Carbon dating range
The Conventional Radiocarbon Age BP is calculated using the radiocarbon decay equation: Where -8033 represents the mean lifetime of 14C (Stuiver and Polach, 1977).Aon is the activity in counts per minute of the modern standard, Asn is the equivalent cpm for the sample. A CRA embraces the following recommended conventions: correction for sample isotopic fractionation (delta C13) to a normalized or base value of -25.0 per mille relative to the ratio of C12/C13 in the carbonate standard VPDB (more on fractionation and delta C13); Three further terms are sometimes given with reported radiocarbon dates. All are expressed in per mille notation rather than per cent notation (%).Another standard, Oxalic Acid II was prepared when stocks of HOx 1 began to dwindle. The ratio of the activity of Oxalic acid II to 1 is 1.29330.001 (the weighted mean) (Mann, 1983). There are other secondary radiocarbon standards, the most common is ANU (Australian National University) sucrose.The ratio of the activity of sucrose with 0.95 Ox was first measured by Polach at 1.50070.0052 (Polach, 1976b:122).Thus, %Modern becomes a useful term in describing radiocarbon measurements for the past 45 years when, due to the influx of artificial radiocarbon into the atmosphere as a result of nuclear bomb testing the 'age' calculation becomes a 'future' calculation.If the sample approaches D14C = -1000 per mille within 2 standard deviations, it is considered to be indistinguishable from the laboratory background, ie, not able to be separated with confidence from the laboratory countrates which result from a sample which contains no radionuclide. An example of a minimum age is 50, 000 yr (Gupta and Polach, 1985).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 is calculated using: Figure 1: Decay curve for C14 showing the activity at one half-life (t/2).
If a sample age falls after 1950, it is termed greater than Modern, or Where Aabs is the absolute international standard activity, 1/8267 is the lifetime based on the new half life (5730 yr), Y = the year of measurement of the appropriate standard.
This is an expression of the ratio of the net modern activity against the residual normalised activity of the sample, expressed as a percentage and it represents the proportion of radiocarbon atoms in the sample compared to that present in the year 1950 AD.
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.