Carbon dating range
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.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.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).
Samples whose age falls between modern and background and are given finite ages.
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.
The Oxalic acid standard was made from a crop of 1955 sugar beet. The isotopic ratio of HOx I is -19.3 per mille with respect to (wrt) the PBD standard belemnite (Mann, 1983). T designation SRM 4990 C) was made from a crop of 1977 French beet molasses.
The Oxalic acid standard which was developed is no longer commercially available. In the early 1980's, a group of 12 laboratories measured the ratios of the two standards.
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.