Carbon ion stripper for NSTRI tandem accelerator
摘要
The tandem accelerator project at the Nuclear Science and Technology Research Institute (NSTRI) aims to design and construction a 1.7 MV accelerator for AMS applications. Alongside the design and construction of the high voltage power supply for the intended accelerator, the accelerating tube including the low-energy section, the stripper section, and the high-energy section is also being developed. This paper focuses specifically on the design of the stripper section for a carbon ion beam. In the design of this section, by excluding interactions involving charge states of (q, q–2) and (q, q + 2), a total of 14 interactions between charge states (q, q + 1) and (q, q–1) have been considered. After deriving the interaction cross-sections at the desired energy, the optimal geometry was introduced, and the charge exchange efficiency was calculated for different gas throughputs. At a gas throughput of 9 Standard Cubic Centimeter per Minute (SCCM), an equilibrium thickness of 2.3 × 1016 cm−2 was achieved, which ensures charge exchange equilibrium for the formation of various carbon charge states within the designed geometry. Results indicate that for charge states 1+, 2+, and 3+, the 14C/12C stripping ratios are 1.22, 1.01, and 0.89 respectively, while the corresponding ratios for 14C/13C are 1.52, 1.09, and 0.76. These variations highlight the importance of considering isotopic mass effect on stripping efficiency in AMS-based radiocarbon measurements. The optimized geometry of the stripper section was constructed and experimentally tested. Pressure measurements taken in different regions showed excellent agreement with the gas-flow simulation results, validating the accuracy of the simulation. Furthermore, the calculated carbon charge-state fractions were validated with experimental measurements from another reference at 1.7 MeV in nitrogen under equilibrium-thickness conditions. The results reveal a close match between the calculated fractions in this study and the experimental data, further supporting the reliability of the model and the credibility of the results. The results demonstrate that this optimized geometry and structure are capable of achieving the required atomic density for reaching the equilibrium thickness of the carbon beam, while the overall length is reduced to approximately one-third of that in conventional stripper designs.