Radiocarbon direct counting

The first entry in Table 4 refers to the accelerator (AMS) experiments noted earlier in which we investigated (a) the feasibility of direct atom counting given only a few micrograms of carbon (using the new international radiocarbon dating... 

Expression and Interpretation of Results. Archaeological interpretation of a radiocarbon age may depend critically on the error associated with that age. Errors are commonly expressed as a variance range attached to the central number (e.g., 2250 80 years). The 80 years in this example may correspond to the random error for a single analytical step. Both decay and direct-atom counting are statistical in nature, and lead to errors that vary as the square root of the number of counts. The error may also be expressed as the overall random experimental error (the sum of individual errors.) Overall random error can be determined only by analyzing replicate samples. 

In the more than 30 years that have followed, radiocarbon studies have evolved through two generations of instrumental methods. Libby employed solid carbon counting (combined with an application of the anticoincidence principle to reduce background count rate) to establish the fundamental validity of the method (6). This goal was achieved in December 1949 with the publication of the famous Curve of Knowns, which demonstrated that the residual content of a series of samples was directly related to their age (7). 

Radiometric measurement of the C levels therefore, while requiring sophisticated instrumentation, is nonetheless the method of choice in our view. The normal procedure used in radiocarbon C analyses is to combust the sample, recover the resultant CO2, and convert it first to acetylene and then to benzene, as described above for the cinnamic aldehyde analysis. However, caffeine cannot be treated in this fashion due to its tendency to sublime from the combustion chamber to the catalyst traps before combustion is complete. Thus, a direct liquid scintillation counting technique was required, and the low-level liquid scintillation counting apparatus described by Noakes (18) was used to make all measurements. 

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