E/Q and ME/Q2 contributions to machine background in sequential injection radiocarbon AMS

Routine radiocarbon machine background values in the range of 10−16 (corresponding to ∼65-70 ka in the radiocarbon timescale) are routinely achieved at the sequential injection AMS system of CEDAD, University of Salento, Lecce, Italy. Although these background values are adequate for dating purposes and well below the sample-processing induced contamination, in recent times efforts have been made to identify the factors limiting the maximum achievable sensitivity of the AMS spectrometer and eventually extend the measured background below the current limits. Different possible sources of machine background have been thus investigated. Dedicated experiments allowed to quantitatively estimate the contributions of both ME/Q2 and E/Q interferences to machine background and the corresponding attenuation factor of the AMS spectrometer. In particular our results indicate that the possibility to gate the 14C detector with the sequential injection of the three carbon masses results in a significant improvement of sensitivity mainly because of the reduction of the contributions to machine background of the ambiguities resulting from the injection of 12C and 13C negative beams. A comparison of the results obtained at CEDAD with those obtained in similar experiments with different experimental set-up are presented.