Science |
| Accelerator Mass Spectrometry (AMS) Radiocarbon Method |
| Until about ten years ago¸ dating the rate of decay (beta counts) from different radiocarbon samples had a calculated radio that gave only approximations. Using carbon 14 (C14) and carbon 12 (C12)¸ scientists could detect and count individual decay events with radiocarbon. They would observe the emission of beta particles to determine the rate of radioactive breakdown and so estimate the number C14 atoms remaining in the sample. Since only a small number of C14 atoms break down over the many hours of the sample count¸ the samples had to be large enough to provide an adequate number of beta counts.
Back in the 1960´s¸ archaeologists collected handfuls of charcoal from hearths in plastic bags¸ the rule being¸ the larger the sample¸ the better. Scientists could not date small objects like maize cobs or tiny wood fragments embedded in the sockets of prehistoric bronze spearheads. Furthermore¸ minute samples such as seeds can easily move upward or downward into other occupations layers¸ either through human agencies such as trampling¸ or through natural phenomena such ass burrowing animals. The development of a new radiocarbon method based on accelerator mass spectrometry (AMS) in 1983 revolutionary radiocarbon chronologies and the study of early food production. An accelerator mass spectrometer can date the age of sample material by counting the number of C14 atoms present. Rather than counting decay events (beta counts¸) researchers estimate the remaining C14 by directly counting C14 atoms. By doing this ¸ they can date samples 1¸000 times smaller than the handful of charcoal used a generation ago. The development of small¸ high–energy mass spectrometers solved a major problem¸ that of background noise from ions or molecules of a similar mass to the C14 masking their presence. The new instruments filter out background¸ as a proportion of the sample´s atoms are propelled through an accelerator. A magnet bends the beam¸ so lighter atoms turn more sharply than heavier ones and move to the inside of the diverging beam. A filiter blocks the passage of all charged particles except those of atomic mass 14. The accelerator pushes the stripped beam through a second beam–bending magnet filtering out any last non–C14 particles. A magnetic lens focuses the beam as a C14 detector counts the number of remaining the calculation of the sample age. |