Improving a gas ion source for 14C AMS

For more than 4 years, gaseous samples of 1-50 μg carbon have been routinely measured with the gas ion source of the small AMS (Accelerator Mass Spectrometer) facility MICADAS (MIni CArbon DAting System) at ETH Zurich. The applied measurement technique offers a simple and fast way of 14C measurements without the need of sample graphitization. A major drawback of gaseous 14C measurements, however, is the relatively low negative ion current, which results in longer measurement times and lower precision compared to graphitized samples. In December 2009, a new, improved Cs sputter ion source was installed at MICADAS and we began to optimize conditions for the measurement of gaseous samples. 12C− currents from the new ion source were improved from initially 3 to 12-15 μA for routine measurements and the negative ion yield was increased by a factor of 2, reaching 8% on average during routine operation. Moreover, the new measurement settings enable a doubled CO2 flow, thus substantially reducing measurement times. The achieved performance allows closing the sample size gap between gaseous and solid samples and makes the gas ion source a promising tool for dating with a measurement precision of 5‰ on samples as small as 50 μg carbon.