COSIMA: Cometary Secondary Ion Mass Analyser
COSIMA (Cometary Secondary Ion Mass Analyser) is a secondary ion mass spectrometer equipped with a dust collector, a primary ion gun, and an optical microscope for target characterization. Dust from the near comet environment is collected on a target. The target is then moved under a microscope where the positions of any dust particles are determined. The cometary dust particles are then bombarded with pulses of indium ions from the primary ion gun. The resulting secondary ions are extracted into the time-of-flight mass spectrometer.
COSIMA will perform in-situ measurements on individual dust particles emitted by the target comet and collected by COSIMA dust collector subsystem. From the resulting data it will be possible to determine:
- The elemental composition of solid cometary particles to characterize comets in the framework of the solar system chemistry
- The isotopic composition of key elements in solid cometary particles such as H, C, Mg, Ca, Ti in order to establish boundary conditions for models of the origin and evolution of comets and thereby of the solar system
- The chemical states of the elements
- Variations of the chemical and isotopic composition between individual particulate components
- Changes in composition that occur as functions of time ("short-term variations") and orbital position
- The variability of the composition of different comets by comparing the results to those obtained previously from comet Halley
- The presence of an organic component that is not associated with a rocky phase
- The molecular composition of the organic phase of the solid cometary particles
- The molecular composition of the inorganic phase of the solid cometary particles
- The chemical state of the organic matter characterized by its saturation degree oxidation state and bond types
which in turn will allow:
- Comparison of the composition of the solid particles to the elemental and isotopic composition of the neutral and ionised atmosphere of the comet
- Gaining insight into the molecular composition of the inorganic phase of the particulate matter
- Assessment of the exobiological relevance of the cometary organic matter as possible organic precursor material
- Evaluation of the relation of the association of inorganic phases and mineral components in cometary matter to the formation of prebiotic organic molecules on the early Earth
The core of the COSIMA instrument is a time-of-flight (TOF) secondary ion mass spectrometer (SIMS) equipped with a dust collector, a primary ion gun, and an optical microscope (COSISCOPE) for target characterization. Once one of the targets on the target wheel has been exposed to cometary dust it is moved in front of the microscope and imaged under shallow angle illumination provided by light emitting diodes. On-board image evaluation detects the presence and location of dust particles with diameters exceeding a few µm and calculates their position relative to the target reference point. Once the presence of features of interest is established, the target is moved in front of the mass spectrometer. Three nanosecond duration pulses of indium-115 with an energy of 10 keV and about 10 µm in diameter from the primary ion gun hit the selected feature. Secondary ions from the cometary matter are extracted by the secondary ion extraction lens (SIL) into the TOF section. After passing deflection plates for beam steering the ions travel through a field free section. Next they pass a two stage reflector, return through the drift section to the ion detector. Its main element is a single stage microsphere plate, where the ions are detected. The arrival time of each ion is measured with an accuracy of about 2 ns.
Precision in the timing of the primary ion pulses, the correct selection of the dimensions and the voltages of the mass spectrometer and the accurate measurement of the secondary ion flight time are needed to obtain high mass resolution in the COSIMA instrument. A mass resolution of 2000 is achieved for ions having a flight time of 16 µs, which occurs for ion masses of above 28 Daltons (atomic mass units).
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Last Update: 18 May 2015