This section contains basic information about radiocarbon dating and the calibration method used in the Centre for Isotope Research, University of Groningen.
Radiocarbon (C14) is a radioactive isotope of carbon, which is naturally produced high in the atmosphere. Living plants, animals and people have in general the same C14 content as the atmosphere they live in. The carbon exchange with the environment ceases after death, whereupon the C14 concentration diminishes in time due to radioactive decay. The basic principle of the radiocarbon dating method is the determination of the age of carbon-containing remains of organic matter (trees, peat, bone, charcoal etc.) by measuring the residual amount of C14 left in the sample in relation to the half life of C14, which is 5730 years. The radiocarbon content of samples can be measured by two methods based on different principles:

• By measuring the C14 radioactivity (the conventional dating method; at
  Groningen marked with a laboratory code number GrN: Mook and Streurman 1983; Mook and Waterbolk 1985) and
• By measuring the C14 concentration (by means of AMS, a form of mass
  spectrometry; at Groningen marked with a laboratory code number GrA: Gottdang et al. 1995; Van der Plicht 1997).

At the Centre for Isotope Research (C.I.O.) of the University of Groningen both techniques are used for the determination of age of carbon-containing samples. The radiocarbon content can be measured accurately depending on the quality of the sample. However, the result of the analysis in radiocarbon years is related to historical ages in a somewhat complicated way. Calibration is the establishment of the relation between radiocarbon age (reported in BP, Before Present defined as 1950 AD) and historical age (BC/AD). Calibration takes into account C14 natural fluctuations in the past and other variables. The ideal samples for calibration are tree rings because they can be dated absolutely by means of dendrochronology. Tree-ring chronologies are now available back into the Preboreal. A C14 calibration curve has been constructed from 9908 BC to the present (Stuiver et al. 1998). Calibration means finding the correct calendar date corresponding to the measured BP-age through a curve. It is the wiggle shape of the calibration data, which are due to solar fluctuations, that makes the calibrated dates usually less precise. For instance, wiggles can cause a C14 date to correspond to more than one calendar date. Illustrations of this principle can be found in van der Plicht and Mook (1987).
In addition one needs to take the measurement error into account. Calibration can be accomplished by just using a large, good print of the calibration curve. Recently, programmes working on Personal Computers have been distributed for calibration purposes. An example is CAL25 (van der Plicht 1993) upgraded with the INTCAL98 data (Stuiver et al 1998). Many C14 laboratories have at present adopted this last programme. A user-friendly calibration programme is provided on the internet by Oxcal.

The radiocarbon method may be used for different purposes, but if used for establishing an absolute chronology then certain strict requirements need to be met:

• The sample needs to come from a closed archaeological context or stratigraphic layer.
• The sample needs to come from a context with artefacts pertaining to a specific cultural phase or pertaining to a transitional period, from one phase to another.
• The sample should not be contaminated with older or younger carbon (ash, crease, lacquer, consolidants etc.).
• A short-lived sample needs to be used - i.e. seeds, animal bones, human bones (only if reservoir effects are measured as well: see Lanting and van der Plicht 1998), nuts, twigs etc.
• Preferably more than one C14 determinations per context or phase need to be made.
• A compendium of C14 dates per phase and possibly per site needs to be set up.
• The archaeological context together with the details of the radiocarbon determination need to be published.

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