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- The primary job of FEEPS is to obtain nearly instantaneous all-sky measurements of how many electrons of different energies and different arrival directions are present. The instrument relies on solid state detectors made of silicon, a semiconductor much like those used in computer electronic systems. Whenever a charged particle hits the detector, it initiates a current that can be used to measure the energy of the original particle. There are two FEEPS instruments per spacecraft, and together they provide 18 views in different directions simultaneously, giving rise to the "fly's eye" in the instrument's name. FEEPS has two sets of sensors, one for electrons and one for ions. The solid state detectors within each of the electron "eyes" are covered by a 2-micrometer aluminum foil, which keeps out the ions. The detectors for the ion views, on the other hand, have no aluminum foil and are exceedingly thin so that electrons generally pass through without leaving a detectable signal.
FEEPS development was led by The Aerospace Corporation of El Segundo, Calif.
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