Investigation of gas stripping at 4MV for high mass negative ions

Abstract

For AMS analysis of actinides using the ANTARES tandem accelerator, we currently achieve an accelerator transmission of 4-5%, when injecting UO– ions and analysing the 5+ charge state at a terminal voltage of 4MV with argon stripper gas. The present study was undertaken to better understand the stripping process for such heavy ions and, if possible, find a means to achieve higher transmission and therefore improved overall efficiency. We have measured charge state distributions at 4MV terminal voltage using negative ions ranging from carbon to uranium oxide. The stripper gas density was varied over a wide range to observe the onset of equilibrium stripping conditions. Using a 12º electrostatic deflector after the accelerator, charge states from 1+ to 8+ have been measured. The results show several interesting features: (i) for heavy ions such as iodine, uranium and thorium, there is a high probability of multiple electron loss in a single collision with the argon gas, leading to the mean charge state for I, Th and U ions being greater than 2.5 even at very low gas stripper pressure; (ii) with increased stripper pressure, scattering losses dominate before equilibrium stripping conditions are achieved. Further experiments are underway using helium and xenon stripper gases. Helium is of particular interest, as in earlier studies [1] performed externally to an accelerator, helium was shown to give the highest equilibrium charge state for uranium at low energy. (c) 2011 AMS12