Carbon background and ionization yield of an AMS system during ~(14)C measurements of microgram-size graphite samples

摘要

For ~(14)C AMS measurements of samples at the microgram level, ion source related effects start to play a role, while generally the lower sample size limit is set by the carbon background introduced during chemical preparation procedures. Measurements of about 800 graphite targets in the mass range of 1-100 μg were performed within 25 AMS beam-times during the last three years at VERA, revealing a dependency of measured ~(14)C~(3+)/~(12)C~(3+) ratios on ~(12)C~(3+) currents. This dependency can be accounted for by assuming a background current, which was determined for each AMS measurement by least square fitting. ~(12)C ion currents extracted from microgram graphite samples were typically (1.0 ± 0.5) μA / μg C. On average a ~(12)C~(3+) background current of (0.14 ± 0.14) μA with F~(14)C = 0.22 ± 0.46 (skewness γ_1 = 3.0) was deduced with significant variations between single measurements. The determination of this background current for each AMS measurement of microgram graphite samples allowed to apply a quantitative correction and thereby to improve the AMS measurement precision. Furthermore, the yield of graphitization and ionization in a Cs sputter ion source of graphitized microgram CO_2 samples was investigated. No dependency on the cathode target geometry was observed for 9 differently shaped cathode types.