(descriptions excerpted from [SIMPSONETAL1992A])
 
  Instrument Overview
  ===================
    The COSPIN Low Energy Telescope (LET) measures the flux, energy
    spectra and elemental composition of solar energetic particles
    and low energy cosmic ray nuclei from hydrogen up to iron.  The
    instrument covers an energy range from ~ 1 to ~ 75 MeV/n, using
    a double dE/dX vs.  E telescope.  Comprehensive on-board
    particle identifier electronics and an event priority system
    enable rare nuclei to be analyzed in preference to the more
    common species.  Isotope separation for light nuclei such as He
    is also achieved.
 
    The LET sensor and associated analogue and digital electronics
    are mounted in the central portion of the SIM-1 package,
    between the Anisotropy Telescopes and the Digital Processing
    Unit.  The instrument aperture is protected by a hinged cover
    that was opened by telecommand after launch.  A gas purge
    system was also included to provide additional protection
    against detector contamination during ground testing and the
    ascent phase of the launch.
 
 
  Detectors
  =========
    The LET sensor consists of a four-element solid-state detector
    telescope surrounded by a cylindrical plastic scintillator
    anticoincidence shield, together with associated analogue
    electronics.  Detectors D[1] and D[2] are large-area (6 cm^2)
    surface barrier devices having nominal thicknesses of 30
    microns (D[1]) and 100 microns (D[2]), while D[3] and D[4] are
    2000 micron-thick Li-drifted devices of 10.0 and 12.5 cm^2
    active area, respectively.  D[4] forms part of the
    anticoincidence shield.  The aperture of the telescope is
    covered by two thin foils, an inner Ti foil (2 microns) and an
    outer Kapton foil (8 microns), included for electrical
    screening and thermal control purposes, respectively.  The
    telescope geometrical factor, defined by two circular
    collimators mounted in front of D[1] and D[2] in order to
    reduce unwanted edge effects, has a value of 0.58 cm^2 sr for
    the coincidence channels.  Low resolution single-detector only)
    measurements of protons and alpha particles are also made.  In
    this case, the geometrical factor is ~ 9.1 cm^2sr.
 
    The signals from detectors D[1], D[2] and D[3] are fed into
    individual amplifier chains consisting of a charge-sensitive
    preamplifier followed by a parallel combination of three pulse-
    shaping voltage amplifiers (PSA).  To accommodate the large
    dynamic range required, separate low- and high-gain PSA have
    been used, selected via an analogue switch controlled by the
    third (fast) PSA in conjunction with a discriminator.  The
    selected outputs are fed into a common 10 bit (1024 channel)
    ADC that provides pulse height information.  The fast PSA are
    followed by a number of threshold discriminators, the outputs
    of which are used in the coincidence logic to define a series
    of counting rate channels.
 
    In addition, the outputs of the slower PSA are fed into a
    Particle Identifier (PI) circuit that provides both counting
    rate and event priority information.  The LET PI circuit makes
    it possible to obtain the counting rates corresponding to
    groups of nuclear species, and comprises a set of analogue
    function generators and discriminators that divide the
    instrument response into regions of different nuclear charge.
    The actual analogue functions used are of two types, namely
 
          E[1]+ b[i]E[2] = a[i]                 (1)
 
      and
 
          E[2](b[i] + E[i]) = a[i]              (2)
 
    where E[1], E[2] and E[3] are the energies deposited in
    detectors D[1], D[2] and D[3], respectively, and a[i], b[i] are
    constants.  For particles stopping in D[2], the loci of points
    satisfying equation (1) for given pairs of (a[i],b[i]) values
    define boundaries on the [DELTA]E ([is defined as]E[1]) vs.
    residual E ( E[2]) diagram that separate the tracks
    corresponding to different elements or groups of elements into
    so-called 'charge groups'.  The charge groups defined for the
    D[1] - D[2] range are: protons; (He^-3, He^-4); (Li, Be, B);
    (C, N, O); Z >= 10 nuclei.  In a similar way, charge group
    boundaries for particles stopping in D[3] are defined by
    equation (2) with suitable pairs of (a[i], b[i]) values.  A
    different functional form is required in this case because of
    the pronounced curvature of the [DELTA]E vs.  residual E tracks
    resulting from the large thickness of D[3] relative to D[2].
    The same charge groups as before are defined for the D[1] -
    D[2] - D[3] range, with the addition of a high-Z group
    corresponding to Z >= 20.
 
    Each of the charge groups has an associated counting rate
    register which is incremented each time a valid event within
    the group is observed.  In the spacecraft prime (real time
    telemetry) mode, accumulation intervals are 32 s for the proton
    and alpha particle rate channels and 128 s for the heavy ion
    rate channels.  In addition to providing counting rate
    information, the PI output is used to generate a 4-bit event
    code that controls the Event Priority System.  The latter is
    included in order to maximize the LET PHA data-collection
    efficiency for the rarer nuclear species in the cosmic ray
    flux.  Such a system is needed because the LET telemetry
    allocation (52 bits per s at the highest bitrate) limits the
    number of PHA events that can be transmitted to a maximum of 1
    per second.  Each pulse-height analyzed event is assigned a 4-
    bit priority value on the basis of its event code via
    predefined sequences stored in a ROM.  Only the pulse heights
    corresponding to the highest-priority event occurring within
    every 1 s sampling period are transferred to the telemetry
    stream.  The sequence of priority assignments for all event
    codes is changed periodically in order to prevent biasing
    effects.  In addition to pulse height data and proton, alpha
    and heavy ion rates, the LET data frame contains digital status
    information and analogue housekeeping values, as well as
    counting rate data for the individual detectors.  Housekeeping
    information includes instrument voltages, the detector leakage
    currents and temperature values for the detector telescope and
    electronics.  A summary of the LET digital data channels is
    given in Table 4.
 
