Introduction

The RTOF (Reflectron Time Of Flight) spectrometer is designed to complement the DFMS by extending the mass range and increasing the sensitivity of the full instrument package. TOF instruments have the inherent advantage that the entire mass spectra are recorded at once, without the need of scanning the masses through slits.

Giotto-Mission

A RTOF-type instrument was successfully flown on the GIOTTO mission to measure atoms and molecules ejected from a surface during impact of fast cometary dust particles. The ROSINA RTOF will include two similar and independent source-detector systems, one for cometary ions and one for cometary neutrals, using the same reflector. This configuration guarantees high reliability by almost complete redundancy.

ToF-Principle

A Time of Flight spectrometer operates by simultaneous extraction of all ions from the ionisation region into a drift space such that ions of a given m/q are time-focused at the first time focus plane (TF) at the beginning of the drift section. Hereby the temporal spread of such an ion packet is bunched from about 800 ns at the exit of the ionisation region to about 3 ns (mass = 28 amu). Those very short m/q ion bunches are then imaged onto the detector by the isochronous drift section. Because different m/q bunches drift with different velocities, the drift length determines the separation of bunches. The reflector incorporates the isochronity in the drift section.

Mass Resolution

Mass resolution is determined by the drift time and the temporal spread of the ion packets. Unlike other types of spectrometers, TOF spectrometers have no limit to the mass range. In practice the mass range is limited by the electronics, e.g., by the size of the signal accumulation memories.
ROSINA-RTOF has a mass resolution of up to Dm/m = 4600.

ROSINA-RTOF Components

The RTOF consists of four main components: the ion sources, the ion optics, the reflector, and the detectors. Two different channels are used in this spectrometer: one which ionises the gas in an ion source and stores the ions, and one that pulses the incoming cometary ions onto the TOF path. The two sources are mechanically very similar, with one source optimised for the gas mode (Storage Source) and one source optimised for the ion mode (Ortho Source). Ions pass through the ion optics section and enter the drift section where they are reflected with an electric field in the ion reflectors (Reflectron and Hard Mirror) and return to two separate detectors. Thus, there are two independent mass spectrometers in a single structure.

With a storage ion source - a source that stores the continuously produced ions until their extraction into the TOF section - with high transmission in the TOF section and with a sensitive detector, it is possible to record a very large fraction (greater than 60%) of all ions produced. These factors contribute to the overwhelming sensitivity of TOF instruments. Another reason to use TOF instruments in space science is their simple mechanical design and easy operation. Their performance depends on fast electronics rather than on mechanical tolerances.