The composition of the impact-plasma, produced by dust particles hitting an Au-target was measured, using a model of the HELIOS-mlcrometeoroid-detector. The 2 MV dust accelerators of the MPI für Kernphysik, Heidelberg, and the NASA Ames Research Center were used to accelerate particles consisting of Al, Al2O3, SiO2, Soda-Lime-Glass, Polystyrene and Kaolin to velocities between 2 km/sec and 15 km/sec. Fe-projectiles could be accelerated up to 40 km/sec. The masses of the dust grains were between 10−15 g and 3 × 10−10 g. The experiments showed, that because of the characteristic features of the measured spectra it is possible to separate noise events from impacts even at a high noise background. The smallest particles (m 10−15 g) triggering the experiment produce spectra well above the noise level (more than a factor 10) because of the high sensitivity of the ion-detector (multiplier).
During a Rendezvous-type cometary mission it will be possible to collect dust particles on exposed surfaces. A secondary-ion-mass-spec trometer is proposed for the subsequent chemical investigation of such dust collections. An ion gun shoots inert gas or oxygen-ions onto the sample. The energy of these primary ions can be chosen between 1 and 5 keV. The positive and negative secondary ions emitted from the dust surfaces are then analysed with a quadrupole mass filter. The mass range of the instrument will be 1-150 amu. The sensor is able to meas ure (a) nearly all elements in this mass region, (b) molecules and organic components present in the dust, (c) isotopic ratios of inter esting elements. Results of a first series of simulation studies are reported.
DISCUSSIONBrownlee: What is the precision of isotopic measurement in the light of hydride formation and other molecular interferences?
Fechtig:There is indeed a problem; particular isotopes could be masked. For example 12 CH on mass 13 masks 13 C. It is possible, however, to vary the energy of the sputter ions. Then 12 CH has different energy patterns compared to 13 C. Thus it is in principle possible to measure these isotopes to a lower accuracy.
Morfill:Isotopic resolution of aluminium (A£ 26 ) is also of great astrophysical significance. What is the sensitivity of the detector there? Fechtig:As shown in one of the figures, the sensitivity for At is reasonably good. Taking into account the solar abundance of At we expect to be able to see it even on samples with <1% dust coverage.
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