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.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).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.
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).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.Beukens (1994) for instance has stated that this means the limit of the range for his Isotrace laboratory is 60 000 yr which is very similar to the conventional range.Figure 1: This gif shows the comparison in radioactivity between a sample, or unknown (green area) , a modern standard (dark blue) and a background (small red peaks) derived from beta decay. A radiocarbon measurement, termed a conventional radiocarbon age (or CRA) is obtained using a set of parameters outlined by Stuiver and Polach (1977), in the journal Radiocarbon.
This is calculated through careful measurement of the residual activity (per gram C) remaining in a sample whose age is Unknown, compared with the activity present in Modern and Background samples. Thus 1950, is year 0 BP by convention in radiocarbon dating and is deemed to be the 'present'.