Radiocarbon dating with accelerator mass spectrometry

A Tandetron accelerator mass spectrometer(AMS), installed at Radioisotope Center, Nagoya University in 1982, has been used to measure natural-level 14C in geological and archaeological samples. The 14C/13C ratio of a modern sample is routinely measured with a precision of ± 1 % using a carbon-silver target containing only 2 - 5 mg of carbon in a measurement lasting 5 hours. Errors for older samples become larger owing to the smaller counts of 14C. The accuracy of a single 14C/13C measurement was estimated as ± 1.3 %. The 14C background of the Tandetron AMS was estimated by using natural graphite samples older than 108 years, after treating it with 1.2N HCl and further heating it in vacuum at 800 deg C. The apparent age of natural graphite was measured as 60000 years. Age measurements were performed on small fragments of fossil shells found at 17 different horizons of the 40-m-long cored sediment in Holocene. The sediment was clearly divided into 5 different horizons according to the sedimentation rates calculated using the 14C ages. (author)