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REQUIRED READING:
- McNutt et al.  (2007) [MCNUTTETAL2007]
- SOC Instrument Interface Control Document ICD, originally delivered with
  this archive
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      This PEPSSI description was adapted from the New Horizons
      website, the SOC Instrument ICD, and McNutt et al. (2007)
      [MCNUTTETAL2007].
 
 
  INSTRUMENT OVERVIEW
  ===================
 
 
    PEPSSI is a medium-energy particle spectrometer.
 
 
    SPECIFICATIONS
    --------------
 
      NAME:                    PEPSSI
      DESCRIPTION:             Medium energy particle spectrometer
      PRINCIPAL INVESTIGATOR:  Ralph McNutt, APL
      ENERGY RANGE:            25-1000 keV (protons)
                               60-1000 keV (atomic ions)
                               25-500 keV (electrons)
      FIELD OF VIEW:           160 x 12 deg
      ANGULAR RESOLUTION:      25 x 12 deg
      ENERGY RESOLUTION:       0.25 keV
 
      PEPSSI (Pluto Energetic Particles Spectrometer Science Investigation)
is a hockey-puck-size (7.6cm diameter by 2.5cm thick), time-of-flight (TOF)
spectrometer that measures ions and electrons over a broad range of energies
and pitch angles.  Particle composition and energy spectra are measured for H
to Fe from ~15 keV/nucleon to 1 MeV/nucleon and for electrons from 15 keV to
700 keV.  The PEPSSI instrument traces its heritage back to the MESSENGER
Energetic Particle Sensor (EPS) instrument. EPS/PEPSSI was developed with the
support of a NASA Planetary Instrument Definition and Development (PIDDP)
grant aimed at designing a low-mass, low-power sensor that can measure
energetic pickup ions produced near planets and comets (Andrews et al., 1998
[ANDREWSETAL1998]; McNutt et al., 1996 [MCNUTTETAL1996]).  The overall PEPSSI
instrument weighs 1.5 kg and uses a maximum of 1.4 W of power.
 
 
  Scientific Objectives
  =====================
 
    Summary
    -------
 
      1.  Determine the escape rate of Pluto's atmosphere.
 
      2.  Measure the interaction of the solar wind with Pluto's
          ionosphere.
 
      3.  Determine the source and nature of energetic particles
          found near Pluto.
 
 
    Details
    -------
 
      PEPSSI's primary objective is to determine the mass, energy spectra,
and directional distributions of energetic particles, with a resolution able
to discriminate between the various types of species expected at Pluto.
 
      PEPSSI will also search for neutral atoms that escape Pluto's atmosphere
and subsequently become charged by their interaction with the solar wind.
 
      Finally, PEPSSI will determine the Composition and density of pick-up
ions from Pluto, which indirectly addresses the atmospheric escape rate.
 
 
  Detector
  ========
 
    Summary
    -------
 
 
      A time-of-flight (TOF) section feeds a solid-state silicon detector
(SSD) array. Each SSD has 4 pixels, 2 dedicated to ions, and 2 for electrons.
PEPSSI's field of view (FOV) is fan-like and measures 160 degrees x 12
degrees, divided into six angular sectors of 25 degrees x 12 degrees each.
Ions entering the PEPSSI FOV generate secondary electrons as they pass
through entrance and exit foils in the TOF section, providing 'start' and
'stop' signals detected by a microchannel plate (MCP).
 
