Page 11 USGS 12 USGS 14, 16, 17 Willard F. Libby 18 University of Arizona 21 Massachusetts Institute of Technology 22 Graduate Research Center of the Southwest 23 Graduate Research Center of the Southwest 25 Robert L. Fleischer 26 Perkin-Elmer Corporation 30 University of Texas 32 Graduate Research Center of the Southwest 36 Massachusetts Institute of Technology 40 Des Bartlett, Armand Denis Productions 43 Anthropology Department, SMU 44 USGS 47 USGS 59 Author’s photo courtesy Graduate Research Center of the Southwest Footnotes [1]Words appearing in SMALL CAPITAL LETTERS are defined in the Glossary beginning on page 49. [2]The process of natural radioactive decay is described in the Appendix beginning on page 52. [3]The U. S. War Department program during World War II that developed the first nuclear weapons. [4]Drawn to scale, the whole age of man is represented by less than the width of the line. [5]For more information on the structure of atoms, see _Our Atomic World_, a companion booklet in this series. [6]From the _Chart of the Nuclides_, prepared by David T. Goldman, Knolls Atomic Power Laboratory, August 1964. [7]This decay process proceeds in a series of steps, during which 6 alpha particles and 4 beta particles are emitted. (See Appendix.) [8]Named after their creator, John Napier, a Scottish mathematician (1550-1617), who also invented the decimal point. [9]It is difficult to determine the half-life of ¹⁴C exactly. In the early days of ¹⁴C dating, in order not to delay continued work, an arbitrary value of 5568 years was chosen and this value is still used in calculations. [10]This means that uranium decays through successive steps in which the entire series emits eight alpha particles. (See Appendix.) [11]Remember, this enormous period of time is a measure of the _rate_ of spontaneous fission, _not_ of the age of ²³⁸U. [12]The rhenium-osmium scheme is shown below the dotted line because the method is still in an early experimental stage and its general utility is not yet established. [13]For more on this family of elements, see _Rare Earths, The Fraternal Fifteen_, a companion booklet in this series. [14]For a fuller explanation of the fission process, see _Our Atomic World_, another booklet in this series. [15]Neutrons that have had their speed reduced by passing through a moderator (graphite, for example) which is built into every reactor to accomplish this very thing. For more about how this is done, see _Nuclear Reactors_ and _Research Reactors_, companion booklets in this series.