Coulomb explosion of BeO

Full text: Molecular negative ions used in AMS, most notably BeO-, may undergo so-called "Coulomb explosion" when they are dissociated in the stripper. In this process, both components of the molecule are stripped to positive ions while still at molecular separation. They therefore repel each other, and the Coulomb potential energy is converted into kinetic energy of their relative motion. Somewhat counter-intuitively, this can lead to a significant spread in energy after the second stage of acceleration with implications for transmission. The situation is most extreme for foil stripping, which strips the two components to higher average charge states, but it is also present for gas stripping (see Nottoli et al., and Suter abstract to this conference). We have made a series of measurements of this phenomenon using foil, gas+foil, and gas-only stripping of BeO^- using the 14UD accelerator at the ANU operating at a terminal voltage of 8.6 MV. Energy distributions of ⁹Be²⁺, ³⁺, ⁴⁺ and ¹⁶O⁵⁺ ions were measured under different stripper conditions. These measurements will be presented and discussed in terms of a model of the Coulomb explosion process.