Ion Compostion Subsystem (ICS)
The ICS is designed to measure ion fluxes and ion compositon above ~50 keV with the geometry factor and angular, temporal and species resolution appropriate to the scientific requirements of the GEOTAIL mission. The ICS
(block diagram)
contains two identical sensors, oriented above and below the ecliptic plane for complete angular coverage. Each sensor head
(cross section diagram)
contains a thin-foil, solid state detector TOF telescope. A front collimator with an electron sweeping magnet precedes a thin, grid-mounted foil which is the front element of the telescope. A silicon surface barrier solid-state detector is located approzimately 6 cm behind the foil. Energetic ions passing through the front foil and striking the back solid detector emit low energy secondary electrons from both foil and silicon detector surfaces, and these electrons are mapped onto separate microchannel plates by electrostatics optics within the telescope. The signals produced by the microchannel plates define the ion TOF (and thus velocity) between the fron foil and the rear solid state detector. Ion energy is measured in the rear detector. Since measurement of ion velocity and energy determines the ion mass, ICS is capable of measuring the spectra and dynamics of all ion species over the GEOTAIL orbit.
Energy coverage is a function of species with a lower threshhold of ~10 keV/nucleon. The ICS produces composite ion energy spectra, composite velocity spectra, and species-resolved spectra. The GEOTAIL orbit covers a range of particle environments, from the very low-flux regions of the lobes to the substantial fluxes in the near-earth plasma sheet in active times. The geometry factor of each ICS head is therefore adaptive, from 0.1 cm**2 sr to less than 0.01 cm**2 sr including both an electronic variation of the effective area of each microchannel plate, and an ability to mechanically vary, by command, the telescope's physical aperature. Temporal resolution ranges from 0.2 sec to 96 sec depending on the individual rate channel
(rate channel diagram)
sampling frequency, and data is sectored into 16 azimuthal angular lines. The ICS Head assemblpy feeds signals into the ICS Analog Electronics box which processes signals into the center EPIC DPU subsystem.
