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.All D14C values are normalized to the base value of -25.0 per mille with respect to the standard carbonate (VPDB).
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.
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.
Ninety-five percent of the activity of Oxalic Acid from the year 1950 is equal to the measured activity of the absolute radiocarbon standard which is 1890 wood.
This is the International Radiocarbon Dating Standard.
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.
Should the activity of the sample be indistinguishable from the background activity at 1 standard deviation, it is released as background.