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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 (%).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.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.This is calculated through careful measurement of the residual activity (per gram C) remaining in a sample whose age is Unknown, compared with the activity present in Modern and Background samples. Thus 1950, is year 0 BP by convention in radiocarbon dating and is deemed to be the 'present'.
Should the activity of the sample be indistinguishable from the background activity at 1 standard deviation, it is released as background.Background samples usually consist of geological samples of infinite age such as coal, lignite, limestone, ancient carbonate, athracite, marble or swamp wood.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.The ratio of the activity of sucrose with 0.95 Ox was first measured by Polach at 1.50070.0052 (Polach, 1976b:122).Later inter-laboratory measurements put the ratio at 1.5081 (Currie and Polach, 1980).