    Also included in the instrument is an In-Flight Calibrator
    (IFC) circuit that checks, on command, the electrical
    characteristics of the instrument by delivering a sequence of
    pulses having well-defined amplitudes to all amplifier inputs.
 
 
  In-flight Performance
  =====================
    The LET aperture cover was opened by means of the pyrotechnic
    actuators on October 19.  On COSPIN turn-on four days later,
    all housekeeping data indicated that the instrument status was
    nominal, and the scientific data showed that a moderate solar
    flare event was in progress at the time of switch-on.
 
    Another example of LET data acquired in flight soon after turn-
    on . . .  covers a 4-day period starting at 00:00 hrs UT on 7
    November 1990, at which time a series of impulsive solar flare
    events occurred on the Sun.  Such events are known to produce
    an enrichment in the isotope ^(3)He relative to the usually
    more abundant 4He.  Both isotopes were present throughout the
    period and were well resolved by the LET.
 
    In addition to the initial flight data from the Ulysses
    instrument presented above, essentially identical LET sensors
    that were flown on the Phobos 1 and 2 space probes have
    provided a wealth of data on the solar and interplanetary
    energetic particle populations during the rising phase of solar
    cycle 22 [Marsden et al., Nucl. Instr.  and Meth.  290, 211,
    1990; Marsden et al., Planet.  Space Sci., 39, 57, 1991].
    Finally, a substantial database on the performance of the LET
    has been accumulated during ground testing; the instrument has
    been extensively calibrated at particle accelerator facilities,
    with exposures to both heavy ion and proton beams [Kamermans et
    al., Nucl. Instr.  and Meth.  171, 87, 1980; LeBorgne et al.,
    Report ESA STM-224, 1981; Marsden et al., Nucl. Instr.  and
    Meth.  221, 619, 1984].
 
    The in-flight performance of the COSPIN LET instrument has
    matched that expected from ground calibration and test.  While
    the majority of single detector counting rates are dominated by
    counts produced by the RTG neutron and gamma ray background,
    all coincidence channels show good immunity to RTG induced
    backgrounds.  An exception is the low energy 0.3 - 1.5 MeV
    electron channel, which is dominated by the RTG contribution
    except during large solar particle events and, we expect, in
    Jupiter's magnetosphere.  The performance in flight so far
    demonstrates that the experiment is capable of making important
    contributions to our knowledge of the particle populations in
    the heliosphere during the exploratory out-of-ecliptic Ulysses
    mission.
 
 
  Measured Parameters
  ===================
    Table 4.  Low energy telescope data channels.
 
      Name          Primary    Energy   Geom.    Avg   Sectors  PHA
                    Particle   Range    Factor   Time           Event
                    Type      (MeV(/n)) (cm^2sr) Res(s)         Code
 
      L1/P1         proton     0.9-1.2   9.1      32*    --      --
      L2/P2         proton     1.2-3.0   9.1      32*    --       1
      L3/P3         proton+    1.8-3.8   0.58     32*    --       3
      L4-L11/P3S    proton+    1.8-3.8   0.58     32      8      --
      L12/P4        proton+    3.8-8.0   0.58     32*    --       4
      L13-L20/P4S   proton+    3.8-8.0   0.58     32      8      --
      L21/P5        proton     8.0-19.0  0.58     32*    --       5
      L22/A1        He         1.0-5.0   9.1      32*    --       2
      L23/A2        He+        1.9-3.7   0.58     32*    --       6
      L24/A3        He+        3.7-8.4   0.58     32*    --       7
      L25/A4        He+        8.4-19    0.58     32*    --       8
      L26/H1        Li,Be,B+   1.9-4.9   0.58    128*    --       9
      L27/H2        Li,Be,B+   4.9-26    0.58    128*    --      10
      L28/H3        C,N,O+     2.6-7.1   0.58    128*    --      11
      L29/H4        C,N,O+     7.1-39    0.58    128*    --      12
      L30/H5        Z>=10+     3.0-9.0   0.58    128*    --      13
      L31/H6        10<=Z<=20+ 9.0-50    0.58    128*    --      14
      L32/H7        Z>20+      12-75     0.58    128*    --      15
      L33/E1        electron   0.3-1**   --      128*    --      --
      L34-L38/S1-S5 --         --        --      128*    --      --
 
      * Rigid interval synchronized to telemetry format
      + Particle identification by hyperbolic discriminator
        thresholds (I1-I9)
      ** Dominated by RTG-induced background in quiet times