Addition of charge state selection after the terminal stripping for reduced background in AMS
Grazia Scognamiglio1, Matthias Klein1, Dirk Mous1
1High Voltage Engineering Europa B.V., Amersfoort, The Netherlands
Fragments from injected molecules, which undergo charge exchange in the second acceleration stage of tandem accelerators, are an important contribution to the measurement background in AMS systems. The latest sub-MV AMS machines designed by High Voltage Engineering Europa B.V. are equipped with vacuum insulated accelerators featuring charge state selectors (CSS) in the terminal . The CSS consists of a magnetic or electrostatic deflector that selects a specific charge state after the stripper, filtering out background contributions originating from charge exchange processes in the high-energy acceleration section.
The effectiveness of the CSS method has been demonstrated on the ¹⁴C-dedicated 4102Bo-AMS system, which is equipped with a tandem accelerator operating at 210 kV terminal voltage. After the stripping process in the accelerator, ions in the 1+ charge state have the highest yield and are selected for the AMS measurement. Along ¹⁴C, isobaric ¹²CH₂ and ¹³CH molecules are injected into the accelerator and break-up in the stripper. A fraction of ¹²C and ¹³C molecular fragments undergoes 2+ → 1+ charge exchange in the second acceleration stage and increases the ¹⁴C/¹²C background to the mid to high 10-¹⁵ level, depending on the molecule content in the ion beam. A magnetic CSS in the accelerator terminal suppresses other charge states than 1+, removing these background contributions and enabling the measurement of a ¹⁴C/¹²C blank ratio in the 10-¹⁶ regime.
Following the results obtained with the ¹⁴C-dedicated system, HVE will apply the CSS method also to the multi-element 4103Bo-AMS system based on a 300 kV terminal voltage vacuum-insulated accelerator. The CSS is added to improve the measurement background especially for actinide AMS. Actinide measurements use the 3+ charge state but suffer the interference of 4+ molecular fragments that charge exchange to 3+ in the high-energy acceleration stage. The CSS removes the ions in the 4+ charge state, thereby preventing the 4+ → 3+ charge exchange processes responsible for the dominant class of background.
In this presentation, we will discuss the design and the measurement results obtained at the ¹⁴C-dedicated 4102Bo-AMS system and the design of the new 300 kV tandem accelerator for the multi-element 4103Bo-AMS system.
 European Patent EP 18177774.9.
Grazia Scognamiglio is an AMS physicist of High Voltage Engineering. She graduated in Physics at the University of Naples and she got a Ph.D. diploma at the University of Seville with a thesis on low-energy AMS. At HVE, she mostly works on the design and testing of compact AMS systems.