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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.
In an earlier section we mentioned that the limit of the technique is about 55-60 000 years.The terms "%Modern", or "pm C" and D14C are shown related in this diagram along with the Radiocarbon age in years BP (Before 1950 AD).If the reservoir corrected conventional radiocarbon age calculated is within the past 200 years, it should by convention be termed 'Modern' (Stuiver and Polach, 192).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).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.
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.
Background samples usually consist of geological samples of infinite age such as coal, lignite, limestone, ancient carbonate, athracite, marble or swamp wood.
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.
Obviously, the limit of the method differs between laboratories dependent upon the extent to which background levels of radioactivity can be reduced.