Instrument Overview
  ===================
    The Alpha Particle X-Ray Spectrometer (APXS) is part of the Athena
    payload of the two Mars Exploration Rovers (MER). The APXS sensor
    head is attached to the turret of the Instrument Deployment Device
    (IDD) of the rover. The APXS is a very light-weight instrument for
    determining the major and minor elemental composition of Martian
    soils, rocks, and other geological materials at the MER landing
    sites. The sensor head has simply to be docked by the IDD on the
    surface of the selected sample. X-ray radiation, excited by alpha
    particles and x-rays of the radioactive sources, is recorded by a
    high-resolution x-ray detector. The x-ray spectra show elements
    starting from sodium up to yttrium depending on their
    concentrations. The backscattered alpha spectra, measured by a ring
    of detectors, provide additional data on carbon and oxygen. By means
    of a proper calibration, the elemental concentrations are derived.
    Together with data from the two other Athena instruments mounted on
    the IDD, the samples under investigation can be fully characterized.
    Key APXS objectives are the determination of the chemistry of
    crustal rocks and soils and the examination of water-related
    deposits, sediments, or evaporates. Using the rock abrasion tool
    (RAT) attached to the IDD, issues of weathering can be addressed by
    measuring natural and abraded surfaces of rocks.
 
    Information in this instrument description is taken from The New
    Athena Alpha Particle X-Ray Spectrometer (APXS) for the Mars
    Exploration Rovers mission paper [RIEDERETAL2003]. See this paper
    for more details.
 
 
  Scientific Objectives
  =====================
    The chief scientific objective of the APXS is:
 
    1) to determine the major and minor elemental composition of Martian
    soils, rocks, and other geological materials at the MER landing
    sites
 
 
  Calibration
  ===========
    During the calibration campaign in the Max Plank Institute Mainz
    laboratory many geostandards were measured. These standards are
    powdered geological samples, whose elemental concentrations are
    certified by qualified institutions. The two flight instruments
    containing their flight sources were calibrated using 11
    validated samples (8 geostandards and 3 meteorites) and a set of
    oxide and metal standards.
 
    A check of the performance of the instrument after the landing on
    Mars will be done making use of the internal calibration target on
    the doors and the Compositional Calibration Target (CCT) that is
    mounted on the rover chassis in reach of the IDD. The x-ray spectrum
    of the internal calibration target shows gold, nickel, and copper
    lines. The target consists of a set of thin layers of gold, Kapton
    (carbon), and nickel on top of the copper-beryllium body of the
    doors. Energy calibration, FWHM, and linearity can be checked by
    evaluation of the copper and gold lines and comparison with
    pre-launch data.
 
    Contamination of the beryllium entrance window of the x-ray
    detector will be noticeable by an intensity reduction of the
    low-energy M lines of gold compared to the L lines. Energy
    calibration can be checked with the position of the gold peak (a
    peak and not a step because of the small thickness of the Au layer).
    The carbon step provides an additional check of consistency.
 
    The CCT consists of a magnetite plate. This target was designed
    for the needs of the MB, but, it can also be used by the APXS to
    check its FWHM usually determined for the 6.4-keV line of iron.
    As the target is mounted on the outside of the rover, it will
    eventually be covered with dust, but the line shape of the Fe
    line will not be affected.
 
 
  Operational Considerations
  ==========================
    There are a few considerations that must be taken into account
    to acquire the best data possible:
 
    1) To search for trace elements, two to four hours are
       sufficient for the x-ray mode. The search for carbon requires
       at least eight hours for the alpha mode as the alpha sensitivity
       is low. X-ray and alpha mode always operate together.
 
    2) To properly make an APXS measurement, the sensor head has to
       be correctly 'docked' at the selected sample by the IDD. The
       nominal APXS docking procedure is the following:
 
      Use images taken by the rover's front cameras: Navcams
      (navigation cameras) and/or Hazcams (hazard avoidance cameras)
      to calculate the IDD positioning
 
      Open the doors by pressing the APXS contact ring against the
      CCT or any other solid surface of the rover
 
      Position the APXS contact ring up against a selected sample
      area with a positional accuracy of 10 mm and 10 degrees on a
      target not previously contacted, or 4 mm and 3 degrees for a
      target previously contacted by any one of the IDD instruments
 
      After data acquisition, the APXS doors are closed by rotating
      the turret past a roller until the release lever is actuated
 
    3) For touch-and-go operations, optimum resolution for the x-ray
       detector is obtained at temperatures of 248 K, and the shortest
       measurement time should be at least 15 minutes
 
 
  Detectors
  =========
    The sensor head is packaged in a cylindrical enclosure 53 mm in
    diameter and 84 mm in length and terminates in an insulating flange
    of 68 mm x 68 mm. The front part facing the sample contains the
    xray detector, mounted on the axis of the instrument, a cylindrical
    source holder with six alpha sources, and six rectangular alpha
    detectors. The coaxial arrangement of sources and detectors for
    alpha particles and x-rays assures that both detectors 'see' the
    same intensity distribution across the sample.
 
