Calibration of the DIDSY-IPM dust detector and application to other impact ionisation detectors on board the P/Halley probes

Abstract

Results from calibration tests with the DIDSY-IPM dust detector, flown on the Giotto mission, are reported and applied to the impact ionisation dust detectors SP-1 and SP-2, flown on the two Vega Halley probes. The DIDSY-IPM detector is the most sensitive subsystem of the Dust Impact Detection System on board the Giotto spacecraft. It consists of two impact ionisation detectors: sensor A is open, whereas sensor B is covered by a 2.5 μm thick aluminised Mylar film to measure the density of cometary particles. A geometrically very similar detector (SP-1) and an impact ionisation detector of different sensor geometry (SP-2) were flown on board the two Russian Vega spacecrafts. Impact experiments with the DIDSY-IPM detector were performed at the dust accelerators of the Max-Planck-Institut in Heidelberg and the Technical University in Munich. Iron, silicate and carbon particles with masses between 10−15 g and 10−6 g and speeds up to 70 km s−1 were used for sensor calibrations. At constant speeds a linear dependency between the impact charge and the particle mass was found. At an encounter speed of 69 km s−1 (Giotto) the charge yield Q/m is 700 C g−1 for iron particles and 3000 C g−1 for carbon and silicate particles. At 78 km s−1 (Vega) the yields are about twice as high. The penetration limit of the sensor B film is 6.510−15 g for iron particles, 1.310 14 g for silicate particles and 3.210−14 g for particles with a density of 1 g cm −3 at an encounter speed of 69 km s −1. For particles with a density of 1 g cm −3 the penetration limits of the SP-1 films (with thicknesses of 0.6μm and 2 μm) are 7.010-16 g and 1.410−14 g for Vega 1 and Vega 2 respectively. Due to the relatively fast risetimes (<5μs) of the charge signals the DIDSY-electronics has a nonlinear response if the impact charge is larger than 10−9 C.