Magnetospheric ion composition spectrometer onboard the CRRES spacecraft

Abstract

The magnetospheric ion composition spectrometer (MICS) in the CRRES scientific payload utilizes time-offlight and energy spectroscopy in combination with an electrostatic entrance filter to measure the mass A , energy E, and ionic charge Q of particles with energies between 1 keV/charge and 430 keV/charge. An advanced ogive design of the electrostatic filter system provides a narrow angle of acceptance and high sensitivity. Incident particles are postaccelerated prior to entering the detection segment in order to improve the resolution at the lower end of the useful energy range. The principle features of the MICS spectrometer are described in some detail. Selected data gathered in-flight are shown as an illustration of the instrument performance in the operational orbit. I. Introduction T HE magnetospheric ion composition spectrometer (MICS) in the payload of the Combined Release and Radiation Effects Satellite (CRRES) belongs to a class of advanced instruments which provide full characterization of incident ions by determining their mass A (in amu), charge Q, and velocity V (magnitude and direction) as independent parameters. The particle's identity is derived from a time-offlight Tand energy E measurement, and the ionic charge Q is obtained from an electrostatic energy per charge E/Q filter which serves as the entry element of the spectrometer. Upon leaving the E/Q filter the ion's energy is increased by a postaccelerating voltage to improve the instrument resolution at low particle energies. The MICS energy range extends from 1.2 keV/charge up to 426.5 keV/charge, and ion species are identified from hydrogen to iron. The obtainable mass resolution A/dA is a complex function of the particle mass and energy. For a given ion species the mass resolution increases as a function of the energy per mass ratio E/A. A typical ratio A /cL4 = 8 is obtained for oxygen ions with energies above 100 keV. Despite this only moderate mass resolution, atoms and molecules, even isobaric structures, can be discriminated by a peculiarity of the MICS detection technique: Fragmentation of swift molecules in the thin START foil of the TIE spectrometer leads to groups of particles which travel with the original velocity but each fragment's energy is apportioned to its mass. The resulting statistical distribution in (E, T) space can be used to identify the presence of molecules.