New insights in impact ionization and in time-of-flight mass spectroscopy with micrometeoroid detectors by improved impact simulations in the laboratory
The present thesis deals with the impact ionization processes appearing at hypervelocity impacts of microparticles on solid surfaces. With a newly developed dust particle source, applied to the Heidelberg Dust Accelerator facility, aluminium, carbon, sodium contaminated carbon, iron and latex projectiles were shot on the micrometeorite detectors CDA and CIDA. The projectile masses cover a range of 10^-18 to 10^-12 kg, the projectile densities a range of 1100 - 7900 kg/m^3 and the impact speeds a range of 2 - 70 km/s. The resulting data show different impact ionization processes depending on the impact speeds: for low speeds (v < 6 km/s) dominates surface ionization of target contaminants, for high speeds (v > 18 km/s) volume ionization of the target and projectile material is dominating. The charge yield in the intermediate impact speed regime shows a reduced increase due to energy consumption by melting and vaporization processes. Time-of-flight mass spectroscopy, provided by both instruments, enabled the investigation of the chemical composition of the impact plasma depending on the projectile type and on the impact speed. Alkaline ions (Na, K) dominate the mass spectra at low impact speeds. For higher impact speeds appear target ions (Rh), projectile material related ions and hydrogen ions (H). The systematics of characteristic atomic and molecular ion species in the mass spectra helps to get clues on the chemical nature of unknown projectiles in space. An improved model on specific ion yields can be given. Furthermore a lot of results which are important for the instrument calibration and the understanding of flight data, like global charge yields, rise times and sensitivities were obtained. First measurements on the inner instrument wall region are provided.
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