Acknowledgments

We are grateful to PPARC for their continued support of this research REFERENCES

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Analysis of micro-craters on metal targets formed by hyper velocity impacts

Y. Hamabea, S. Sasaki2, H. Ohashib, T. Kawamura0, K. Nogami0, H. Yanod, S. Hasegawad, and H. Shibata6

aEarth and Planetary Science, University of Tokyo, 3-7-1 Hongo, Bunkyo, Tokyo 113-0033, JAPAN.

''Department of Ocean Science, Tokyo University of Fisheries, Tokyo 108-8477, JAPAN.

cDepartiment of Physics, Dokkyo University School of Medicine, Tochigi 321-0293, JAPAN

institute of Space and Astronautical Science, Kanagawa 229-0022, JAPAN

'Research Center for Nuclear Science and Technology, University of Tokyo, Tokyo 1130032, JAPAN

This paper reports the impact craters formed by Ag microparticles impacts, comparing with the TOF-MS (Time-Of-Flight Mass Spectrometer) spectra obtained simultaneously. The TOF spectra suggested that ions were hardly produced on A1 target compared to Mo and Au targets. To determine the reason for this and to select the best material that ionizes the impacting dust particles, we observed impact craters on these targets with SEM and STM. Our results suggest that the ionization depends on the materials and that the combination of target and projectile materials causes the differences of crater shape and the Ag residue distribution.

1. INTRODUCTION

The production of plasma by hypervelocity impacts has been studied with the experimental investigations. Previous works suggested that the ionization required higher impact velocity above lOkm/s [1,2], and that the threshold velocity depended on the characteristics of projectile or target materials such as the ionization energy and the temperature [3]. However, ion formation is not explained only by the simple physical processes because of the complicated energy division. The division ratio of the impacting energy is related to some factors such as the differences in their density, melting point, ductility, and so on.

2. IMPACT EXPERIMENTS OF MICROPARTICLES AND TOF-MS SPECTRA

Impact experiments of microparticles were performed with a linear type TOF-MS at HIT (High Fluence Irradiation Facility, University of Tokyo) in Japan [4], Silver particles around 0.5-2.0p.m in diameter were accelerated up to 2-7km/s, and they impacted on a metal target (Al, Mo and Au) with the incident angle of 45 degrees. Ions are produced by the hypervelocity impacts of Ag projectiles and accelerated in the accelerating region between the target surface and the front Cu mesh. After ions are extracted from the target region, they fly in the drift region of length 141mm and achieved at the detector (MSP; Micro Sphere Plate).

It is expected that ions both from the projectile and the target are detected. However, we could hardly obtain Ag+ and Al+ for the impacts of Ag projectiles on Al target. On the contrary, Mo+, Au+ and Ag+ were sometimes detected for Mo and Au targets, especially when the impact velocity exceeded above 3km/s. The ion peaks from projectile (Ag+) and target (Al+, Mo+, and Au+) were not obtained repeatedly. The peaks of Na+ and K+, the contaminants in the target or projectile materials, were detected in the most spectra.

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