Dust Particle Detector (DUCMA)

Description

This instrument, DUCMA (Dust Counter and Mass Analyzer), was designed to measure the fluxes of impacting dust particles with masses above the four threshold values of 1.5E-13, 9E-13, 9E-12, and 9E-11 g. The dust detector consisted of 75 sq cm of a 28-micrometer polarized polymer film, with conducting surfaces. This film, of polyvinylidene fluoride, had a permanent electric dipole moment per unit volume ("electret") with the polarization direction normal to the film surface. Destruction of the polarization along the path of the impacting particle produced a fast current pulse which was detected by a charge-sensitive amplifier. The magnitude of the signal was a known function of the mass and the velocity of the impacting particle. Because all of the dust particles impact with the flyby speed of 78 km/s, the electronically set thresholds corresponded to known mass values. There was a second similar detector, called the veto detector, which was protected from direct dust impacts and responded only to acoustic noises generated by activities of the spacecraft. This veto detector had two output channels, set at a low and a high threshold value. The ten counting rates of the instrument also included four single counting rate channels from the four thresholds of the dust detector, and the same four channels in anti-coincidence with either the low or the high level of the veto detector. The instrument was self-calibrating either periodically or upon command, and operated normally even when a portion of the detector was destroyed by large dust particles. It was calibrated at the Heidelberg dust accelerator up to velocities of 12 km/s. The counting rates for dust particles required no correction up to 1E4 particles/s, and only a small correction for 5E4 particles/s. The instrument was to be powered throughout the mission, so that after Venus encounter it was to search for interplanetary dust particles during the cruise mode. The detector was described by J. A. Simpson and A. J. Tuzzolino in Nuclear Instruments and Methods, in press, 1985. A description of the entire instrument is to be published in Nuclear Instruments and Methods, 1985 or 1986.