    Use of a high resolution x-ray detector (silicon drift detector with
    10 mm^2 active area, a 5 micron thick Be-window and an energy
    resolution of about 160 eV @ 5.9 keV) permits high quality
    measurements. The advanced detector versions were provided by KETEK,
    Munich, Germany.
 
    There is a second group of detectors, identical to the alpha
    detectors, but not exposed to alpha particles from the sample.These
    detectors measure the background contribution to alpha spectra due
    to cosmic radiation at the surface of Mars and high energy gamma
    background of the Cm sources as well as the Moessbauer source. The
    field of view for the x rays is delineated by means of a collimator
    in front of the detector: the collimator is formed by two apertures
    made from Zr, one immediately in front of the detector and one in
    the central orifice of the source holder.
 
    The alpha sources (6 pellets) are contained in a source holder that
    attaches to the sensor head with a spring loaded bayonet-style
    mechanism. This permits quick and easy exchange of the sources
    without the need to disassemble the sensor head. The sources are
    covered with 2.5 micron thick titanium foils, turning them into
    'quasi-closed' sources (hermetically sealed sources are under
    development, but were not available for this mission). The foils
    prevent contamination of samples with source material, emitted from
    the sources as a result of 'recoil sputtering', and the same time
    reduce the energy of the alpha particles from 5.80 MeV to 5.17 MeV,
    thereby avoiding a resonance in the 12C(alpha, alpha')12C reaction
    at ca. 5.7 MeV (Figure 6). This measure, together with an optimized
    design of the source-collimator-detector geometry, significantly
    reduces the background signals from carbon and oxygen in the Martian
    CO2-atmosphere. Nevertheless, this background signal remains the
    limiting factor for the determination of carbon in the samples.
 
 
  Electronics
  ===========
    The main electronics consists of the analog signal conditioning
    segments (6-pole Gaussian filter amplifiers, threshold
    discriminators and peak detectors), an analog multiplexer, a 16-bit
    analog-to-digital converter and an 8-bit microcontroller.
 
    Control logic determines the presence of a relevant signal and
    generates an interrupt in the microcontroller. To avoid additional
    noise in the analog signal chain, the microcontroller is kept in
    idle mode until the analog signal is processed and buffered in the
    peak detectors. Selection of the appropriate multiplexer input,
    conversion of the signal amplitude to a digital number and
    registration of the signal by incrementing the number of counts in
    the corresponding amplitude channel of the respective detector is
    then handled by the microcontroller. Conversion time is typically
    200 microseconds. A digital temperature compensation routine that
    minimizes the influence of temperature changes during long
    measurements adds another 100 microseconds. For a mean count rate
    of 100 Hz, a total dead time of below 5 % is achieved.
 
    The microcontroller is equipped with a watchdog circuit that
    performs a soft reset in case of an abnormal program flow. The data
    are stored in 32 Kbyte SRAM that are buffered by a battery located
    on the main electronics board.
 
    The interface for commanding the instrument and transfer of data
    consists of an RS 422 serial link. Power is provided to the
    instrument directly from the board battery (nominally 28 V);
    voltages required by the electronics (5 V digital, +/-5 V analog
    and +/-12 V analog) are generated by its own power converter and
    filters.
 
    The x-ray spectrum is divided into 512 channels. The lower threshold
    is fixed at ~850 eV. This is sufficient to detect Na at 1040 eV. The
    upper energy limit is about 16 keV. The spectral range includes the
    K lines up to Zr and the L and M lines of higher Z elements. It also
    contains elastic scattered Pu lines at 14.3 keV and 12.6 keV, as
    well as inelastic scattered peaks. The alpha and background spectra
    use 256 channels and range up to about 6 MeV.
 
 
  Location
  ========
    The APXS sensor head is attached to the turret of the Instrument
    Deployment Device (IDD) of the rover.
 
 
  Measured Parameters
  ===================
    The APXS measures x-ray radiation and backscattered alpha spectra.
    These measurements can determine the elemental composition of the
    target on which it is docked.