EPE Instrument on IMP-8

  1. Overview

    2. The Energetic Particle Experiment (EPE) aboard IMP-8 was one of the first low energy (tens of keV) magnetic/solid state detector ion-electron separation and analysis systems flown in space. The Principal Investigator is D. J. Williams. Further instrumental details are summarized by Williams [NOAA Tech. Rpt. ERL 393-SEL 40, 1977] and in the EPE Table.

    The EPE cleanly separates ions and electrons in the energy range for 30 keV through several MeV. In addition to the magnetic deflection system, complementary particle observations are obtained from a low-noise detector (~18 keV discriminator level) and a thin (~5 micron) detector.

    One full 15 degree viewing cone perpendicular to the spacecraft spin axis (nearly normal to the ecliptic plane) and two full 13 degree viewing cones 45 degree to the spin axis are used for the EPE particle observations. Angular distributions are measured by obtaining 8 or 16 samples (depending on particle type and energy) per satellite spin period. Orientations of the 16, 22.5-degree (full-angle) sectors of the EPE on IMP-8 are illustrated in EPE sector look directions. A complete energy-angular distribution is obtained every 20.4 seconds.

    The EPE particle detector assembly consists of a main magnet-detector assembly and two auxiliary detector heads. All detectors are fully depleted, surface barrier, solid state detectors, and are operated with bias voltages 1.25-1.5 times the values required for full depletion. To minimize radiation damage effects, all detectors directly exposed to ion fluxes are mounted with the aluminum contact exposed to the incoming beam.

  2. Main Magnetic Detector Assembly

    3. The EPE main magnetic detector assembly consists of (i) a three-element telescope (detectors A, B, and C), (ii) a sweeping magnet that keeps low energy electrons (Ee<=200-300 keV) away from the telescope, and (iii) two detectors (D and E) to detect the swept electrons. The telescope covers the proton energy range 50 keV<=Ep<=25 MeV and the alpha particle range 2.2 MeV<=Ea<=35 MeV. Detectors D and E cover the electron energy range 30 keV<=Ee<=200 keV.

    The detector ABC telescope has the following geometric factors: detector A, 1.11e-02 cm^2-sr; detectors AB, 1.07e-02 cm^2-sr; detectors ABC, 0.97 e-02 cm^2-sr. Accounting for measured energy and angle-dependent efficiencies, approximate detector D and E geometric factors are 1.5e-03 and 0.9e-03 cm^2-sr, respectively.

  3. Auxiliary Detectors

    4. The EPE auxiliary detectors are designated F and G. Detector F is a selected low-noise device employing a single discriminator in the 15-20 keV region to measure low energy protons and electrons. Detector G is a very thin (~5 micron) device employing discriminator levels to measure alpha particle and Z>=3 ion intensities.

    EPE detector discriminator levels, logics, energy passbands, measured particle species, and number of samples per satellite spin are given in the EPE Table [after Williams, 1977]. Calibrations of the EPE were performed at the Goddard Space Flight Center and the Naval Research Laboratory accelerator facilities.


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