Alpha Particle X-ray Spectrometer (APXS)
The Alpha Particle X-Ray Spectrometer (APXS) is part of the Athena payload of the two Mars Exploration Rovers (MER). The APXS sensor head is attached to the turret of the Instrument Deployment Device (IDD) of the rover. The APXS is a very light-weight instrument for determining the major and minor elemental composition of Martian soils, rocks, and other geological materials at the MER landing sites. The sensor head has simply to be docked by the IDD on the surface of the selected sample. X-ray radiation, excited by alpha particles and x-rays of the radioactive sources, is recorded by a high-resolution x-ray detector. The x-ray spectra show elements starting from sodium up to yttrium depending on their concentrations. The backscattered alpha spectra, measured by a ring of detectors, provide additional data on carbon and oxygen. By means of a proper calibration, the elemental concentrations are derived. Together with data from the two other Athena instruments mounted on the IDD, the samples under investigation can be fully characterized. Key APXS objectives are the determination of the chemistry of crustal rocks and soils and the examination of water-related deposits, sediments, or evaporates. Using the rock abrasion tool (RAT) attached to the IDD, issues of weathering can be addressed by measuring natural and abraded surfaces of rocks.
Information in this instrument description is taken from The New Athena Alpha Particle X-Ray Spectrometer (APXS) for the Mars Exploration Rovers mission paper [RIEDERETAL2003]. See this paper for more details.
The chief scientific objective of the APXS is:
- To determine the major and minor elemental composition of Martian soils, rocks, and other geological materials at the MER landing sites
During the calibration campaign in the Max Plank Institute Mainz laboratory many geostandards were measured. These standards are powdered geological samples, whose elemental concentrations are certified by qualified institutions. The two flight instruments containing their flight sources were calibrated using 11 validated samples (8 geostandards and 3 meteorites) and a set of oxide and metal standards.
A check of the performance of the instrument after the landing on Mars will be done making use of the internal calibration target on the doors and the Compositional Calibration Target (CCT) that is mounted on the rover chassis in reach of the IDD. The x-ray spectrum of the internal calibration target shows gold, nickel, and copper lines. The target consists of a set of thin layers of gold, Kapton (carbon), and nickel on top of the copper-beryllium body of the doors. Energy calibration, FWHM, and linearity can be checked by evaluation of the copper and gold lines and comparison with pre-launch data.
Contamination of the beryllium entrance window of the x-ray detector will be noticeable by an intensity reduction of the low-energy M lines of gold compared to the L lines. Energy calibration can be checked with the position of the gold peak (a peak and not a step because of the small thickness of the Au layer). The carbon step provides an additional check of consistency.
The CCT consists of a magnetite plate. This target was designed for the needs of the MB, but, it can also be used by the APXS to check its FWHM usually determined for the 6.4-keV line of iron. As the target is mounted on the outside of the rover, it will eventually be covered with dust, but the line shape of the Fe line will not be affected.
There are a few considerations that must be taken into account to acquire the best data possible:
- To search for trace elements, two to four hours are sufficient for the x-ray mode. The search for carbon requires at least eight hours for the alpha mode as the alpha sensitivity is low. X-ray and alpha mode always operate together.
- To properly make an APXS measurement, the sensor head has to be correctly 'docked' at the selected sample by the IDD. The nominal APXS docking procedure is the following:
- Use images taken by the rover's front cameras: Navcams (navigation cameras) and/or Hazcams (hazard avoidance cameras) to calculate the IDD positioning
- Open the doors by pressing the APXS contact ring against the CCT or any other solid surface of the rover
- Position the APXS contact ring up against a selected sample area with a positional accuracy of 10 mm and 10 degrees on a target not previously contacted, or 4 mm and 3 degrees for a target previously contacted by any one of the IDD instruments
- After data acquisition, the APXS doors are closed by rotating the turret past a roller until the release lever is actuated
- For touch-and-go operations, optimum resolution for the x-ray detector is obtained at temperatures of 248 K, and the shortest measurement time should be at least 15 minutes
The sensor head is packaged in a cylindrical enclosure 53 mm in diameter and 84 mm in length and terminates in an insulating flange of 68 mm x 68 mm. The front part facing the sample contains the xray detector, mounted on the axis of the instrument, a cylindrical source holder with six alpha sources, and six rectangular alpha detectors. The coaxial arrangement of sources and detectors for alpha particles and x-rays assures that both detectors 'see' the same intensity distribution across the sample.