 
   Details
   -------
 
 
      PEPSSI is a compact particle telescope with a time-of-flight (TOF)
section and a solid-state detector (SSD) array.  A mechanical collimator
defines the acceptance angles for the incoming ions and electrons.  The TOF
section is axially symmetric; entrance and exit apertures are 6 mm wide with
an azimuthal opening angle of 160 degrees.  The entry and exit apertures are
covered by thin (~7E-6 g/cm**2) polyimide/aluminum and (~10E-6  g/cm**2)
palladium/carbon foil mounted on high-transmittance stainless-steel grids,
respectively.  The foil thickness and composition is a compromise to minimize
the energy threshold, secondary electron production, and scattering of
particles in the foil while blocking UV from the direct Sun and Lyman-alpha
background.  PEPSSI measures the ion TOF using secondary electrons generated
as the ion passes through the entrance and exit foils in the spectrometer.
Total energy is measured by the SSD array.  Each of the six SSDs has two
pixels, three of the SSDs are dedicated for ion measurement.  The other three
have one pixel covered with ~1E-6 m Al absorber, to block low energy ions and
permit measurements of electrons.  The fan-like collimator together with the
internal geometry defines the acceptance angles.  The FOV is 160 degrees by
12 degrees with six angular sectors of 25 degrees each; the total geometric
factor is ~0.15 cm**2-sr.  As an ion passes through the sensor, it is first
accelerated by the potential of ~3 kV on the front foil prior to hitting it.
The ion generates secondary electrons at the foils, which are then
electrostatically steered to well-defined separate regions on a single micro
channel plate (MCP), providing 'start' and 'stop' signals for the TOF
measurements (from 1 ns to 320 ns).  The segmented MCP anode, with one start
segment for each of the six angular entrance segments, allows determination
of the direction of travel even for lower-energy ions that do not give an SSD
signal above threshold.
 
      The combination of measured energy and TOF provides unique particle
identification by mass and particle energy in the range: for protons from 15
keV to 1 MeV; for heavy (CNO) ions from 80 keV to 1 MeV.  Lower-energy (>3
keV) ion fluxes are measured by TOF and pulse-height analysis (PHA) of the
signal they produce in the MCP, providing particle identification and
velocity spectra at these energies as well.  Molecular ions, expected from
Pluto's atmosphere, will break up in the foil prior to their full detection,
but will be detected as high-mass events.  Internal event classification
electronics determine the mass and produce an eight-point energy spectrum for
each of four species for six arrival directions.  Energetic electrons are
measured simultaneously in the dedicated electron pixels in the range from 20
to 700 keV.  Only protons with energies > 300 keV (expected to be very rare
at Pluto) can penetrate the absorbers on these pixels, and even those would
be eliminated by on-board MCP coincidence requirements and ground comparisons
with the simultaneously measured ion flux.
 
 
  Electronics
  ===========
 
 
    Extensive uses of miniaturization and custom electronic in the design
allow PEPSSI to weigh less than 1.5 kg and consume less than 1.4 W.  PEPSSI
is made up of six modular 10cm x 10cm slices.  They consist of:
 
      1) Energy board;
      2) High Voltage Power Supply (HVPS);
      3) TOF board;
      4) Digital processing board;
      5) Common event processor board; and
      5) Low Voltage Power Supply (LVPS) board.
 
    See the SOC Instrument ICD, originally archived with this volume, and
McNutt et al. (2007) [MCNUTTETAL2007] for further details.
 
 
  Operational Modes
  =================
 
 
    The PEPSSI instrument can operate in two modes: Normal and Diagnostic. On
the spacecraft, each event generates a PHA record.  This record is classified
by event type: Electron, High-Energy Ion (or 'Hi-Ion' or 'Triple'), or
low-energy ion (or 'Low-Ion,'  'Double,' or 'TOF-only').  In diagnostic mode,
events are not classified; alternatively, all events are 'diagnostic events'.
 Events of a given type are further classified into 'Rate Boxes' by their
energy and/or time of flight (TOF).  Thus each event has a type, a rate box,
and a detector in which it occurred.
 
 
    Bad Time Intervals (BTIs)
    -------------------------
 
 
      Various instrument conditions can make the PEPSSI data difficult or
impossible to use for scientific purposes.  Powering down, ramping the high
voltage power up or down, running in diagnostic mode, etc. will all make the
PEPSSI data unusable for standard analysis.  The PEPSSI_BTI.TAB file contains
a table of 'Bad Time Intervals' which should not be used for science
analysis.  It should be noted that the entire 'Launch' phase of PEPSSI data
is classified as a BTI.  While not actually a BTI, the period between 2007
day 144 and day 178 should be treated with caution as well.  The PEPSSI Rate
Box tables were changed on day 144 and calibration and analysis of this
period is still preliminary.
 
 
    See the SOC Instrument ICD, originally archived with this volume, and
McNutt et al. (2007) [MCNUTTETAL2007] for details.
 
