An Impact Mass-Spectrometer for the Halley-Probe

Extract

The relative speed between Comet Halley and the presently planned Halley Probe will be approx. 55 km/s. At such a speed the method of impact ionization mass-spectrometry is perfectly suited to analysing cometary dust particles with masses from 10⁻¹⁶g to 10⁻¹⁰g. First results are reported by Dalmann et al. If a micrometeoroid hits the sensitive target, ions from both particle and target material are produced. The total number of ions is registered with a charge-sensitive amplifier and this signal Q allows the determination of the particle mass m (Q ∼ m). By an electric potential difference of 3 kV, ions are drawn into a field-free drift-tube. The ions are separated in time in the TOF-Spectrometer due to their different masses. The mass spectrum is registered as output current of the particle multiplier at the end of the TOF-tube. It gives information about the chemical composition of the detected particle. Characteristics of such spectra recorded with the Impact-Spectrometer currently used in the laboratory are the following: a) mass resolution m/Δm > 100 at 23 amu, b) flight-time (t ∼ 14 μs at 100 amu; the corresponding Δt ∼ 70 ns between 99 and 100 amu).