Design of a secondary ionization target for direct production of a C− beam from CO2 pulses for online AMS

摘要

We designed and optimized a novel device “target” that directs a CO2 gas pulse onto a Ti surface where a Cs⁺ beam generates C⁻ from the CO2. This secondary ionization target enables an accelerator mass spectrometer to ionize pulses of CO2 in the negative mode to measure ¹⁴C/¹²C isotopic ratios in real time. The design of the targets were based on computational flow dynamics, ionization mechanism and empirical optimization. As part of the ionization mechanism, the adsorption of CO2 on the Ti surface was fitted with the Jovanovic–Freundlich isotherm model using empirical and simulation data. The inferred adsorption constants were in good agreement with other works. The empirical optimization showed that amount of injected carbon and the flow speed of the helium carrier gas improve the ionization efficiency and the amount of ¹²C⁻ produced until reaching a saturation point. Linear dynamic range between 150 and 1000 ng of C and optimum carrier gas flow speed of around 0.1 mL/min were shown. It was also shown that the ionization depends on the area of the Ti surface and Cs⁺ beam cross-section. A range of ionization efficiency of 1–2.5% was obtained by optimizing the described parameters.