Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:epps-eps.mess::1.1
NAME EPPS ENERGETIC PARTICLE SPECTROMETER
TYPE
DESCRIPTION The Energetic Particle Spectrometer (EPS) is part of the MESSENGER Energetic Particle and Plasma Spectrometer (EPPS) system, which which also includes the Fast Imaging Plasma Spectrometer (FIPS). EPS covers the energy range of 25 to > 500 keV for electrons, and 10 keV/nucleon to ~3 MeV total energy for ions. The desired throughput for FIPS charged particle and EPS event processing is 5 kHz. The Johns Hopkins University/Applied Physics Laboratory constructed the EPS instrument. The EPS determines the distributions of the higher-energy magnetospheric ion and electrons, including the composition of the ions, to characterize the nature of the planetary field of Mercury. It does this by measuring the energy and velocity of the particles and then using a look-up table to determine the mass and therefore the species of particle. The measured species for the EPS include H, He, CNO, Fe, and electrons. Electrons are measured by solid- state detectors behind absorbing aluminum flashing. The EPS sensor consists of a 60-mm diameter, tuna-can-like cylinder, in which a start foil and stop foil, wrapped around opposite curved sides of the cylinder, constitute the TOF chamber. An incoming particle hits the start foil and scatters one or more electron, which is attracted to the start-anode ground. The particle continues and hits the stop foil, scattering other electrons, which are then attracted to the stop-anode ground. The solid-state detectors outside of, but wrapped around the curved face of, the stop foil, then detect the particle and measure the energy state. The detectors are arranged so that each detector senses the events within a given range of incidence angles. Each of the 6 detector modules is composed of 4 pixels: large and small ion and large and small electron. This provides 24 detector elements. At any one time, 12 of the 24 elements are used (6 ion and 6 electron detectors). Each of the 6 EPS detector modules also maintains its own spectrum via 64 16-bit bins. 63 bins will count the particle/energy combinations of interest, and 1 will count the remaining background events that do not fall in the particle/energy combinations of interest. The spectra are accumulated over a time set by ground command, after which they are compressed and reported in telemetry. The EPS system also includes 32 16-bit rate counters and 3 24-bit rate counters that are read by the EPPS software every n seconds (n specified by command). EPS status and housekeeping data such as voltages, currents, and temperatures are also periodically sampled. The EPPS instrument is described in full detail in Andrews, et al., 2007.
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REFERENCES Andrews, G.B., T.H. Zurbuchen, B.H. Mauk, H. Malcom, L.A. Fisk, G. Gloeckler, G.C. Ho, J.S. Kelley, P.L. Koehn, T.W. LeFevere, S.S. Livi, R.A. Lundgren, and J.M. Raines, The Energetic Particle and Plasma Spectrometer instrument on the MESSENGER spacecraft, Space Science Reviews, 131, 523-556, 2007.