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Laboratory
for ChemoMetrics,
Vienna (Austria) MS - Chemometrics - COSIMA |
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COSIMA |
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[ Aims | People | Results | Presentations | Pictures | ROSETTA | COSIMA Instrument
| Comet
Wirtanen | Literature
] |
Last update 2000-12-04 |
Summary
Principal
investigator of COSIMA is Jochen KISSEL
(Max-Planck-Institute for Extraterrestrial Physics, Garching, Germany). In
several research teams the different parts of the instrument and the software
are developed; an overview of the groups is contained in the homepage of the Finnish group.
COSIMA
is one of the instruments on the orbiter of Rosetta, a space
mission planned by the ESA to comet Wirtanen.
COSIMA stands for Cometary Secondary Ion Mass Analyzer. It is a
time-of-flight mass spectrometer (TOF) for secondary ion mass spectrometry
(SIMS). [Picture
of a preliminary model].
COSIMA
is dedicated to collect and analyze cometary grains in situ and to determine
their organic and inorganic components.
The intimate mixture
of organic and inorganic material in cometary grains is considered to has been
essential – after impact on Earth – in the development of self-reproducing
molecules and probably in the start of life.
A prominent objective of
COSIMA is the search for organic chemical precursor molecules in cometary
grains; for instance unsaturated nitrogen-containing compounds are
expected.
A contribution to data interpretation by chemometric methods is
developed in the project MS-Chemometrics-COSIMA at the Laboratory for ChemoMetrics (LCM), Vienna
University of Technology.
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/ Summary / More Details
More Details
The COSIMA
instrument has the following main functional hardware elements: the dust
collector, the target manipulator, the COSISCOPE - a microscope for target
inspection, the primary ion source, and the TOF mass spectrometer with an ion
reflector. Mass of the COSIMA instrument is about 20 kg, and electrical power
consumption is 20 W.
The target manipulator unit is a mechanical device operating
with stepper motors and grasping tongs to put targets to the dedicated
positions for dust collection, storage, cleaning or analysis.
Cometary
dust particles can be analyzed by COSIMA in the size range of 10 µm and
above; the expected impact speeds are up to a few 100 m/s. Particles will be
trapped in metal black layers of a few 10 µm thickness, placed on top of square
shaped 1x1 cm2 metal plates of 0.5 mm
thickness. Part of these substrates will be made of gold and silver blacks,
formed by vapor condensation and deposition of these metals under a reduced
Argon atmosphere of a few hectopascal. Other substrates will be made of
electrochemically coated platinum, and in a few cases of palladium black. Such
blacks are not only optically black, but also in the SIMS image, since the
loose structure largely suppresses the transmission of secondary ions.
The target imaging device, COSISCOPE, will image an entire target on 1024x1024 pixels, which
gives a pixel resolution of about 14 µm. The detection of collected cometary
grains on the targets and their location by coordinates is essential because
the effective area for SIMS experiments is only 200x200 µm2.
The primary
ion beam system consists of an ion emitter, a chopper, a buncher, and a
focusing lens. The primary ions are generated in one
of two liquid metal ion sources, filled with isotopically clean 115In (113In
< 0.1%) and accelerated into the ion optics. Two separate emitters are implemented for a limited redundancy.
During normal operation, one emitter is used for
sample analysis, and the other for target cleaning by ion sputtering. Typical
operating data will be: 10 keV accelerating voltage, 1000 ions per pulse, pulse
frequency 2 kHz, spot size 20µm diameter, and pulse length 3 ns (50 ns without
buncher).
TOF:
During analysis the target is maintained at ground potential (0 V);
accelerating voltage for secondary ions is 1 kV. After a first field-free drift
region of 540 mm length the ions enter a two-stage ion reflector (with a total
length of 70 mm) which is able to compensate most of the different initial
kinetic energies, so that a working mass resolution of 1000 to 4000 can be
expected. This mass resolution allows a good separation of inorganic and
organic ions up to about 300 Dalton. From the exit of the reflector the ions
travel in a field-free region for 510 mm before arriving at the detector. Post
accelerating of ions will be typically 9 kV; the time resolution of the used
micro sphere plate detector is about 1 ns. For the measurement of flight times
a time resolution of 2 ns and a measurement range of 250 µs will be applied.
All electronics
are housed in the electronics box, with the exception of a few fast switching
circuits of the primary ion beam system. This is done to prevent out gassing
products to directly interfere with the high sensitive TOF-SIMS instrument. The
COSIMA experiment operates from a single power line of nominally +28 V from the
ROSETTA power system.
During the active
phase of the mission at the comet the distance to earth will be about 2 AU
and the signal round trip time about 30 minutes. The reaction time for normal
operational procedures via ground loop may be of the order of one week. These
conditions require a large degree of autonomy of the instrument.
Measurements: The data for each primary ion pulse are added into
time bins. After about 1000 shots the data should be significant for a first evaluation
to characterize the site under analysis. Two peak lists are generated
containing the absolute peak intensities (number of ions) per integer mass for
both, inorganic and organic ions. These lists are used for further data
evaluation. An automatic comparison of peak list tables is necessary for the
detection of cometary grains (target versus grain signals) or to decide whether
a newly measured site differs from those already analyzed.
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[ Aims | People | Results | Presentations | Pictures | ROSETTA | COSIMA Instrument
| Comet
Wirtanen | Literature
] |
Last update 2000-12-04 |
