Mössbauer Spectrometer (MB)
Moessbauer (MB) spectroscopy is a powerful tool for quantitative mineralogical analysis of Fe-bearing materials. The miniature MB spectrometer MIMOS II is a component of the Athena science payload to be launched to Mars in 2003 on both Mars Exploration Rover missions. The instrument has two major components: (1) a rover-based electronics board which contains power supplies, a dedicated central processing unit, memory, and associated support electronics and (2) a sensor head that is mounted at the end of the instrument deployment device (IDD) for placement of the instrument in physical contact with soil and rock. The velocity transducer operates at a nominal frequency of ~25 Hz and is configured with two 57Co/Rh MB sources. One source (~5 mCi landed intensity), together with a reference target (alpha-Fe2O3 plus alpha-Fe0) and PIN diode detector in transmission geometry, are internal to the sensor head and is used for instrument calibration. The other source (~150 mCi landed intensity), together with four PIN diodes in backscatter measurement geometry, irradiates Martian surface materials with a beam diameter of ~1.4 cm after passing through a collimator. Physical contact with surface materials is sensed with a switch-activated contact plate. The contact plate and internal reference target are instrumented with temperature sensors. Assuming ~18% Fe for Martian surface materials, experiment time is 6-12 hours during the night for quality spectra (i.e., good counting statistics); 1-2 hours is sufficient to identify and quantify the most abundant Fe-bearing phases. Data stored internal to the instrument for selectable return to Earth include MB and pulse-height analysis spectra (256 channels each) for each of the five detectors in up to 13 temperature intervals (65 MB spectra), engineering data for the velocity transducer, and temperature measurements. The total data volume is ~150 kByte. The mass and power consumption are ~500 g (~400g for the sensor head) and ~2 W, respectively.
The scientific measurement objectives of the MB investigation are to obtain for rock, soil, and dust (1) the mineralogical identification of iron-bearing phases (e.g., oxides, silicates, sulfides, sulfates, and carbonates), (2) the quantitative measurement of the distribution of iron among these iron-bearing phases (e.g., the relative proportions of iron in olivine, pyroxenes, ilmenite and magnetite in a basalt), and (3) the quantitative measurement of the distribution of iron among its oxidation states (e.g., Fe2+, Fe3+, and Fe6+). Special geologic targets of the MB investigation are dust collected by the Athena magnets and exterior and interior rock and soil surfaces exposed by the Athena Rock Abrasion Tool and by trenching with rover wheels, respectively.
Information in this instrument description is taken from The Athena MIMOS II MB Spectrometer Investigation paper [KLINGELHOFERETAL2003]. See this paper for more details.
The chief scientific objectives of the MB are:
- to identify iron-bearing mineral phases (e.g., oxides, silicates, sulfides, sulfates, and carbonates), in rock, soil, and dust,
- to quantitatively measure the distribution of iron among these iron-bearing phases (e.g., the relative proportions of iron in olivine, pyroxenes, ilmenite and magnetite in a basalt),
- to quantitatively measure the distribution of iron among its oxidation states (e.g., Fe2+, Fe3+, and Fe6+), and
- to distinguish between magnetically ordered and paramagnetic phases and provide, from measurements at different temperatures, information on the size distribution of magnetic particles.
It is necessary to calibrate both the drive velocity and the detectors of the MB. The interpretation of acquired MB spectra is impossible without knowing the drive velocity precisely at any given time. MB drive velocity calibration for MIMOS II is rather straightforward and done in three different ways, thus ensuring redundancy. Prior to flight each individual drive system was calibrated by measuring in backscattering mode an alpha-iron foil standard. A maximum drive velocity was preset by firmware. Fitting the acquired MB spectrum using the well known parameters of the alpha-iron foil then yielded the real velocity. This procedure was repeated at different temperatures.
During the mission, the magnetite CCT (Compositional Calibration Target) will be measured in several runs to verify the functionality of MIMOS II. The well known MB parameters of magnetite can be used for velocity calibration again. These kind of measurements have been done already in the lab with the flight units as a function of temperature, to be used as reference for the measurements on Mars.
The primary method for velocity calibration is the internal reference target and detector configured in transmission measurement geometry. The reference target is a mixture of alpha-Fe0 (metallic iron, 30% enriched 57Fe) and alpha-Fe2O3 (hematite, 95% enriched 57Fe), and its MB spectrum is measured automatically during each backscattering measurement. Each component of the reference target has well-known MB parameters, so that fitting of reference spectra enables velocity calibration for each individual measurement done in backscatter geometry, ensuring that the actual drive velocity is always well-defined, regardless of prevailing environmental conditions.
Careful energy calibration on each detector was done to achieve optimal detection rate. Each sensor head was temperature cycled (153 K - 293 K). During cycling, energy spectra were measured. As a result of analysis of these spectra, optimal firmware parameters were calculated for each detector and each temperature window. During operation, instrument firmware will adjust those parameter depending on temperature, thus ensuring best detector performance.
