Dinosaur tracks imprinted in rock in Navajo Canyon, Arizona, arouse the professional interest of this scientist. Fossil traces of extinct prehistoric creatures were for a long time the best clues to the age of rock formations.
A scientist using liquid nitrogen to freeze carbon dioxide gas made from a sample of ancient material that he is preparing for age determination by the carbon-14 technique.
For example, a bit of a rafter from a prehistoric cliff-dwelling or a remnant of charcoal from an ancient fire may be analyzed for its remaining ¹⁴C content, and its age determined accurately within the margin of a few hundred years. This fixes the time at which the wood for the rafter or the firewood was broken or cut from the living tree, and hence the period in which the men lived who used the wood.
The most useful samples for carbon-14 age determination are charcoal, wood, and shells.
Carbon-14 Counting
Carbon-14 measurements are made by taking a known amount of carbon, reducing it to a gas, and then counting the ¹⁴C disintegrations in the gas. This may sound simple, but in reality the measurement process is a formidable undertaking, because the amount of the ¹⁴C isotope in the carbon is so extremely small. (The remainder of the carbon, of course, consists of other isotopes—¹²C or ¹³C, which are stable.)
There are two basic techniques. The carbon can be: