Formula for carbon 14 dating
Much of the information presented in this section is based upon the Stuiver and Polach (1977) paper "Discussion: Reporting of C14 data". 1890 wood was chosen as the radiocarbon standard because it was growing prior to the fossil fuel effects of the industrial revolution.
A copy of this paper may be found in the Radiocarbon Home Page The radiocarbon age of a sample is obtained by measurement of the residual radioactivity. T (National Institute of Standards and Technology; Gaithersburg, Maryland, USA) Oxalic Acid I (C). The activity of 1890 wood is corrected for radioactive decay to 1950.
d14C represents the per mille depletion in sample carbon 14 prior to isotopic fractionation correction and is measured by: D14C represents the 'normalized' value of d14C.
'Normalized' means that the activity is scaled in relation to fractionation of the sample, or its delta C13 value.
By measuring the activity of a background sample, the normal radioactivity present while a sample of unknown age is being measured can be accounted for and deducted.
In an earlier section we mentioned that the limit of the technique is about 55-60 000 years.
Should the activity of the sample be indistinguishable from the background activity at 1 standard deviation, it is released as background.
According to Stuiver and Polach (1977), all laboratories should report their results either directly related to NBS Oxalic acid or indirectly using a sub-standard which is related to it.
Beukens (1994) for instance has stated that this means the limit of the range for his Isotrace laboratory is 60 000 yr which is very similar to the conventional range.
Figure 1: This gif shows the comparison in radioactivity between a sample, or unknown (green area) , a modern standard (dark blue) and a background (small red peaks) derived from beta decay. A radiocarbon measurement, termed a conventional radiocarbon age (or CRA) is obtained using a set of parameters outlined by Stuiver and Polach (1977), in the journal Radiocarbon.
Standard errors released with each radiocarbon assay (see below) are usually rounded by convention (Stuiver and Polach, 1977).
Again, not all laboratories subscibe to these conventions, some do not round up ages.
The Conventional Radiocarbon Age BP is calculated using the radiocarbon decay equation: Where -8033 represents the mean lifetime of 14C (Stuiver and Polach, 1977).