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.Standard errors released with each radiocarbon assay (see below) are usually rounded by convention (Stuiver and Polach, 1977).Again, not all laboratories subscibe to these conventions, some do not round up ages.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.
'Normalized' means that the activity is scaled in relation to fractionation of the sample, or its delta C13 value.
Much of the information presented in this section is based upon the Stuiver and Polach (1977) paper "Discussion: Reporting of C14 data". 1890 wood was chosen as the radiocarbon standard because it was growing prior to the fossil fuel effects of the industrial revolution.
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.
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.
In order to make allowances for background counts and to evaluate the limits of detection, materials which radiocarbon specialists can be fairly sure contain no activity are measured under identical counting conditions as normal samples.