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In an earlier section we mentioned that the limit of the technique is about 55-60 000 years. 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. 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. The scale represents log E (energy).

Conventional radiocarbon ages (BP)

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. 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.

The Conventional Radiocarbon Age BP is calculated using the radiocarbon decay equation:

t=-8033 ln(Asn/Aon)

Where -8033 represents the mean lifetime of 14C (Stuiver and Polach, 1977). Aon is the activity in counts per minute

of the modern standard, Asn is the equivalent cpm for the sample. 'ln' represents the natural logarithm.

A CRA embraces the following recommended conventions:

� a half-life of 5568 years;

� the use of Oxalic acid I or II, or appropriate secondary radiocarbon standards (e.g. ANU sucrose) as the modern

radiocarbon standard;

� correction for sample isotopic fractionation (deltaC13) to a normalized or base value of -25.0 per mille relative to

the ratio of C12/C13 in the carbonate standard VPDB (more on fractionation and deltaC13);

� the use of 1950 AD as 0 BP, ie all C14 ages head back in time from 1950;

� the assumption that all C14 reservoirs have remained constant through time.

Three further terms are sometimes given with reported radiocarbon dates. d14C, D14C and deltaC13.

All are expressed in per mille notation rather than per cent notation (%).

d14C represents the per mille depletion in sample carbon 14 prior to isotopic fractionation correction and is measured

by:

In an earlier section we mentioned that the limit of the technique is about 55-60 000 years. 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. 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. The scale represents log E (energy).

Conventional radiocarbon ages (BP)

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. 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.

The Conventional Radiocarbon Age BP is calculated using the radiocarbon decay equation:

t=-8033 ln(Asn/Aon)

Where -8033 represents the mean lifetime of 14C (Stuiver and Polach, 1977). Aon is the activity in counts per minute

of the modern standard, Asn is the equivalent cpm for the sample. 'ln' represents the natural logarithm.

A CRA embraces the following recommended conventions:

� a half-life of 5568 years;

� the use of Oxalic acid I or II, or appropriate secondary radiocarbon standards (e.g. ANU sucrose) as the modern

radiocarbon standard;

� correction for sample isotopic fractionation (deltaC13) to a normalized or base value of -25.0 per mille relative to

the ratio of C12/C13 in the carbonate standard VPDB (more on fractionation and deltaC13);

� the use of 1950 AD as 0 BP, ie all C14 ages head back in time from 1950;

� the assumption that all C14 reservoirs have remained constant through time.

Three further terms are sometimes given with reported radiocarbon dates. d14C, D14C and deltaC13.

All are expressed in per mille notation rather than per cent notation (%).

d14C represents the per mille depletion in sample carbon 14 prior to isotopic fractionation correction and is measured

by: