NSSDCA ID: 2003-027A-05
Mission Name: SpiritThe Alpha Particle X-ray Spectrometer (APXS) is designed to determine the elemental chemistry of rocks and soils on the surface of Mars. The APXS sensor head is mounted on the rover arm, or instrument deployment device, and is operated by placing the sensor directly on the sample. The APXS works by exposing the target to energetic alpha particles and X-rays emitted by a radioactive 244Cm source in the sensor head and measuring the energy spectrum of backscattered alpha particles and emitted X-rays from the target. Using this technique abundances of all rock-forming elements except hydrogen can be measured.
The APXS sensor head contains six 244Cm sources with a total source strength of 30 mCi. Each source is covered with 3 micrometer aluminum foils that reduce the energy of the emitted alpha particles from 5.8 MeV to 5.2 Mev, which serves to suppress the atmospheric carbon dioxide background. Collimators in front of the sources give a field of view 38 mm in diameter at 29 mm working distance. Six alpha detectors surround the source and inside these is a high-resolution silicon drift X-ray detector. A pair of doors protects the sensor from martian dust. When the sensor head is put in place, the doors swing open to expose the source and detectors. The instrument electronics are housed in the warm electronics box.
The X-ray detector can measure major elements such as Mg, Al, Si, K, Ca, and Fe and minor elements including Na, P, S, Cl, Ti, Cr, and Mn. The FWHM of the x-ray detector is 160 eV at 6.4 keV. The alpha detector is better suited for lighter elements, particularly C and O. FWHM for detections at 5.8 MeV is less than 100 keV. The detection limit is about 0.5 to 1 weight percent. The time for a full measurement is at least 10 hours. The x-ray mode alone takes significantly less time than this. Most data accumulation is planned for nighttime when temperatures are lowest, giving the best spectral resolution.
Questions and comments about this experiment can be directed to: Dr. David R. Williams
Name | Role | Original Affiliation | |
---|---|---|---|
Prof. Raymond E. Arvidson | Deputy Principal Investigator | Washington University | arvidson@wunder.wustl.edu |
Dr. Rudolf Rieder | Lead Investigator | Max-Planck-Institut fur Chemie | rieder@mpch-mainz.mpg.de |
Dr. Steven W. Squyres | Principal Investigator | Cornell University | sws6@cornell.edu |
Diagram of the APXS, drawing of the suite of instruments on the end of the rover arm, and the APXS on Earth
APXS Technical Briefing (PDF file)
NASA Selects 28 Participating Scientists for Mars Rover Mission
- NASA Press Release 29 May 2002