Alpha Proton X-ray Spectrometer
This
instrument is a foreign-provided derivative of instruments flown on the Russian Vega and
Phobos missions and identical to the unit that was planned for flight on the Russian Mars
'96 missions. Accordingly, the instrument has extensive, applicable flight heritage. With
the mobility provided by the microrover and a deployment mechanism, the APX Spectrometer
not only acquires spectra from the ubiquitous martian dust, but more importantly, is
deployed to distinct rock outcroppings, thereby analyzing the native rock composition for
the first time. The alpha and proton spectrometer portions are provided by the Max Planck
Institute, Department of Chemistry, Mainz Germany. The x-ray spectrometer portion is
provided by the University Of Chicago.
Alpha Proton X-Ray Spectrometer Front View On
Rover
This elemental composition instrument consists of alpha particle
sources and detectors for back-scattered alpha particles, protons and X-Rays. The APX
Spectrometer will determine elemental chemistry of surface materials for most major
elements except hydrogen. 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. Such an instrument can identify and determine the amounts of
most chemical elements.
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.
Alpha Proton X-Ray
Spectrometer Sensor Head
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 addition of a third detector for X-Rays Results in a significant
extension of the accuracy and sensitivity of the instrument, particularly for the heavier,
less abundant elements. Alpha sources produce characteristic X-Rays for a range of
elements, giving an instrument sensitivity that can approach the ppm level.
The APXS sensor head is mounted external to the Rover chassis on a
deployment mechanism (described below). This mechanism places the APXS in contact with
rock and soil surfaces. The APXS electronics are mounted within the rover, in a
temperature-controlled environment.