Alpha Particle X-ray Spectrometer (APXS)

PI: Ralf Gellert, University of Guelph, Canada

The APXS (Alpha-Particle X-ray Spectrometer) for MSL is an improved version of the APXS that flew successfully on Pathfinder and the Mars Exploration Rovers Spirit and Opportunity (Rieder et al., 1997, 2003; Gellert et al., 2006). The MSL APXS takes advantage of a combination of the terrestrial standard methods Particle-Induced X-ray Emission (PIXE) and X-ray fluorescence (XRF) to determine elemental chemistry. Curium-244 sources irradiate a sample with alpha particles and X-rays, permitting X-ray spectroscopy to determine the abundance of major elements down to trace elements, from sodium to bromine and beyond.

The instrument consists of a main electronics unit in the rover's body and a sensor head mounted on the robotic arm. Measurements are taken by deploying the sensor head towards a desired sample, placing the sensor head in contact or hovering typically less than 2 cm away, and measuring the emitted X-ray spectrum for 15 minutes to 3 hours without the need of further interaction by the rover. Fifteen minutes or less is sufficient to retrieve major elements above ~ 0.5% abundance, such as Na, Mg, Al, Si, Ca, Fe, or S. Longer data acquisition is needed to determine trace elements like Ni, Zn or Ge at abundances of 20 to 100 ppm. The MSL APXS uses the same 10 mm-squared SDD X-ray detector as MER. The full-width at half maximum at 5.9 keV is ~140eV at low temperature, an improvement from ~155 eV for MER.

On MER, the elements detected by the APXS in rock and soil samples are typically Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Zn, and Br (e.g., Rieder et al., 2004; Gellert et al., 2006). Elevated levels of Ge, Ga, Pb, and Rb were found in some of the MER samples (e.g., Clark et al., 2007).

The sampled area is about 15 mm in diameter. Low Z (atomic number) element X-rays stem from the topmost 5 micrometers of the sample, higher Z elements like Fe are detected from the upper ~ 50 micrometers. Sample preparation is not needed; the APXS results average the composition over the sampled area and the oxide abundances measured are re-normalized to 100%. However, on MSL, a dust removal tool (brush) is available to remove loose material from a rock surface before making an APXS measurement.

The major improvements and changes compared to the MER APXS are:

• Improved sensitivity by a factor 3, giving a full chemical analysis within ~ 3 hours
• Additional improved sensitivity for high Z elements by increased X-ray source strength
• Operable during Martian day due to Peltier cooler for the X-ray detector (maximum ambient temperature for good resolution X-ray spectra is increased to -5C from -40C on MER)
• Basaltic calibration target mounted on the rover, dedicated for the APXS
• No alpha channel (no Rutherford back-scattering spectroscopy)
• Compressed short duration X-ray spectra ( ~ 10 seconds ) can be used to steer the arm movement in a "proximity mode"

The main objective of the APXS is to characterize the geological context of the rover surroundings and to investigate the processes that formed the rocks and soils. The high precision and low detection limits, especially for salt forming elements like S, Cl, and Br, allow identification of local anomalies and guided in situ sample selection for the analytical instruments of MSL. The rover observation tray for processed samples will allow the APXS to provide additional characterization of the samples collected and prepared for the analytical instruments, connecting the analytical instrument results with the in situ samples. MSL sample preparation with the brush will allow in situ APXS investigations of thin alteration rinds or near-surface layers or veins which cannot be collected by the drill for the analytical instruments.

The APXS has been calibrated using standard geological samples in the laboratory. The data analysis is theoretically well understood and delivers unambiguous element identification and accuracy on the order of ~10%, mainly caused by microscopic sample heterogeneity. The APXS data analysis is fast and allows a quick turnaround of results used for tactical rover operation.

The elemental data can be used to extract normative mineralogy either from scratch or using constraints from the mineralogy provided by CheMin. A newly developed method (Campbell et al., 2008) using the backscattered peaks of the primary X-ray radiation allows detecting of X-ray invisible compounds like bound water or carbonates, if present in significant amounts ( greater than ~5% by weight).

The MSL APXS is funded by the Canadian Space Agency, with MDA Corporation as prime subcontractor. Funding for the science team comes from CSA, NASA, and the University of Guelph.