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.
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:
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.
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.
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
| Home | About RCD | Tritium | Industrial Applications | Environmental Applications | Sampling Equipment | Contact Us |