Conversion surfaces for neutral particle imaging detectors

Low-energy neutral particles must be ionized before they are analyzed by mass spectrometric means. Within the limitations regarding space, weight and power consumption onboard a spacecraft surface ionization has been identified as the only viable ionization technique. In the study presented here molecular oxygen and hydrogen ions were scattered at grazing incidence from several diamond-like carbon (DLC) surfaces in the energy range from 190 to 2400 eV. Most surfaces were hydrogen terminated. For incident positive oxygen and hydrogen ions, scattered negative ion fractions of up to 28% and more than 6%, were measured, respectively. These values are among the highest ever reported, especially for oxygen. Furthermore, though these surfaces are amorphous, due to improved polishing techniques angular scattering distributions of the scattered beam have been observed, which were comparable to distributions of scattering experiments with single-crystal surfaces. Therefore, these DLC surfaces are strong candidates to work as conversion surfaces in a neutral particle imaging detector for the BepiColombo mission.