Use of a high resolution x-ray detector (silicon drift detector with 10 mm^2 active area, a 5 micron thick Be-window and an energy resolution of about 160 eV @ 5.9 keV) permits high quality measurements. The advanced detector versions were provided by KETEK, Munich, Germany.
There is a second group of detectors, identical to the alpha detectors, but not exposed to alpha particles from the sample. These detectors measure the background contribution to alpha spectra due to cosmic radiation at the surface of Mars and high energy gamma background of the Cm sources as well as the Moessbauer source. The field of view for the x rays is delineated by means of a collimator in front of the detector: the collimator is formed by two apertures made from Zr, one immediately in front of the detector and one in the central orifice of the source holder.
The alpha sources (6 pellets) are contained in a source holder that attaches to the sensor head with a spring loaded bayonet-style mechanism. This permits quick and easy exchange of the sources without the need to disassemble the sensor head. The sources are covered with 2.5 micron thick titanium foils, turning them into 'quasi-closed' sources (hermetically sealed sources are under development, but were not available for this mission). The foils prevent contamination of samples with source material, emitted from the sources as a result of 'recoil sputtering', and the same time reduce the energy of the alpha particles from 5.80 MeV to 5.17 MeV, thereby avoiding a resonance in the 12C(alpha, alpha')12C reaction at ca. 5.7 MeV (Figure 6). This measure, together with an optimized design of the source-collimator-detector geometry, significantly reduces the background signals from carbon and oxygen in the Martian CO2-atmosphere. Nevertheless, this background signal remains the limiting factor for the determination of carbon in the samples.
The main electronics consists of the analog signal conditioning segments (6-pole Gaussian filter amplifiers, threshold discriminators and peak detectors), an analog multiplexer, a 16-bit analog-to-digital converter and an 8-bit microcontroller.
Control logic determines the presence of a relevant signal and generates an interrupt in the microcontroller. To avoid additional noise in the analog signal chain, the microcontroller is kept in idle mode until the analog signal is processed and buffered in the peak detectors. Selection of the appropriate multiplexer input, conversion of the signal amplitude to a digital number and registration of the signal by incrementing the number of counts in the corresponding amplitude channel of the respective detector is then handled by the microcontroller. Conversion time is typically 200 microseconds. A digital temperature compensation routine that minimizes the influence of temperature changes during long measurements adds another 100 microseconds. For a mean count rate of 100 Hz, a total dead time of below 5 % is achieved.
The microcontroller is equipped with a watchdog circuit that performs a soft reset in case of an abnormal program flow. The data are stored in 32 Kbyte SRAM that are buffered by a battery located on the main electronics board.
The interface for commanding the instrument and transfer of data consists of an RS 422 serial link. Power is provided to the instrument directly from the board battery (nominally 28 V); voltages required by the electronics (5 V digital, +/-5 V analog and +/-12 V analog) are generated by its own power converter and filters.
The x-ray spectrum is divided into 512 channels. The lower threshold is fixed at ~850 eV. This is sufficient to detect Na at 1040 eV. The upper energy limit is about 16 keV. The spectral range includes the K lines up to Zr and the L and M lines of higher Z elements. It also contains elastic scattered Pu lines at 14.3 keV and 12.6 keV, as well as inelastic scattered peaks. The alpha and background spectra use 256 channels and range up to about 6 MeV.
The APXS sensor head is attached to the turret of the Instrument Deployment Device (IDD) of the rover.
The APXS measures x-ray radiation and backscattered alpha spectra. These measurements can determine the elemental composition of the target on which it is docked.