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

For 14C 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 14C3+/12C3+ ratios on 12C3+ currents. This dependency can be accounted for by assuming a background current, which was determined for each AMS measurement by least square fitting. 12C- ion currents extracted from microgram graphite samples were typically (1.0 ± 0.5) μA / μg C. On average a 12C3+ background current of (0.14 ± 0.14) μA with F14C = 0.22 ± 0.46 (skewness γ1 = 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 CO2 samples was investigated. No dependency on the cathode target geometry was observed for 9 differently shaped cathode types.