Instrument Overview
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      The detector is a 28 Km thick film of polarised
      polyvinylidene fluoride (PVDF), covered on each
      face with a metallic conducting coating.  A dust
      particle impacting the detector will displace a
      small volume of polarised material in the bulk of
      the detector, which then results in a fast
      depolarisation signal whose amplitude is a known
      function of the particle mass and velocity.
      Electronic circuits measure the pulse height and
      ac- cumulate pulse events above four different
      threshold levels.  Since the relative impact
      velocity is known (i.e.  the comet-spacecraft
      velocity), the mass is determined direct- ly from
      the known mass/velocity relationship for these
      detectors.  The detector assembly consists of the
      dust detector (M), and a small anti-coincidence
      detector (V), mounted perpendicular to the
      direction of arrival of the dust particles.  The
      purpose of the small detector is to detect very
      large mechanical shocks on the spacecraft which
      might trigger the most sensitive level of the dust
      detector.  Further technical details are given in
      the table below, where the values for the
      'differential dust mass ranges' are published in
      Simpson et al.  'Ducma Measurements of Comet Halley
      Dust Mass Spectra Bases on Post-Encounter Dust
      Calibrations', Asteroids, Comets, and Meteors III
      (1990, Uppsala).  This instrument was selected one
      year before launch and could still be considered at
      this late stage because it did not require any
      direct telemetry or telecommand inter- face with
      the spacecraft.  Its inclusion was facilitated
      because it was allowed to share the data format and
      telecommands initially allocated to the ING
      instrument.
 
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 Table. Dust Particle Detector and Mass
 Analyser characteristics
 
 Impact detector           28 micro m**2 PVDF foil
 Detector area             75cm**2
 Maximum count rate        10**5/s
 Differential dust mass
 range
                           1.5x10**-13 to 5.6x10**-11 g,
                           5.6x10**-11 to 1.6x10**-9 g,
                           1.6x10**-9 to 4.0x10**-8 g,
                           and greater than 4.0x10**-8 g
 
 Integral dust mass range   >4.0x10**-8 g
 Integration time for flux
 measurements at
 encounter                  1 s
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