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|
Preliminary composition results were obtained [RIEDERETAL1997B] using just X-ray energy spectra and calibration curves of peak areas versus concentration for several standards measured for each element. Conversion of combined alpha, proton, and X-ray energy spectra to elemental abundances requires further instrument calibration, which is still underway. Alpha and proton modes will be recalibrated at Martian pressures.
Refinements of the calibration have been reported in meetings [BRUECKNERETAL1998], [BRUECKNERETAL1999], [DREIBUSETAL1999], and [WAENKE1999].
Alpha and proton modes provided excellent data on the Martian rocks and soils throughout the mission.
On the order of 10 hours integrated measurement time produces high-quality alpha and proton spectra, and about 3 hours are required for high-quality X-ray spectra. All modes (alpha, proton, X-ray) are measured simultaneously when the APXS is powered on and measuring.
Other parameters measured are the temperature of the X-ray preamplifier in the sensor head, temperature of the alpha detector preamplifier in the rover's warm electronics box (°C), and the sampling duration of the alpha, proton, and X-ray measurements (hh:mm:ss).
TargetsPDS Welcome to the Planets: Mars
PDS High Level Catalog: Mars
Instrument HostMars Pathfinder Rover
InstrumentMars Pathfinder Rover Cameras
Data Set DescriptionsAPXS Raw Data
APXS Oxide Abundances
Rover Camera Raw Data
Rover Camera Calibrated Images and Mosaics
Rover Engineering Data