A GASEOUS ION-SOURCE FOR ROUTINE AMS RADIOCARBON DATING

The negative ion source developed at Oxford for AMS radiocarbon dating directly on carbon dioxide has now been in routine operation for about six months. The advantages and limitations of the source have become clear. The chief advantages, apart from the obvious simplification of sample preparation, are: (1) Reproducible currents of about 12-mu-A which can be stabilised. (2) High overall efficiency (circa 3-4% of total gas sample into the accelerator) of C- ion production from the admitted CO2. (3) Samples as small as 100-mu-g can be run at 10-mu-A. The limitations found are: (1) Cross contamination within the source of 0.05%. (2) Cross contamination in the gas handling of 0.2% which is routinely corrected for in the data analysis. (3) Background current before the admission of gas and after cleaning the targets by sputtering for 15 min represents circa 0.5% modern in C-14. For each sample this background is measured and corrected for. When running continuously the source can measure about 60 samples (including standards) within a week to a statistical accuracy of about 0.4% for modern material.