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. It uses curium-244 sources for 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. At the end of the measurement, the rover retrieves the science data of 32 kilobytes, containing up to 13 consecutively taken spectra and engineering data. The internal APXS software splits the total measurement into equal time slots with an adjustable cycle time parameter. This allows us to check for repeatability and to select spectra with sufficient spectral quality.

The MSL APXS can activate an internal Peltier cooler for the X-ray detector chip. This results in a sufficient spectral resolution (FWHM) of below 200 eV at 6.4 keV below ~ -5 °C and best FWHM of < 150 eV below ~ -15 °C environmental temperature. Compared to the APXS on MER, where the best FWHM was reached at temperatures below ~ -45 °C, this allows a significantly larger operational time window for APXS analysis.

The MSL APXS has approximately 3 times the sensitivity for low Z (atomic number) elements and approximately 6 times for higher Z elements than the MER APXS. A full analysis with detection limits of 100 ppm for Ni and ~ 20 ppm for Br now requires 3 hours, while quick look analysis for major and minor elements at ~ 0.5% abundance, such as Na, Mg, Al, Si, Ca, Fe, or S, can be done in 10 minutes or less.

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). A comparison of spectra from the MER and MSL APXS is shown on the figure below.

A direct comparison of spectra taken of the reference material BCR with the MER and MSL APXS instruments. The MSL energy range has been extended to about 25 keV, where additional Compton and Rayleigh backscattered X-ray peaks can be identified. The overall sensitivity (signal per time) is increased by about a factor of three for low Z elements and ~ 6 for high z elements above Ti due to added 30 mCi sealed Cm244 sources. The peak to background ratio is comparable. No significant additional background from the MSL MMRTG is expected.

The sampled area is about 1.7 cm in diameter when the instrument is in contact with the sample. A standoff results in gradually lower intensity and an increased diameter of the measured spot. Low Z element X-rays stem from the topmost 5 microns of the sample, higher Z elements like Fe are detected from the upper ~50 microns. Sample preparation is not needed; the APXS results average the composition over the sampled area and the oxide abundances measured are renormalized 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.

Schematic diagram of the APXS sensor head, showing the relationship between the radioactive sources, sample surface, and silicon drift X-ray detector.

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

• Improved sensitivity by a factor 3 giving full analysis within ~3 hours
• Additional improved sensitivity for high Z elements by increased X-ray source strength
• Operable during Martian day by using Peltier cooler for the X-ray detector
• Basaltic calibration target mounted on the rover (on the robotic arm azimuth actuator housing), dedicated for the APXS
• No alpha channel (no Rutherford Backscattering 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. Another important aspect of the APXS investigation will be to relate the chemical composition of the MSL landing site and the results from the MSL payload to what has been found by the previous landed missions, which used similar X-ray spectroscopy methods.

The APXS will be fully calibrated using standard geological samples in the laboratory. An onboard basaltic rock slab, surrounded by a nickel plate, will be used periodically to check the performance and calibration of the instrument. The data analysis is theoretically well understood and delivers unambiguous element identification and accuracy on the order of ~10%, mainly limited by microscopic sample heterogeneity (i.e., grain size effects). The APXS data analysis is fast and allows a quick turnaround of results used for tactical rover operations.

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 detection 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.