What Is Carbon-14?

Carbon-14 (14C), a radioactive isotope of carbon (half-life 5570 years, max β- energy 160 keV), is produced naturally in the upper atmosphere through the reaction of neutrons from cosmic rays with nitrogen (14N n,p 14C). The 14C rapidly oxidises to form 14CO2 and, as such, lies at the head of the food chain for all living organisms. The level of 14C in atmospheric CO2 has remained essentially constant over ancient times but with certain perturbations due to variations in the cosmic ray output from the sun. An additional major variation occurred in the 1950s and 1960s when a large input of 14C and tritium was made into the atmosphere during the atmospheric nuclear weapons trials, with the level of 14C in the Northern hemisphere rising to nearly twice its normal level in 1963/4.


Carbon-14 Measurement Techniques

Three methods are currently offered by RCD for carbon-14 measurement, benzene synthesis, direct absorption and accelerator mass spectrometry (AMS). AMS is the preferred method for samples with low carbon content, i.e. below 0.5g carbon. Full processing is carried out at RCD in the first two methods whereas the smaller samples, which require the third method, are processed to CO2 then sent to the AMS laboratory for measurement.

The main features of the techniques are:

  1. Benzene synthesis - combustion/acidification of the sample to CO2 followed by conversion to benzene through the stages of lithium carbide and C2H2. Liquid scintillation counting of the benzene sample in an optimised low level liquid scintillation counter, e.g. Wallac Quantulus, to achieve an overall precision of ± 1% (ie ± 2.5 Bq/kg carbon). Sample size required 1g to 4g elemental carbon.

  2. Direct absorption - combustion/acidification of the sample to CO2 followed by absorption of the CO2 in a suitable scintillant for immediate liquid scintillation counting. Precision of the measurement is between 5% and 10% (ie ± 12.5 to ± 25 Bq/kg carbon). Sample size required 0.5g to 1g carbon.

  3. Accelerator mass spectrometry - combustion/acidification of the sample to CO2 (at RCD) followed by conversion of the sample to graphite at an accelerator laboratory and measurement of the 14C content. Precision of the method is better than ± 1% (± 2.5 Bq/kg carbon) and the sample size required is 5mg to 10mg carbon.

The main difference between the methods is the size of sample required for measurement, but the second technique, direct absorption, offers considerably lower precision than the other two and is, therefore, generally only of use in situations where the question is simply whether a sample is ancient (0% 'modern') or modern (> 100% 'modern') but not where finer detail is necessary to help unravel mixtures of sources or provide an actual age.


Carbon Dating (Carbon-14 Dating, Radiocarbon Dating)

The technique for dating once living material containing carbon was proposed by W.F. Libby and first tested in 1948. Its basis is that all carbon life forms assimilate the radioactive isotope of carbon, carbon-14, whilst alive but that after death the amount accumulated in the organism decays away at the rate consistent with the 14C half-life - originally thought to be 5568 years, now corrected to 5730 years.
In theory therefore, the age of a sample can be determined from a measurement of the 14C remaining in the ancient sample. In practise this residual measured value requires further correction (calibration) to convert the apparent age to an actual calendar date.

Suitable materials for carbon dating include: wood, charcoal, ivory, straw, hair etc.

For samples which can yield between 1g and 4g of carbon (eg ~15g wood taken from a house timber or 5-10g charcoal from an archaeological site) the dating procedure via benzene synthesis can be carried out in-house by RCD. The process usually takes 4-6 weeks to complete (Please phone to discuss any contemplated dating project with RCD before commencing any sample taking).

For samples yielding significantly smaller amounts of carbon (eg hydrological samples or art history treasures) please refer to the requirements given in Carbon Dating For Art History.


Carbon-14 And Landfill Sites

Modern landfill sites containing organic waste produce landfill gases (including CH4) which carry the 14C levels appropriate to the time the material was living. If the waste contains a high proportion of recent organic material, eg garden waste, then a clearly measurable difference exists between CH4 originating from decomposition of matter in the landfill and CH4 from coal measures or oil products which do not contain 14C.

Please see Landfill Gas Isotope Characterisation