Pulsed operation of a SNICS ion source – ionization efficiency and ion output
Dr Martin Martschini1, Alfred Priller1, Peter Steier1, Robin Golser1
1University of Vienna, Faculty of Physics – Istope Physics, VERA Lab, Vienna, Austria
Accelerator mass spectrometry (AMS) aims at highly efficient and fast detection of minute amounts of radioisotopes and thus crucially depends among others on the attainable negative ion output from a cesium sputter ion source. Generations of scientist have thus experimented with variations of sputter matrix composition, ion source settings and other ion sourcery magic to achieve the best performance of their AMS ion sources.
In 2020, Blahins et.al.  reported strong ion current enhancement for negative ions when operating their Peabody Scientific Model PS-120 in a pulsed mode. They switched the cathode voltage with 10Hz repetition rate and a duty cycle of 10% between the standard operating value of −3 kV and an idle value of −200V, primarily with the aim to make their cathodes last longer during experiments with pulsed lasers.
At the Vienna Environmental Research Accelerator VERA, we have subsequently investigated possible ion output enhancement factors for C−, C₄−, Al−, AlO−, CaF₃− and SrF₃−, by a pulsed operation of one of our NEC-MC-SNICS II ion sources under AMS-conditions. The cathode voltage was switched periodically between −5 kV for regular sputtering and a reduced value of −800 V with a Behlke GHTS-60A High Voltage Push-Pull Switching Unit. A pulse generator allowed variation of both the repetition rate (1-10Hz) and the duty cycle (5-90%).
When operating the ion source with regular AMS settings such that a ¹²C− output of 30-40 μA DC was obtained, all of the above ion species showed a peak ion current in pulsed mode similar to the DC ion current of that species. A study of ionization efficiency of Al− from Al₂O₃ sputter targets likewise revealed no enhancement of ionization efficiency by pulsing the source. However, current enhancement of up to 400% for C− and especially C₄− were observed in pulsed mode when strongly reducing the cesium supply to the source. Presumably, longer Cs accumulation times enhance the ion current only in a state of cesium scarcity in the source. Since these are unfavorable and unusual conditions in AMS, no benefits for AMS could be identified.
 J. Blahins, T. Leopold, A. Apsitis, U. Berzins, A. Ubelis, J. Rohlén, D. Lu, and D. Hanstorp, Operating a cesium sputter source in a pulsed mode, Review of Scientific Instruments 91 (2020) 023322.
Dr. Martin Martschini holds a postdoc position at the VERA-AMS facility in Vienna and focuses his research on the technical development of the AMS technique. During his PhD (finished 2012), he managed to make VERA the first 3-MV-accelerator capable of separating the isobars 36S and 36Cl down to environmental levels. This was mainly achieved by a thorough assessment of the physics of gas ionization detectors. Already then, he was involved in the ILIAMS project and, in his first postdoc years, took over the project leadership during commissioning of the ion-laser-interaction setup at the test bench. After a postdoc position at the 14MV-HIAF-AMS-facility of the Australian National University in Canberra, he has now rejoined the VERA team and works on AMS detection of previously inaccessible trace isotopes with the ILIAMS technique.