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The decay constant of a radioactive nuclide is defined by the relationship dNdt=λN where N is the number of atoms of the nuclide under consideration. Because radioactive decay is statistical in nature this relationship is approximate. However, geo-chronological work always involves the decay of a sufficient number of atoms of the parent nuclide to produce measurable quantities of the daughter; in this case the statistical fluctuations are of negligible importance. The decay constant is related to the half-life t 1/2 by the equation:  
and therefore any experimental method which determines the decay constant also determines the half-life. If the parent nuclide decays in two or more alternative ways (branching decay) there are as many decay constants as there are branches, and the relationship may be written  
The only branching decay of major importance in geochronology is that of K40 which decays into Ca40 (β decay) and into Ar40 (electron capture).

A discussion has been published [1] of the exact way in which the decay constants enter into the age equations, the errors in the calculated age resulting from possible errors in the decay constants, and experimental problems encountered in measuring the various decay constants.

Historically, geochronological measurements preceded counting techniques in determining good values for the decay constants of U235 [2], K40 [3], Rb87 [4], and Re187 [5,6], However, at the present time, with the exception of Re187, the counting methods should be superior, and only results obtained by counting experiments will be discussed.

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