Instrument Overview
    ===================
 
      The acoustical detector consists of three
      piezo-electric elements mounted on a mem- brane
      (3), two of these sensors (4) are connected to two
      identical recording circuits in order to improve
      the overall reliability of the system; the third
      element (5) is used as a stimuli source for
      calibration purposes.  The membrane is mounted in a
      frame (l) which damps the mechanical oscillations
      and improves the counting rate.  The detector
      assembly is fixed to the electronics box (6) by
      means of three acoustical insulators (2).  Each
      piezo-electric element delivers a signal which is
      fed into a narrow-band amplifier working at a
      frequency of about 160 kHz.  The output signal is
      split into l6 channels, which have their
      sensitivity thresholds logarithmically distributed
      across the whole dynamic range.  Four identical
      impact plasma detectors are mounted at the
      periphery of the acoustical detector.  They are
      associated in pairs and connected to identical
      electronic circuits.  The entrance to each detector
      is protected against the environmental plasma by a
      system of deflecting electrodes and grids (7,8).
      The ions and electrons generated by each particle
      impact are separated and collected by a grid (9)
      and a target (10), bet- ween which a potential
      difference of 2 kV is applied.  The electron pulse
      detected by the lower electrode constitutes the
      input signal, which is analysed in several channels
      with different sensitivity thresholds.  The
      threshold sensitivity of the dust particle acoustic
      detector in terms of the transferred momentum is
      given by Pmin ~ 2x10**-5 gcm/s.  Taking into
      account that the recoil momentum may exceed the
      particle momentum mV by 5 to 10 times, the minimum
      mass of recorded particles is < 3x10**-13 g.  The
      measurement range n (> m)x3x10**-13 to ~ 2x10**-6 g
      is divided into 16 channels.  With the relative
      width delta/m ~ 2.82, sensitive area S = 500 cm**2,
      accumulation time for one spectrum is 1 s, which
      corresponds to the spatial resolution ~ 80 km over
      the flyby trajectory.  The counting rate of small
      mass particles is about 4095 1/s.  The ionization
      detector is capable of recording particles with
      mass 3x10**-16 to 3x10**-11g.  The number of
      channels is 6, with relative width delta/m = 10,
      sensitive area - 40 cm**2.  The limit counting rate
      of small mass particles is ~ 65,500 1/s.  The
      accumulation time for one spectrum is 1 s.  The
      characteristic features of SP-2 are listed in the
      table below.
 
 
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Table. Dust Particle Impact Detector characteristics
 
Detector                 Piezo-electric          Impact plasma
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Total sensor area        500 cm**2               40 cm**2
Maximum count rate       4095 s**-1              65500 s**-1
Integration time         1 s                     1 s
Mass resolution,m/deltam 2.82                    10
Number of channels       16                      6
Dynamic range            2x10**-6  -3x10**-3 g   3xl0**-16  -3x10**-11 g
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