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The discovery of radioactivity in 1896 by Henri Becquerel, the isolation of radium by Marie Curie shortly thereafter, the discovery of the radioactive decay laws in 1902 by Ernest Rutherford and Frederick Soddy, the discovery of isotopes in 1910 by Soddy, and the development of the quantitative mass spectrograph in 1914 by J. Thomson all formed the foundation of modern isotopic dating methods.

But it was not until the late 1950s that all the pieces were in place; by then the phenomenon of radioactivity was understood, most of the naturally occurring isotopes had been identified and their abundance determined, instrumentation of the necessary sensitivity had been developed, isotopic tracers were available in the required quantities and purity, and the half-lives of the long-lived radioactive isotopes were reasonably well known.

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Some of the isotopic parents, end-product daughters, and half-lives involved are listed in Table 1.

Trees undergo spurts in growth in the spring and summer months while becoming somewhat dormant in the fall and winter months.

When a tree is cut down, these periods are exhibited in a cross section of the trunk in the form of rings.

Simply counting the number of rings will give one a fairly good idea of the age of the tree.

Periods of heavy rain and lots of sunshine will make larger gaps of growth in the rings, while periods of drought might make it difficult to count individual rings. When a given quantity of an isotope is created (in a supernovae, for example), after the half-life has expired, 50% of the parent isotope will have decomposed into daughter isotopes.