Radiocarbon Dating for Forensic Science

Tom Higham   Oxford Radiocarbon Accelerator Unit.

Radiocarbon dating is something of a blunt instrument in the modern period (between c. 1650-1950 AD), but is much more useful for the post-1950 period. From the mid-1950s and over the course of a decade, the atmospheric radiocarbon concentration increased dramatically because radiocarbon was produced artifically through thermonuclear bomb testing. Atmospheric nuclear testing resulted in a doubling of radiocarbon concentration over natural proportions by 1963 in the northern hemisphere. Since then, with occasional perturbations, the levels of 'bomb carbon' have declined as this artificial radiocarbon enters the biosphere. In 2007 we are now almost back to the levels of radiocarbon in the early 1950s.

One advantage of this nuclear bomb signal is that it is possible to derive radiocarbon ages for material that was once alive in this very recent period. Radiocarbon can therefore be useful in providing ages for forensic specimens.

Advantages of Accelerator Mass Spectrometry

Accelerator measurement differs from the conventional method in that it enables samples a thousand times smaller to be dated. The ability to obtain a radiocarbon date using only a small quantity of material has made possible the direct dating of forensic samples with the minimum of damage. Consequently, the last ten years have seen a great deal of interest in radiocarbon dating this kind of material.

The precision of the measurements is similar to conventional radiocarbon measurements and for the post-1950 era can provide results to within two years at best, if the material dated is not subject to the vagaries of bone collagen turnover effects or other sample related problems.

Sample Material and Type

A variety of organic materials is suitable for dating in human forensic cases including:

* Human bone (although much depends on the age of the human, since bone turnover rates slow with age, influencing the accuracy).

* Human hair 

* Human skin

* Clothing of an organic nature, such as cotton.

In addition, radiocarbon may be used in examining issues related to wildlife crime and the trafficking of illegal animals and birds. Suitable sample types include;

    * Animal or bird bone.

    * Antler, tusk and horn.

    * Hair.

    * Feathers.

    * Egg carbonates.

The quantity of material required depends strongly on the composition; this table gives approximate values for some common materials. In some cases, substantially smaller quantities can be used.

 

Material	Quantity
Bone/ivory	200mg - 2g
Egg carbonate	30 - 50 mg
Hair/feather	20 - 40 mg
Skin	20 - 40 mg

Oxford Radiocarbon Accelerator Unit (ORAU)

There are several accelerator facilities worldwide performing radiocarbon analyses, with about six in Europe. The Oxford Laboratory is unique in putting the majority of our effort into the dating of archaeological samples. We have an international reputation for this and have specialised both in developing chemical treatment processes, and in providing archaeological advice, scrutiny and interpretation.

Much of the expertise in this area is directly applicable to forensic science questions. We measure about 1000 dates per year in house with a further 1000 AMS measurements on samples prepared elsewhere. About 25-40 of these are related to forensic cases, mostly for local UK police forces investigating human remains found by the general public to determine whether they are of forensic interest or not. ORAU has considerable expertise in these areas and encourages potential submitters to contact us.

In addition to our dating programme, we undertake research into methods that improve the accuracy and usefulness of radiocarbon dating. We have relevant interest and expertise in these areas amongst others:

    * chemical pre-treatment of bone

    * removal of preservatives and contaminants from objects

    * stable isotope analysis and interpretation - including the identification of contaminants

    * dating of extremely small samples

    * radiocarbon calibration

    * general interpretation of radiocarbon dating evidence

We perform regular quality control tests both through the main international laboratory inter-comparisons (TIRI, FIRI etc), informal laboratory inter-comparisons and with regular in-house measurements on known-age material. We are also ISO-9001 accredited.