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Radiocarbon dating

A method of obtaining age estimates on organic materials which has been used to date samples as old as 75,000 years. The method was developed immediately following World War II by Willard F. Libby and coworkers, and has provided age determinations in archeology, geology, geophysics, and other branches of science.

Radiocarbon (14C) determinations can be obtained on wood; charcoal; marine and fresh-water shell; bone and antler; peat and organic-bearing sediments; carbonate deposits such as tufa, caliche, and marl; and dissolved carbon dioxide (CO2) and carbonates in ocean, lake, and ground-water sources. Each sample type has specific problems associated with its use for dating purposes, including contamination and special environmental effects. While the impact of 14C dating has been most profound in archeological research and particularly in prehistoric studies, extremely significant contributions have also been made in hydrology and oceanography. In addition, beginning in the 1950s the testing of thermonuclear weapons injected large amounts of artificial 14C (“bomb 14C”) into the atmosphere, permitting it to be used as a geochemical tracer.

Basis of the Method

Carbon (C) has three naturally occurring isotopes. Both 12C and 13C are stable, but 14C decays by very weak beta decay (electron emission) to nitrogen-14 (14N) with a half-life of approximately 5700 years. Naturally occurring 14C is produced as a secondary effect of cosmic-ray bombardment of the upper atmosphere (Fig. 1). As 14CO2, it is distributed on a worldwide basis into various atmospheric, biospheric, and hydrospheric reservoirs on a time scale much shorter than its half-life. Metabolic processes in living organisms and relatively rapid turnover of carbonates in surface ocean waters maintain 14C levels at approximately constant levels in most of the biosphere. The natural 14C activity in the geologically recent contemporary “prebomb” biosphere was approximately 13.5 disintegrations per minute per gram of carbon.  See also: Cosmogenic nuclide; Isotope

 ig. 1  Generation, distribution, and decay of 14C.

 To the degree that 14C production has proceeded long enough without significant variation to produce an equilibrium or steady-state condition, 14C levels observed in contemporary materials may be used to characterize the original 14C activity in the corresponding carbon reservoirs. Once a sample has been removed from exchange with its reservoir, as at the death of an organism, the amount of 14C begins to decrease as a function of its half-life. A 14C age determination is based on a measurement of the residual 14C activity in a sample compared to the activity of a sample of assumed zero age (a contemporary standard) from the same reservoir. The relationship between the 14C age and the 14C activity of a sample is given by the equation below, where t

is radiocarbon years B.P. (before the present), λ is the decay constant of 14C (related to the half-life t1/2 by the expression t1/2 = 0.693/λ), Ao is the activity of the contemporary standards, and As is the activity of the unknown age samples. Conventional radiocarbon dates are calculated by using this formula, an internationally agreed half-life value of 5568 ± 30 years, and a specific contemporary standard. Most laboratories define the contemporary standard value by using one of the standards prepared by the U.S. National Bureau of Standards [NBS; now known as the U.S. National Institute of Standards and Technology (NIST)], or a standard with a known relationship to the NBS/NIST oxalic acid preparations.

Measurement of Radiocarbon

The naturally occurring isotopes of carbon occur in the proportion of approximately 98.9% 12C, 1.1% 13C, and 10−10% 14C. The extremely small amount of radiocarbon in natural materials was one reason why 14C was one of the isotopes which had been produced artificially in the laboratory before being detected in natural concentrations. The routine development of the radiocarbon method was made possible by the development by Libby of a practical method of low-level counting. To detect the very weak beta-decay characteristic of 14C, a means had to be devised to introduce the sample directly into the sensitive volume of a detector. In all of Libby's early work, the sample was converted to solid carbon (amorphous elemental carbon) and deposited on a sleeve which fitted inside a screen-wall type of Geiger counter. The counting rate of an unshielded screen-wall counter was on the order of 500 counts per minute.

Since the activity from 14C decay of a modern sample was expected to be about six or seven counts per minute, the total background counting rate had to be radically reduced. This was accomplished initially by placing the instrument in an iron shield with 20-cm (8-in.) walls. This reduced the activity in the detector to 120 counts per minute, still unacceptably high. The final reduction was made possible by enclosing the sample counter in a ring of smaller Geiger counters. The sample counter and the outer guard ring were connected together electronically so that any pulse from any of the outer Geiger tubes would inactivate the sample counter for about 10−3 s. This anticoincidence system reduced the background in the center detector to about five counts per minute.

With this system, the maximum age that could be measured was about 23,000 years and required the use of 10–12 g (0.35–0.42 oz) of carbon from sample materials. Because of self-absorption of the weak betas in the sample, the efficiency of the detector was only about 5%. Because of this and the susceptibility of the carbon black to contamination from airborne radioactive fallout, the solid carbon technique was replaced by either gas counters or liquid scintillation systems.  See also: Low-level counting; Radioactivity