# 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 (%).

All D14C values are normalized to the base value of -25.0 per mille with respect to the standard carbonate (VPDB).

D14C is calculated using: Figure 1: Decay curve for C14 showing the activity at one half-life (t/2).

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.

A time-independent level of C14 activity for the past is assumed in the measurement of a CRA.

The activity of this hypothetical level of C14 activity is equal to the activity of the absolute international radiocarbon standard.

If the reservoir corrected conventional radiocarbon age calculated is within the past 200 years, it should by convention be termed 'Modern' (Stuiver and Polach, 192).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).It is vital for a radiocarbon laboratory to know the contribution to routine sample activity of non-sample radioactivity.Obviously, this activity is additional and must be removed from calculations.