The Centre for Isotope Research (CIO) conducts radiocarbon-based research, and offers a radiocarbon dating service. Operating continuously since the early 1950s, the CIO now houses the longest-running radiocarbon dating laboratory in the world. The facility recently received a major upgrade, which included the installation of a state-of-the-art MICADAS accelerator mass spectrometer (AMS).
Radiocarbon dating can be used to determine the age of any carbon-containing material (such as bone, wood or shell) that is less than 50,000 years old. The CIO accepts samples for a variety of academic and publicly funded research projects, and our staff work closely with all submitters, advising them on sample selection, pretreatment and data interpretation.
Overview Radiocarbon dates are obtained by measuring the ratio of the unstable isotope, 14C (radiocarbon), to the stable isotope, 12C, in a sample. Radiocarbon is produced naturally, in the upper atmosphere, as a secondary product of the cosmic ray bombardment. After dispersal throughout the atmosphere, it becomes fixed by vegetation through photosynthesis, and is subsequently transmitted up the food chain. However, 14C is unstable, and continually decays away with a half-life of about 5730 years. As a result, the amount of 14C present in dead organic material halves every 5730 years. Once the ratio (14C:12C) is measured, the age of the sample in calendar years can be calculated. The analytical process also incorporates the measurement of other stable isotopes, such as 13C and 15N. General information on the dating method can be found in Toekomst van het Verleden (Dutch).
Sample Selection and Pretreatment
Accurate radiocarbon dating starts with careful sample selection and rigorous chemical pretreatment. Prospective submitters may benefit from advice about which samples to select for the best results. When samples arrive at the laboratory, they are registered and passed to chemical pretreatment specialists. Pretreatment encompasses both physical and chemical cleaning of the sample, and it is a labour intensive and time-consuming process. During pretreatment, the most reliable fraction of the sample (usually a chemically inert structural fibre like collagen or cellulose) is extracted. This product is then purified by CO2 gas through combustion, or acid dissolution in the case of carbonate samples such as shells. Finally, the CO2 gas is reduced to graphite (pure carbon) for 14C:12C measurement on the AMS.
Determination of the 14C Ratio
The AMS measurement process involves separation of the rare isotope, 14C, from the abundant isotope, 12C. In 2017, the CIO began using a MICADAS (180 kV) AMS instrument for this purpose. The instrument first ionises carbon atoms in the graphite sample, accelerates them to high speeds, and deflects them by magnetic and electric fields. The ions of the heavier isotope, 14C, are less perturbed than those of 12C, enabling the two ion beams to be separated. In the acceleration phase, collisions with helium gas are also applied to eliminate any molecular impurities (isobars). At the far end of the accelerator, sensitive instruments detect the arrival of the 14C and 12C ions, allowing the 14C:12C ratio to be estimated.
|Last modified:||27 September 2019 08.35 a.m.|