 
  Measured Parameters
  ===================
 
    Particle energy information, measured by the SSD, is combined with TOF
information to identify the particle's composition. Each particle's direction
is determined by the particular 25 degrees sector in which it is detected.
Event classification electronics determine incident mass and energy, with 12
channels of energy resolution.
 
A typical measurement includes 8-point spectra for protons and electrons and
reduced resolution energy spectra for heavier ions for all six look
directions.
 
    In calibration, the rate, in counts/s, of each energy and/or TOF bin is
converted to flux i.e. differential intensity (1/cm**2-sr-s-keV).
 
 
  Calibration
  ===========
 
 
      The calibration parameters are ideally be determined through a
combination of all of the following:  ground measurements; analysis of the
in-flight calibration alpha-particle source; modeling; intercalibration with
known measurements.  Currently only the final method has been employed, which
has the obvious drawback of not providing an independent determination of the
absolute flux.  Therefore the fluxes provided in CODMAC Level 3 data should
not be used as is to conduct science that is relying on absolute fluxes for
scientific interpretation unless the user determines the fluxes independently.
 
    Brief summaries of the flux and PHA calibrations are given here.  See
McNutt et al. (2007) [MCNUTTETAL2007] and the SOC Instrument Interface
Control Document (distributed with this archive) for details.
 
 
    Flux Calibration
    ----------------
 
      The calibration quantities are energy pass-band (dE = Ehi - Elo, lower
and upper limit of the energies of the particles measured), measurement
efficiency (N, the fraction of valid incident particles that are actually
measured), the geometry factor (G, the measurement of the physical detector
size and solid angle subtended by the field of view).  These values are all
given and applied with uncertainties in the CODMAC Level 3 files.
 
      The differential intensity, j (1/cm**2-sr-s-keV), is calculated in
terms of the counts C, time coverage T (s), geometric factor G (cm**2-sr),
upper and lower energy bounds Ehi and Elo (keV), and detection efficiency N:
 
        j = (C/T)/(G * dE * N),
 
where  E = Ehi - Elo.  The uncertainty values assume Poisson statistics for
C, no error in T, absolute errors in G, Ehi, Elo and relative error in N.
I.e., formally the counts are C = C +/- deltaC, the energies are E = E +/-
deltaE, the geometry factor is G = G +/- deltaG.  The efficiency is N = [N *
epsilon or N / epsilon], where epsilon = deltaN/N, to one sigma confidence.
 
      In this initial delivery of the PEPSSI data from the Launch and Jupiter
phases of the New Horizons mission these values are supplied to convert the
instrument specific data (e.g., count rates) into physical
instrument-independent units (e.g., differential intensity), as well as
computing the physical quantities themselves.  It must be stressed that these
are preliminary values that should not be used without significant effort
from the user to understand their limitations (see the SOC Instrument ICD,
provided with this archive, and McNutt et al., 2007 [MCNUTTETAL2007] for a
description of the method used to calculate differential intensity, also
called flux).
 
 
    PHA Event Calibration
    ---------------
 
 
      The following quantities are provided in the calibrated data products.
The linear calibration constants are in the data labels; see the SOC
Instrument ICD and McNutt et al., 2007 [MCNUTTETAL2007] for details.
 
      * Calibrated Deposited Energy and/or TOF values
 
      * Speed column from the TOF assuming a 6.0cm flight path.
 
      * The PHA_HIGH_ION calibrated data contain additional quantities,
        where each value indicates the Incident energy assuming that
        the event is of that (H, He, O, or S) species:
        - H_Incident_Energy
        - He_Incident_Energy
        - O_Incident_Energy
        - S_Incident_Energy
 
      Events with the multi-hit (cross talk) flag set have been excluded.
Quantities of limited usefulness (such as Heavy Ion Discriminator triggers)
have been excluded.   Because of the difficulty of removing priority scheme
biases from non-N2 PHA data, only N2 (APID == 0x692) PHA data is present in
the calibrated PHA data.
 
      The Rate_Normalized_Weight quantities have had Priority Group artifacts
removed from the PHA data by the procedure described in the SOC Instrument
ICD.  This column is usually used in making histograms of the High Energy Ion
PHA data.
 
 
  Filters, Optics, Locations, Subsystems
  ======================================
 
 
    N/A