A unit that describes the amount of a radioactive substance in terms of the rate at which its atoms are undergoing radioactive decay. Its value was originally set in a fashion that gave ²²⁶radium a specific activity of approximately 1 curie per gram. The unit is usually encountered as the millicurie (a thousandth of a curie) or microcurie (a millionth of a curie). The curie is still in use but is supposed to be replaced by an SI unit, the becquerel; 1 curie = 3.7 × 10¹⁰ becquerel.

The curie was first defined by the International Radium Standards Committee at a meeting of the International Congress on Radiology and Electricity in Brussels in 1910. Among its members the committee included such eminences as Marie Curie, Ernest Rutherford and Frederick Soddy. The primary task of the committee was to define a unit for radioactivity.

The curie was named for the then recently deceased Pierre Curie (1859–1906), French physicist and husband of Marie, and defined as the quantity of “radium emanation” (i.e., ²²²radon, half life 3.8 days) in equilibrium with 1 gram of ²²⁶radium (half life, 1600 years), which requires some explanation. Imagine some ²²⁶radium is put in a sealed bottle. When a radium atom undergoes radioactive decay, an atom of the radioactive gas ²²²radon is produced. Since the gas is also radioactive, eventually the radon atom will decay to produce a third substance. The amount of radon in the bottle will continue to increase as long as the number of radon-destroying disintegrations per second among the radon atoms is less than the number of radon-producing disintegrations among the radium atoms. When enough radon accumulates that those numbers are equal, the radon and radium are said to be in equilibrium. Thereafter the ratio of radium atoms to radon atoms will be constant.

Then as now the curie is a very large unit in relation to the quantities that workers usually handle. Its size is due to the insistence of Marie Curie at the 1910 meeting that she did not want her husband's name on a unit that represented an infinitesimal quantity.

Marie Curie was asked by the committee to prepare a standard, which she did. The first radium standard, a glass tube containing 21.99 milligrams of anhydrous radium chloride, was left with the BIPM in 1913.

In 1930, the International Radium Standard Commission made the curie the equilibrium quantity of any decay product of radium. In practice, however, workers in the field had begun to use a different definition for the curie, applying it to any radioactive substance. To them, a curie was that quantity of any radioactive substance in which 3.7 × 10¹⁰ disintegrations occur each second (which is the number of disintegrations per second in 1 gram of radium). In 1948, the Committee on Standards and Units of Radioactivity of the National Research Council (United States) recommended that this unofficial definition be made the official definition of the curie. At a meeting in Paris in July 1950, the Joint Commission of the International Council of Scientific Unions on Standards, Units and Constants of Radioactivity recommended the above definition.¹

The curie is typically used in working with radioactive isotopes that are used, for example, as tracers. If a substance has a specific activity of 41 millicuries per gram, in a milligram of it will there will be (3.7 × 10¹⁰ × 0.041, ÷ 1000 =) 1,517,000 nuclear disintegrations per second.

1. National Research Council.
A Glossary of Terms in Nuclear Science and Technology.
New York: American Society of Mechanical Engineers, 1955.

Page 41.


Radium Emanation.- Radium resembles thorium very closely in the character of its radio-active disintegration products. It gives a gaseous emanation possessing an almost completely similar physical and chemical nature to the thorium emanation, resisting absorption by all known reagents,* condensing and becoming non-volatile at low temperatures, and exhibiting to an even more marked degree than the thorium emanation the property of being retained by the dry solid compounds producing it, and of being liberated therefrom by heat and solution. It is distinguished by a much slower rate of change. Dorn (Abk. der Naturforsch. Ges. für Halle, 1900) first recognised the existence of the radium emanation by testing a sample of radio-active barium prepared from pitchblende in a manner similar to that already described by Rutherford for thorium, and pointed out that its activity lasted a much longer time than in the case of the thorium emanation. It can still be detected several weeks after it has been separated from the radium producing it. Owing to the comparatively slow rate of disappearance, the radium emanation, when it does not succeed in escaping, accumulates in the compound producing it to a much more marked extent than in the case of thorium. It has been shown that the amount of any transition-form capable of accumulating reaches an equilibrium value when the amount produced per second equals the amount disintegrating per second. By definition λ is the fraction of the total amount disintegrating per second. If N0 represents the equilibrium quantity of emanation, and q₀ the quantity produced per second, q₀= λN₀, or N₀/q₀= 1/λ. λ for the thorium emanation is ¹⁄₈₇, and for the radium emanation 1/463,000, or 6,000 times smaller. Hence, if compounds of radium and thorium are taken and kept under conditions in which the radio-active emanation does not escape (most conveniently in the form of solutions in stoppered vessels) for a sufficient length of time for the equilibrium point to be reached in each case, the maximum amount of radium emanation accumulating in the bottle will be 463,000 times the amount produced per second, while in the case of the thorium emanation the maximum amount will be only 87 times the amount produced per second. These relations have been quantitatively verified by actual experiments (Rutherford and Soddy, Phil. Mag., 1903, VI., 5., p. 450).

* M. and Mme. Curie have described some experiments which, when interpreted, seem to indicate that celluloid has the power of absorbing the emanation and retaining it. It would be interesting if further experiments could be carried out on this point.

Frederick Soddy.
Radio-Activity: an elementary treatise from the standpoint of tbe disintegration theory.
London: “The Electrician” Printing & Publishing Company, 1904.
Pages 130 & 131.

The activity of thorium X must always be decaying at the same rate, whether it is present in the thorium compound producing it or whether it is separated from it. The apparent constancy of activity of the radio-elements results from the continuous production of fresh matter possessing continuously diminishing radio-activity. When the increase in radio-activity through the production of new active matter balances the decrease through the decay of the activity of that already formed, the activity remains constant. This point may be termed the radio-active equilibrium. The mystery of the constancy and permanence of the emission of rays from radio-active substances, which from the first riveted attention on the new property, is, therefore, one step, nearer elucidation. The rays in question are always dying down and are always being renewed. The constancy is apparent rather than real, and is due to the balance or equilibrium between opposed processes.

Soddy, as above, pages 92 & 93.

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