The RAPID (Research with Adaptive Particle Image Detectors) is an advanced particle detector for the analysis of suprathermal plasma distributions in the energy range from 20-400 keV electrons, 40 keV-1500 keV hydrogen, and 10 keV/nucleon-1500 keV heavier ions. Novel detector concepts in combination with pin-hole acceptance allow the measurement of angular distributions over a range of 180° in polar angle for electrons and ions. Identification of the ion species is based on a two-dimensional analysis of the particle's velocity and energy. Electrons are identified by the well-known energy-range relationship.
Ions and electrons are detected by two independent sub-instruments, the IIMS (Imaging Ion Mass Spectrometer) for ions and the IES (Imaging Electron Spectrometer) for electrons.
Particles entering the IIMS telescope pass through an entry foil, releasing secondary electrons that are accelerated and detected by a microchannel plate detector. This constitutes the start signal for the time-of-flight analysis. The particle proceeds to a solid-state detector, which emits a signal proportional to the particle's loss of energy within it. Secondary electrons ejected from the surface of the solid-state detector are detected by further microchannel plates, establishing the stop signal. If the particle's energy is sufficiently high, it will pass through the solid-state detector, activating a second back detector. This coincidence signal eliminates the event from further analysis, since the total energy of the particle is thus unknown. The combination of flight time and energy permits the event to be sorted by mass (species) and energy.
The flight path is 34 mm long, the solid-state detector has an area of 75 mm^2 and thickness 300 m. One such IIMS unit covers a range of 60° in the plane containing the spacecraft spin axis. Three such units cover the full 180°. Over one spacecraft rotation the entire unit sphere is scanned.
The ion spectral counts are sorted into 8 energy bins, 12 polar angle segments, 16 azimuthal sectors, and 3 mass ranges. Higher energy and mass precision is obtained with so-called ''direct events''. However, for the CSDS data sets, only omni-directional fluxes in two energy and three mass ranges are delivered.
The IES system is similarly arranged in three units, each covering 60° in the plane of the spacecraft axis, each subdivided into 20° ''pixels''. There are thus 9 microstrip solid-state detectors, one for each polar segment. The charge placed on each detector by an incident particle is stored on a capacitor, and is strobed out after a certain integration time (2-50 µs) for further evaluation. The read-out time is 47 µs. The integration time is selected so that the detectors likely have at most one count.
Internally, the electron energy is sorted into 256 bins from which the output energy channels are selected. These are set relative to the location of zero energy (the so-called ''pedestal'') so that the output channels should all be much the same although the pedestal varies with each strip.
A further complication is the presence of residual charges on the capacitors that lead to a large spike at the pedestal position, the width of which also varies for each strip. In order to monitor the position and width of the pedestal for possible temperature and count-rate dependencies, 2 channels in normal mode and 4 in burst mode are set around the pedestal. This leaves 6 ''science'' energy channels in normal and 8 in burst mode.
The electron spectral counts are also sorted into 16 azimuthal sectors, but only 9 polar segments. Again, for the CSDS data sets, only omni-directional fluxes in two energy ranges are delivered.
This description has been obtained from Section 3.9 of the ''Users Guide to the Cluster Science Data System'', DS-MPA-TN-0015.