Time of flight mass spectra of ions in plasmas produced by hypervelocity impacts of organic and mineralogical microparticles on a cosmic dust analyser

¹ Centre for Astrophysics and Planetary Science, University of Kent at Canterbury, Kent CT2 7NR, UK
² School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton BN1 9QJ, UK
³ Planetary and Space Science Research Institute, The Open University, Milton Keynes, MK7 6AA, UK
⁴ Max-Planck Institute für Kernphysik, Postf. 103980, 69029 Heidelberg, Germany
⁵ School of Life Sciences and Technology, Victoria University, Melbourne City, Australia

Corresponding author: M. J. Burchell, M.J.Burchell@kent.ac.uk

Received: 4 June 2002
Accepted: 16 July 2003

Abstract

The ionic plasma produced by a hypervelocity particle impact can be analysed to determine compositional information for the original particle by using a time-of-flight mass spectrometer. Such methods have been adopted on interplanetary dust detectors to perform in-situ analyses of encountered grains, for example, the Cassini Cosmic Dust Analyser (CDA). In order to more fully understand the data returned by such instruments, it is necessary to study their response to impacts in the laboratory. Accordingly, data are shown here for the mass spectra of ionic plasmas, produced through the acceleration of microparticles via a 2 MV van de Graaff accelerator and their impact on a dimensionally correct CDA model with a rhodium target. The microparticle dusts examined have three different chemical compositions: metal (iron), organic (polypyrrole and polystyrene latex) and mineral (aluminosilicate clay). These microparticles have mean diameters in the range 0.1 to 1.6 μm and their velocities range from 1–50 km s^-1. They thus cover a wide range of compositions, sizes and speeds expected for dust particles encountered by spacecraft in the Solar System. The advent of new low-density, microparticles with highly controllable attributes (composition, size) has enabled a number of new investigations in this area. The key is the use of a conducting polymer, either as the particle itself or as a thin overlayer on organic (or inorganic) core particles. This conductive coating permits efficient electrostatic charging and acceleration. Here, we examine how the projectile's chemical composition influences the ionic plasma produced after the hypervelocity impact. This study thus extends our understanding of impact plasma formation and detection.
The ionization yield normalized to particle mass was found to depend on impact speed to the power (3.4 ± 0.1) for iron and (2.9 ± 0.1) for polypyrrole coated polystyrene and aluminosilicate clay. The ioization signal rise time was found to fall for all projectile materials from a few microseconds at low impact speeds (3 km s^-1) to a few tenths of a microsecond at higher speeds (approximately 16 km s^-1 for aluminosilicate particles and approximately 28 km s^-1 for iron and polystyrene particles). At speeds greater than these the rise time was a constant few tenths of a microsecond independent of impact speed. The mass resolution of the time of flight spectrometer was found to be non-linear at high masses above 100 amu. It was  = 5 for m = 1 amu and 40 for m = 200 amu. However, although at high masses most mass peaks had the resolution quoted, there were also occasional much narrower mass peaks observed, suggesting that at 250 to 280 amu  = 80 to 100. The lower resolutions may be due to closely spaced mass peak signals effectively merging into one observed peak due to the (greater but still finite) resolution found for the isolated mass peaks.
Complex mass spectra have been reproducibly obtained from a number of different projectiles that display many charged molecular fragments with masses up to 250 amu and with periodicities of 12–14 amu. These new studies reveal an extremely strong dependence of the time-of-flight mass spectra on the impact speed, particularly at low velocities (1–20 km s. In some impact velocity regimes it is possible to distinguish time-of-flight spectra originating from organic microparticles from those obtained from iron microparticles. However, such discrimination was not possible at high impact speeds, nor was it possible to distinguish between the time-of-flight spectra obtained for aluminosilicate particles from those obtained for iron projectiles.