Instrument Overview
  ===================
    The Alpha/Proton/X-ray Spectrometer (APXS) is a foreign-provided
    copy of an instrument design flown on the Russian Vega, Phobos,
    and Mars '96 missions.  Thus, the instrument has extensive,
    applicable flight heritage.  The alpha and proton spectrometer
    portions were provided by the Max Planck Institute, Mainz
    Germany.  The X-Ray spectrometer portion was provided by the
    University of Chicago.  Integration of the instrument portions
    was the responsibility of the Principal Investigator.
 
    This elemental analysis instrument consists of nine curium-244
    alpha-particle sources and three detectors for each of three
    types of particles: backscattered alpha particles, protons, and
    X-rays.  The APX spectrometer was placed against rocks and soils
    on Mars to determine the abundances of all elements except
    hydrogen, with a lower detection limit of about 0.1 weight
    percent.  The analytical process is based on three interactions
    of alpha particles with matter: elastic scattering of alpha
    particles by nuclei, alpha-proton nuclear reactions with certain
    light elements, and excitation of the atomic structure of atoms
    by alpha particles leading to the emission of characteristic
    X-rays.  The approach used is to expose material to a radioactive
    source that produces alpha particles with a known energy, and to
    acquire energy spectra of the alpha particles, protons and X-rays
    returned from the sample.
 
    The basis of the alpha mode of the instrument is the dependence
    of the energy spectrum of alpha particles scattered from a
    surface on the composition of the surface material.  The method
    has the best resolving power for the lighter elements (carbon and
    oxygen).  A least-squares analysis of a complex spectrum from an
    unknown sample in terms of a library of known, pure element
    spectra determines the elements and their abundance in the
    sample.  A characteristic of the alpha technique is that, due to
    variations in scattering intensity with atomic number, there is a
    minimum in the scattering probability for elements with atomic
    numbers between 9 and 14.  This includes the important elements
    Na, Mg, and Al, so the alpha mode alone is insufficient for these
    elements.  On the other hand, these elements produce protons when
    bombarded with alpha particles.  The proton spectra for alpha
    particles interacting with elements with atomic numbers from 9 to
    14 are very characteristic of the individual elements, reflecting
    the resonance nature of the nuclear interactions involved.  The
    proton mode allows their detection and measurement.  The alpha
    particles from the radiation sources are also an efficient source
    for production of characteristic X-rays.  The addition of a third
    detector for X-rays therefore results in a significant extension
    of the accuracy and sensitivity of the instrument, particularly
    for the heavier, less abundant elements.  In the X-ray technique,
    characteristic X-rays are emitted when the low electron orbit
    vacancies (in the K- and L-shells) produced by bombardment of
    atoms by alpha particles are filled by electrons from higher
    orbits.
 
    The APXS electronics are mounted in the rover warm electronics
    box in a temperature-controlled environment.  Cables leaving the
    electronics box connect the APXS electronics to the APXS sensor
    head, which contains the radioactive sources and particle
    detectors.  The instrument sensor head is held by a robotic arm
    (deployment mechanism) attached to the back of the rover.  The
    deployment mechanism, which places the APXS in contact with rock
    and soil surfaces, interfaces the APXS with the microrover.  For
    the APXS to conduct a high quality measurement of a rock or soil
    sample, it must be placed with its front aperture ring in contact
    with the sample surface and the axis of the sensor head must be
    within 20 degrees of normal to the surface.  The linkage is
    designed to allow the APXS to be placed at a variety of
    elevations above nominal ground level and at a variety of
    rotational orientations.  The mounting of the APXS to the
    deployment mechanism permits about 20 degrees of compliance
    motion as the APXS is placed in contact with the sample.  Three
    contact sensors on the deployment mechanism bumper ring indicate
    to the rover that the positioning is complete, thereby
    terminating the positioning motions.  The sources and detectors
    are recessed in a cylinder 4 cm behind the bumper and the area of
    rock or soil that is analyzed with the instrument is a circle 5
    cm across.
 
    Instrument Id                   : APXS
    Instrument Host Id              : MPFR
    PI PDS User Id                  : R. Rieder
    Instrument Name                 : ALPHA PROTON X-RAY SPECTROMETER
    Instrument Type                 : SPECTROMETER
    Build Date                      : August 28, 1995
    Instrument Mass                 : 570 grams
    Instrument Length               : 10.5 cm
    Instrument Width                : 8.0 cm
    Instrument Height               : 6.5 cm
    Instrument Manufacturer Name    : Max Planck and Univ. Chicago
 
 
  Scientific Objectives
  =====================
    The Alpha-Proton X-Ray Spectrometer (APXS) will determine the
    dominant elements that make up the rocks and other surface
    materials at the landing site.  A better understanding of these
    materials will address questions concerning the composition of
    the martian crust and the processes that led to the formation and
    alteration of rocks and soils.  This also provides some ground