| DESCRIPTION |
Instrument Overview
===================
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
[KLINGELHOEFERETAL2003]. See this paper for more details.
Scientific Objectives
=====================
The chief scientific objectives of the MB are:
1) to identify iron-bearing mineral phases (e.g., oxides, silicates,
sulfides, sulfates, and carbonates), in rock, soil, and dust,
2) 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),
3) to quantitatively measure the distribution of iron among its
oxidation states (e.g., Fe2+, Fe3+, and Fe6+), and
4) to distinguish between magnetically ordered and paramagnetic
phases and provide, from measurements at different temperatures,
information on the size distribution of magnetic particles.
Calibration
===========
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
(153K 293K). 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.
Operational Considerations
==========================
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 um is a good choice according to
calculations and experience. The energy resolution is ~1.0-1.5 keV
at room temperture, and it improves at lower temperaures. 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.
Location
========
The MB is mounted on the end of the IDD.
Measured Parameters
===================
The Athena Mossbauer 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.
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