Targets for MB analysis for the mineralogical composition of Fe-bearing phases, the relative distribution of Fe and its oxidation states among those phases, are the exposed surfaces of soil and rock, interior regions of rock exposed by the Rock Abrasion Tool, subsurface soil exposed by trenching, and the two magnets mounted on the rover deck. In the case of the magnets, it is know from Viking and Mars Pathfinder that Martian aeolian dust is magnetic, but the composition of the magnetic phase or phases is not known. When the dust buildup on the magnets is sufficiently large, as determined using Pancam, the IDD will be used to place MIMOS II directly against the magnets, providing what should be a definitive identification of the magnetic phases present. Fine-grained material produced by the RAT, if present in sufficient lateral extent and depth, provides a target that is representative of the volume excavated and for which orientation effects are likely not present. The relatively high penetration depth of the 14.4 keV MB radiation means that it may be possible to obtain mineralogical information about unaltered rock without removing exterior rinds or dust coverings with the RAT.
The MB is a less contamination sensitive to dust than the APXS or the Microscopic Imager, which are the other two instruments on the IDD. Therefore, for soft and/or dirty targets the MIMOS II contact sensor may sometimes be used as a 'blind man's cane', helping to establish target location in IDD coordinates so that the other instruments can be placed with less risk of contamination.
In some instances, it may be possible for MIMOS II to achieve a signal-to-noise ratio that is adequate for answering key scientific questions in a time much less than the nominal experiment time of 6-12 hr during the overnight period. This will be particularly true early in the mission when radiation source strength is greatest, and for targets with high Fe contents. Where appropriate, then, the MB may be used in a 'touch and go' mode, in which a short integration is performed at the start of a sol, followed by other rover activities that may include driving.
Detectors and Electronics
The main disadvantage of the backscatter measurement geometry employed by MIMOS II is the secondary radiation caused by primary 122 keV radiation from the decay of 57Co. To reduce the background at the energies of the 14.4 keV gamma-ray and the 6.4 keV X-ray lines, a detector with good energy resolution is required. In addition, an intense main 57Co source and a detector system covering a large solid angle are needed to minimize data acquisition time. Good resolution is even more important should it prove possible to use these detectors for elemental analysis with the X-ray fluorescence technique (i.e., using the pulse height analysis (PHA) spectra that are also acquired as a part of our measurement procedure). For this reason, four Si-PIN-diodes with a 10 x 10 mm2 active area were selected as detectors instead of gas-counters. A detector thickness of about 400-500 microns is a good choice according to calculations and experience. The energy resolution is ~1.0-1.5 keV at room temperature, and it improves at lower temperatures. The efficiencies at 6.4 and 14.4 keV are nearly 100 % and about 70 %, respectively.
The 100 V DC bias voltage for the detector diodes is generated by high frequency cascade circuitry with a power consumption of less than 5 mW. Noise contributions are minimized by incorporating a preamplifier-amplifier-SCA system for each individual detector.
In addition to the four detectors used to detect backscattered radiation from the sample, there is a fifth detector to measure the transmission spectrum of the reference absorber (alpha-57Fe plus alpha-57Fe2O3 Sample and reference spectra are recorded simultaneously, and the known temperature dependence of the MB parameters of the reference absorber can be used to give a measurement of the average temperature inside the sensor head, providing a redundancy to measurements made with the internal temperature sensor.
MIMOS II has three temperature sensors: one on the electronics board in the Rover warm electronics box and two on the sensor head (Analogue Devices AD590). One temperature sensor in the sensor head is mounted near the internal reference absorber, and the measured temperature is associated with the reference absorber and the internal volume of the sensor head. The other sensor is mounted outside the sensor head at the contact ring assembly. It gives the approximate analysis temperature for the sample on the Martian surface. This temperature is used to route the MB data to the different temperature intervals (maximum of 13, with the temperature width software selectable) assigned in memory areas. In case of contact-ring temperature sensor failure, the internal temperature sensor would be used (software selectable).
During measurements, a temperature log is acquired for all three sensors. Temperature measurements are done approximately every 5 min (software selectable: min ~10 sec, max ~40 min). MIMOS II can accumulate up to 256 temperature records corresponding to a total integration time of ~21 hours.
The electronics in the rover body include an internal microcontroller, so that the instrument can collect data independently of the rover computer. The analog signals of the five detector channels are analyzed by discriminators for 14.4 keV and 6.4 keV peaks. MB spectra for the two different energies of 6.4 keV and 14.41 keV are sampled separately.
The MB is mounted on the end of the IDD.
The Athena Moessbauer spectrometer uses a vibrationally-modulated 57Co source to illuminate target materials. Backscattered gamma signals are binned according to the source velocity, revealing hyperfine splitting of 57Fe nuclear levels that provides mineralogical information about the target.