A Computer-assisted Method for the Calibration of Raw Data from the DFMS sensor on Rosetta

The double focussing mass spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument package, consists of an ion source, a mass analyser and a detector assembly consisting of three detectors. The magnetic sector of the analyser provides the mass dispersion needed for use with the position-sensitive microchannel plate (MCP) detector. Ions that hit the MCP release electrons that are recorded using a linear electron detector array with 512 pixels (LEDA). Raw data for a given commanded mass are obtained as ADC counts as a function of pixel number. This contribution describes a computer-assisted approach to address the problem of calibrating such raw data, in other words, how to associate the pixel number to a mass-over-charge ratio (m/Z) and how to convert the ADC counts per pixel to ion counts per second. After calibration, the abundance and identity of ions at the detector are known. Neutral comet gases, however, are ionized in the ion source before they are transferred to the mass analyser and during this process fragmentation of the gas molecules may occur. Our software allows a tentative identification of the neutrals that entered the instrument, given the detected ion fragments and a fragmentation database. By taking into account experimentally determined sensitivities gas number densities are obtained. The instrument characterisation (experimental determination of sensitivities, fragmentation patterns for the most common neutral species, etc.) has been conducted by the consortium using an instrument copy in the University of Bern test facilities.