In AMS, the carbon or "graphite" derived from a sample is compressed into a small cavity in an aluminum "target" which acts as a cathode in the ion source. The surface of the graphite is sputtered with heated cesium and the ions produced are extracted and accelerated in the AMS system. After acceleration and removal of electrons, the emerging positive ions are magnetically separated by mass and the 12C and 13C ions are measured in Faraday Cups where a ratio of their currents is recorded. Simultaneously the 14C ions are recorded in a gas ionization counter, so that instantaneous ratios of 14C to 13C and 12C are recorded. These are the raw signals that are ultimately converted to a radiocarbon age.
From a contemporary sample, about 150 14C counts per second are collected. It is expected then, for a 5,570 year (1 half-life) or 11,140 year old (2 half-lives) sample that 75 or 38 counts per second would be obtained. Although one can simply measure older samples for longer times, they are constantly being consumed by the ion source, so there are practical limits to the minimum sample activity that can be measured, depending on how much material you have. At the present time, for a 1 milligram sample of graphite, this limiting age is about ten half-lives, or 60,000 years, if set only by the sample size. However, limiting ages or "backgrounds" are also determined by process blanks which correspond to the method used to extract the carbon from the sample.
The process blanks contain small but measurable amounts of 14C from contamination introduced during chemical preparation, collection or handling. Organic materials, which require the most processing, are limited to younger ages by their corresponding process blank. Since it is always necessary to subtract the counts due to blanks, from the counts due to sample, it may become a statistical limitation for very old samples (small number of 14C atoms) where we are measuring the difference between very small numbers. Thus, ages are limited by the age of the process blanks (more on that below) and by the statistical uncertainty of the 14C measurement.
The Fraction Modern (Fm) is computed from the expression:
Where B, S and M represent the 14C/12C ratios of the blank, the sample and the modern reference, respectively.
Fraction Modern is a measurement of the deviation of the 14C/12C ratio of a sample from "Modern." Modern is defined as 95% of the radiocarbon concentration (in AD 1950) of NBS Oxalic Acid I normalized to δ13CVPDB=-19 per mil (Olsson, 1970). AMS results are calculated using the internationally agreed upon definition of 0.95 times the specific activity of NBS Oxalic Acid I (SRM 4990B) normalized to δ13CVPDB=-19 per mil. This is equialent to an absolute (AD 1950) 14C/12C ratio of 1.176 ± 0.010 x 10-12 (Karlen, et. al., 1968); all results are normalized to -25 per mil using the δ13CVPDB of the sample (see below). The value used for this correction is specified in the report of final results.
In addition to loss through decay of radiocarbon, 14C is also affected by natural isotopic fractionation. Fractionation is the term used to describe the differential uptake of one isotope with respect to another. While the three carbon isotopes are chemically indistinguishable, lighter 12C atoms are preferentially taken up before the 13C atoms in biological pathways. Similarly, 13C atoms are taken up before 14C. The assumption is that the fractionation of 14C relative to 12C is twice that of 13C, reflecting the difference in mass. Fractionation must be corrected for in order to make use of radiocarbon measurements as a chronometric tool for all parts of the biosphere. In order to remove the effects of isotopic fractionation, the Fraction Modern is then corrected to the value it would have if its original δ13C were -25 per mil (the δ13C value to which all radiocarbon measurements are normalized.)
Atoms of 14C contained in a sample are directly counted using the AMS method of radiocarbon analysis. Accordingly, we calculate an internal statistical error using the total number of 14C counts measured for each target ( ± √n ). An external error is calculated from the reproducibility of multiple exposures for a given target. For example, we may measure the 14C /12C of a sample up to 9 separate times over the course of a 2-day period. The reproducibility of these measurements gives us a good estimate of the true experimental error. The final error is the larger of the internal or external errors.
Aside from the normal statistical errors intrinsic to the counting of 14C events, there are additional statistical errors associated with the corrections applied to the Fraction Modern that we account for. For example, the δ13C correction, from a stable mass spectrometer has an uncertainty of approximately 0.1‰. The error associated with δ13C is calculated by:
This component of the Fm error is then added in quadrature as follows:
Radiocarbon age is calculated from the δ13C-corrected Fraction Modern according to the following formula:
Age = -8033 ln (Fm)
Reporting of ages and/or activities follows the convention outlined by Stuiver and Polach (1977) and Stuiver (1980). Ages are calculated using 5568 years as the half-life of radiocarbon and are reported without reservoir corrections or calibration to calendar years. For freeware programs, we suggest that you look at the following web site for a list of programs that will calibrate radiocarbon results to calendar years (including making reservoir corrections).[ Radiocarbon-Related Information Sources]
There are two situations that limit an age; the first is that the measured Fm is smaller than that of the corresponding process blank measured in the same suite of samples on the AMS. If this is the case, then the reported age will be quoted as an age greater than the age of the process blank. No age is reported greater than 60,000 years. The typical background age for organic combustions is 48,000 years and for inorganic carbon samples, 52,000 years.
One other situation that limits the age (if not already limited by the background age) is the error of the AMS result. If twice the reported error of the Fraction Modern (let's call this 2sigma) is larger than the sample Fraction Modern, then a limiting age is reported. The limiting age is then calculated as -8033 * ln(2sigma) and rounded according to conventions outlined above.
Age > Modern
Since Modern is defined as 95% of the 14C activity for AD 1950, as defined by the oxalic acid standard, sample activities can be substantially greater than Modern, and so the ages are reported as > Modern.
We also report the Δ14C value as defined in Stuiver and Pollach (1977) as the relative difference between the absolute international standard (base year 1950) and sample activity corrected for age and δ13C. The Δ14C is age corrected to account for decay that took place between collection (or death) and the time of measurement so that two measurements of the same sample made years apart will produce the same calculated Δ14C result. Collection year must be specified in question 8 of the submittal form in order for Δ14C results to be calculated.
Where lambda is 1/(true mean-life) of radiocarbon = 1/8267 = 0.00012097
Karlen, I., Olsson, I.U., Kallburg, P. and Kilici, S., 1964. Absolute determination of the activity of two 14C dating standards. Arkiv Geofysik, 4:465-471.
Olsson, I.U., 1970. The use of Oxalic acid as a Standard. In I.U. Olsson, ed., Radiocarbon Variations and Absolute Chronology, Nobel Symposium, 12th Proc., John Wiley & Sons, New York, p. 17.
Stuiver, M. and Polach, H.A., 1977. Discussion: Reporting of 14C data. Radiocarbon, 19:355-363. (pdf)
Stuiver, M., 1980. Workshop on 14C data reporting. Radiocarbon, 22:964-966.