograms. The typical human body contains roughly 0.1 μCi of naturally occurringpotassium-40. A human body containing 16 kg of carbon 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.
As a measure of quantity
Units of activity 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 and so where λ 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
232Th
years
9.1 tonnes
238U
years
2.977 tonnes
40K
years
140 kg
235U
years
463 kg
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 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:
