Instrument Overview
  ===================
    The Energetic Particles Investigation (EPI) instrument onboard
    the Galileo Probe was part of the Lightning and Radio Emission
    Detector (LRD).  The complete instrument consisted of four
    sensors (RF antenna, 2 optical sensors, particle sensor) and a
    central electronic box.  The EPI instrument was a two-element
    telescope using totally depleted silicon surface barrier
    detectors.  The instrument operated during the pre-entry phase of
    the mission, when the Probe was in the configuration with the
    heat shield still protecting the descent module.  Thus, the
    measured particles had to have high enough energies to penetrate
    the heat shield prior to their measurement by one of the 8
    instrument channels.  The instrument was designed to handle
    countrates up to 3 billion counts per second.  The data coverage
    of the investigation consisted of three data samples acquired
    near the equatorial region at 5, 4.1, and 3.2 Rj, and a
    continuous series of measurements (12 data samples) from 2.3 to
    1.25 Rj, which corresponded to a spatial resolution of about 0.02
    Rj in the innermost region prior to Probe entry into the
    atmosphere.
 
    The EPI was manufactured in cooperation between the
    Max-Planck-Institut fuer Aeronomie Katlenburg-Lindau, Technische
    Universitaet Braunschweig, Bell Laboratories, University of
    Arizona, University of Florida, NASA-Ames Research Center and
    University of Kiel.  The weight of the EPI part was 0.5 kg, the
    power consumption was 1 watt for 1 hour and the amount of bits
    for data transmission was 7680 bits.
 
    The EPI instrument was mounted on the descent module with its
    viewing direction towards the thinner rear heat shield and its
    telescope axis fixed with an inclination of 41 degrees with
    respect to the probe's figure axis.
 
 
  Scientific Objectives
  =====================
    The principal objective of this investigation was to study the
    energetic particle populations in the innermost region of the
    Jovian magnetosphere.  To achieve these scientific objectives the
    EPI made omnidirectional measurements to get intensity profiles
    with a spatial resolution of about 0.02 Rj.  In addition to the
    omnidirectional measurement, sectored data were obtained for
    certain channels.  For detailed description see Galileo Mission
    Description.
 
 
  Calibration
  ===========
    There has been a long series of detector calibration measurements
    with radioactive sources over a period of 10 years to determine
    the long-term stability of the detectors under normal and
    thermal-vacuum conditions.  During the whole time from start to
    end of the probe mission, the sensor box of the EPI flight spare
    unit was kept under vacuum conditions to check the behavior of
    the instrument.
 
    For the higher energies, the instrument was calibrated with
    protons at the Harvard Cyclotron (30-150 MeV), with electrons at
    the Lawrence Livermore accelerator (20-30 MeV), and for heavier
    particles (He, O, Si, Ar) at the Berkeley Bevatron (70-340
    MeV/n).  The calibration measurements were performed in part with
    a layer of the original heat shield material in front of the
    aperture in order to determine the influence of the absorber on
    the instrument's response functions.
 
 
  Operational Considerations
  ==========================
    The EPI instrument was operated in the pre-entry phase of the
    mission where it was difficult to get reliable information about
    the magnetic field value from the LRD.  The data acquisition
    scheme is based on the nominal probe spin frequency of 10.5 rpm,
    which was achieved very precisely according to the Mission
    Operations Final Report.
 
 
  Detectors
  =========
    The EPI detectors were totally-depleted, circular silicon surface
    barrier detectors with a thickness of 0.5 mm.  The detectors were
    shielded by a tungsten aperture of 0.27 cm thickness forming an
    opening angle of 73 degrees for detector A and of 44 degrees for
    the coincidence of both detectors.  Having a sensitive area
    diameter of 2.8 mm and a waver diameter of 7.2 mm, the edge
    effect is of special interest for these detectors.  Evidence for
    the influence of the edge effect was found in the energy loss
    distributions recorded during calibration measurements
    [FISCHERETAL1992].  Additional measurements with radioactive
    sources were carried out to determine the detection efficiency of
    the detector edges.  In parallel a mathematical model of the
    detector assembly was developed for the investigation of the
    physical details by means of a Monte Carlo method using the
    computer code GEANT from CERN.  A model of the detection
    efficiency was developed and validated by comparison with
    calibration measurements [FISCHERETAL1992].  The investigation
    revealed that the area of the detector edge was of the same order
    of magnitude as the nominal sensitive area.  This is important to
    know for a correct data evaluation.
 
