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Symbol in

parentheses indicates type of emission;

E.C. = K-electron capture, S.F. = spontaneous fission; y =

years, d = days, h = hours, m

= minutes, s = seconds.

The curie (Ci) is thus the quantity of any radioactive material in which the number of disintegrations is 3.7 X 1010 per second. This is a rather large amount of radioactivity, and smaller quantities are expressed in such units as millicuries (1 mCi = 10-3 Ci), microcuries (1 fCi = 10-6 Ci), and picocuries (1 pCi = 10-12 Ci). Since the curie is a measure of the emission rate, it is not a satisfactory unit for setting safety standards for handling radioactive materials.

Radioactivity originates from natural and man-made sources. Man-made radioactive materials produce artificial radioactivity. The radioactivity produced from nuclear fission in a nuclear reactor is a classic example of artificial radioactivity.

Naturally occurring radioisotopes of higher atomic number elements belong to chains of successive disintegrations. The original element, which starts the whole decay series, is called the parent. The new elements formed are called daughters, and the whole chain is called a family. The parent of a natural radioactive series undergoes a series of disintegrations before reaching its stable form. When a series is in secular equilibrium, one Ci of the parent will coexist with one Ci of each of the daughters. Three series, the uranium, actinium, and thorium, make up most of the naturally radioactive elements found in the periodic table (Figure 11.21.2).

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