Ralf Gellert, principal investigator, University of Guelph, Canada
The Alpha Particle X-Ray Spectrometer (APXS, pronounced apex), is one of the instruments on Curiosity meant for (near) contact with Mars rocks and soils. APXS determines the chemical composition of rocks, soils, and processed samples. 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. Characterising the geological context will help the scientist to make informed choices about acquiring samples for analysis inside the mission’s analytical laboratory instruments: SAM and CheMin. Learning which elements, in what concentrations, are in the targets will help researchers identify processes that formed the rocks and soils in the area of Mars where Curiosity is working.
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.
To determine precisely what elements are in a given sample APXS uses two techniques:
* Particle-Induced X-ray Emission (PIXE)
* X-ray Fluorescence (XRF).
PIXE relies upon the fact that fast moving alpha particles can knock electrons from the lowest energy levels of atoms right out of the atom itself. This leaves an atom with an unstable configuration of electrons, and one of the electrons in the high energy levels of the atom will now drop down to the low level one, emitting an X-ray as it does so. APXS has a detector to capture these X-rays and we can determine the elements in the sample by looking at the energy of the X-rays – each element emits X-rays with very specific energies, an energy signature if you like. PIXE is good for detecting lighter elements, essentially sodium through to calcium.
APXS contains Curium 244 as a source of alpha particles. But it also decays into Plutonium 240 (one of the tracer elements people have looked for in tracking Fukushima emissions) which emits X-rays that can in turn excite X-ray emission in other atoms. This is called X-ray Fluorescence and the idea is pretty much the same as PIXE, accept instead of alpha particles causing the excitation, this time it is incoming X-rays. The two methods turn out to be very complementary as XRF is good for detecting heavier elements, calcium through to zirconium.
The major improvements and changes compared to the MER APXS are:
1. Improved sensitivity by a factor 3 giving full analysis within ~3 hours
2. Additional improved sensitivity for high Z elements by increased X-ray source strength
3. Operable during Martian day by using Peltier cooler for the X-ray detector
4. Basaltic calibration target mounted on the rover (on the robotic arm azimuth actuator housing), dedicated for the APXS
5. No alpha channel (no Rutherford Backscattering spectroscopy)
6. Compressed short duration X-ray spectra ( ~10 seconds ) can be used to steer the arm movement in a “proximity mode”
For the APXS to work effectively you have to be able to get close to the sample. So the emitter and detector part of APXS sits on the end of Curiosity’s robotic arm while the main electronics are inside the body of the rover.
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 the scientist to select spectra with sufficient spectral quality.
Curiosity’s APXS can make measurements in about one-third the time needed for equivalent readings by its predecessors. This improvement in sensitivity results mainly from shrinking the distance between the X-ray detector and the sample by about one-third, to 19 millimeters (0.75 inch).
The spectrometer uses the radioactive element curium as a source to bombard the target with energetic alpha particles (helium nuclei) and X-rays. This causes each element in the target to emit its own characteristic X-rays, which are then registered by an X-ray detector chip inside the sensor head. The investigation’s main electronics package, which resides inside the rover, records all detected X-rays with their energy and assembles the detections into the X-ray spectrum of this sample.
Additional improvement in sensitivity, mainly for heavy elements such as iron, comes from increasing the amount X-rays emitted by the curium. Curiosity’s APXS has about 700 micrograms (in mass) or 60 millicuries (in radioactivity), which is twice as much as Spirit’s or Opportunity’s.
Curium is a synthetic element first identified in a laboratory in 1944. The specific isotope used in all Mars rovers’ APXS instruments is curium 244, which has a half-life of 18.1 years. This makes it ideal for longduration missions, where even after more than seven years of the Opportunity mission, the loss in activity is hardly noticeable.
The APXS instrument determines the abundance of elements from sodium to strontium, including the major rockforming and soil-forming elements sodium, magnesium, aluminum, silicon, calcium, iron and sulfur. In 10-minute quick looks, it can detect even minor ingredients down to concentrations of about one-half percent.
In three hour readings, it can detect important trace elements down to concentrations of 100 or fewer parts per million. The instrument is so good that it makes for high precision and low detection limits. The latter is important where salt forming elements like Sulfur, Chlorine and Bromine, are concerned, as these elements can indicate that the terrain has interacted with water in the past. Being so precise and highly sensitive APXS can identify local anomalies in the terrain. It also can guide the sample selection for the analytical instruments of MSL.
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.
On Spirit and Opportunity, the need for the X-ray detector chip to stay cold, and the length of time necessary for acquiring a measurement, have restricted most APXS measurements to Martian nighttime hours. One change in Curiosity’s APXS is the possibility to activate a solid-state electric cooler (Peltier cooler) for the X-ray detector chip, for use of the APXS during Martian daytime. At MER the APXS was mostly used at night as the environmental temperatures would be much lower, not running the risk of overheating the instrument.
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 APXS data analysis is fast and allows a quick turnaround of results used for tactical rover operations.
The additional X-ray intensity will benefit use of a technique called the scatter peak method, which was developed by physicist Iain Campbell, an APXS coinvestigator at the University of Guelph, Ontario, Canada.
This method extracts the content of elements invisible to X-rays, such as oxygen. It was used to detect and quantify water bound in the minerals of salty subsurface soils examined by Spirit at Gusev Crater.
When the spectrometer is in contact with the target, it examines a patch about 0.7 inch (1.7 centimeters) in diameter. It detects elements to a depth of about 0.0002 inch (5 microns) for low-atomic-weight elements and to about 10 times that depth for heavier elements. 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, a dust removal tool (brush) is available on the Curiosity rover, and could be used to brush some rock surfaces clean before APXS examines them.
Schematic diagram of the APXS sensor head, showing the relationship between the radioactive sources
APXS will use the observation tray on Curiosity for processed samples to check the composition of the samples further. APXS will be able to connect the analytical instrument results with the on site samples. Curiosity is capable of preparing the sample for APXS with a brush, which will make it possible for APXS to investigate near-surface layers or veins without removing them from their site. This is especially handy when such layers can not be collected by the drill to be served up for investigation to the analytical instruments.
The Alpha Particle X-ray Spectrometer (APXS) was built for 17.8 million $ as Canada’s contribution to Curiosity’s instrument suite. APXS was built in Richmond B.C. by MacDonald, Dettwiler and Associates Ltd (MDA). Funding for the science team comes from CSA, NASA, and the University of Guelph.