Carbon dating absolute dating
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).
Later inter-laboratory measurements put the ratio at 1.5081 (Currie and Polach, 1980).
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.
Should the activity of the sample be indistinguishable from the background activity at 1 standard deviation, it is released as background.
Samples whose age falls between modern and background and are given finite ages.
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.
You can get an idea of the relationship between C14 and age at the Carbon Dating calculator page. 1950 was chosen for no particular reason other than to honour the publication of the first radiocarbon dates calculated in December 1949 (Taylor, 19).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).Obviously, the limit of the method differs between laboratories dependent upon the extent to which background levels of radioactivity can be reduced.Amongst accelerator laboratories there has been mooted the theoretical possibility of extended range dating to 75 000 yr , at present this seems difficult to attain because of the problems in accurately differentiating between ions that mimic the mass and charge characteristics of the C14 atom.