 
  Electronics
  ===========
    Detector telescope and analog electronics, for 2 separate
    channels referring to detector A and B respectively, the detector
    bias supply converter, and three housekeeping channels for
    monitoring leakage currents in the two detectors and the ambient
    temperature were contained in the sensor box.  The scaling, data
    processing, and data formatting were executed together with the
    Lightning and Radio Emission Detector (LRD) data in the central
    electronics box.
 
    The energy loss dE/dx in the detectors was used to set up the 8
    channels as shown in the following table:
 
         channel    dE/dx in detector A    dE/dx in detector B
        ------------------------------------------------------
           E1             >0.1 MeV                 N/A
           E2             >0.1 MeV               >0.1 MeV
           E3                N/A                 >0.1 MeV
           P1             >0.6 MeV                 N/A
           P2             >0.6 MeV               >0.6 MeV
           P3             >0.6 MeV               >1.1 MeV
           HE             >2.3 MeV               >2.3 MeV
           HVY            >2.3 MeV               >24.5 MeV
 
    Channels E1, E3 and P1 were single counter channels, all others
    were coincidence channels.
 
    For more details see [FISCHERETAL1980].
 
 
  Operational Modes
  =================
    A total amount of 7680 bits was available for the EPI.  The
    general approach was on the one hand to achieve for comparison
    some measurements in regions of the Jovian magnetosphere where
    already particle observations exist but on the other hand to
    concentrate the main investigation to the innermost edge below 2
    Rj.  The amount of bits was divided into 15 samples each with 64
    words containing 8 bits, i.  e., 60 words for scientific data and
    an additional 4 words for status information.  Starting the first
    data sample at 5 Rj the subsequent samples were positioned at
    4.1, 3.2 and 2.3 Rj.  At 2.3 Rj the continuous sampling started.
    In case the phase locked loop circuit of the LRD was not able to
    detect the magnetic field zero crossing, the data acquisition was
    based on the nominal probe spin frequency of 10.5 rpm.  One Major
    Frame Period (MFP) was 64 seconds.  There were two different
    acquisition schemes for the first four samples down to 2.3 Rj and
    the one following.  The two schemes are shown in the next two
    tables.
 
    Data acquisition scheme for EPI samples at 5, 4.1, 3.2 and 2.3 Rj:
 
        Chan     MFP-1              MFP-2
        -----------------------------------
        E1       E11S               E12
        E2       E21S               E22
        E3       E31S               E32
        P1       P11S               P12
        P2       P21S               P22S
        P3       P31                P32
        HE       HE1S               HE2
        HVY      HVY
 
    E11S, E21S, E31S, P11S, P21S, P22S: Sectorized measurements in 8
    sectors accumulated over 3 spin periods in such a way that always
    the contents of one sector are added to the equivalent sector of
    the next rotation.
 
    HE1S: For alpha particles the measurement is elongated over 8
    spin periods but only divided into four sectors.
 
    E12, E22, E32, P12, P31, P32: Omnidirectional measurements
    accumulated over 3 spin periods.
 
    HE2: Omnidirectional measurement accumulated over 8 spin periods.
 
    HVY: Because of the uncertainty in finding heavy particles the
    investigation period is elongated to two MFP's without
    sectorization.
 
    Data acquisition scheme for EPI samples between 2.1 Rj and the
    innermost edge of the Jovian radiation belt:
 
        Chan     MFP-1              MFP-2         MFP-3         MFP-4
        -------------------------------------------------------------
        E1       E11S                             E13
        E2       E21S               E22           E23           E24
        E3       E31S                             E33
        P1       P11S                             P13
        P2       P21S               P22           P23           P24
        P3       P31                              P33
        HE       HE1S                             HE3
        HVY      HVY1               HVY2          HVY3          HVY4
 
    E11S, E21S, E31S, P11S, P21S: Sectorized measurements in 8
    sectors accumulated over 3 spin periods in such a way that always
    the contents of one sector are added to the equivalent sector of
    the next rotation.
 
    HE1S: For alpha particles the measurement is elongated over 8
    spin periods but only divided into four sectors.
 
    P31, E22, P22, E13, E23, E33, P13, P23, P33: Omnidirectional
    measurements accumulated over 3 spin periods.
 
    HE3: Omnidirectional measurement accumulated over 8 spin periods.
 
    HVY1-HVY4: Omnidirectional measurements accumulated over one MFP.
 
 
  Measured Parameters
  ===================
    The instrument counted charged particles using their energy loss
    in one or two silicon surface barrier detectors to attribute them
    to different channels.