The sensor elements in HEPS are surface-barrier and lithium-drifted silicon detectors arranged in stacks or telescopes. Using pulse-height analysis of the signals from incident particles and coincidence/anti-coincidence conditions among the detectors, high energy resolution over a wide energy range is achieved. The energy range of protons is 0.1 MeV to 160 MeV; that of the electrons is 0.03 MeV to 5 MeV. Particles with energies in these ranges can penetrate the atmosphere down to the stratosphere.
Six telescopes are arrayed at various angles from the spacecraft zenith to nadir in the plane containing the spacecraft velocity vector. Four of these telescopes are mounted at angles of -15 degrees, +15 degrees, +45 degrees, and +90 degrees with respect to the zenith. These telescopes measure both protons and electrons. Two telescopes view near the nadir at -165 degrees and +165 degrees, but measure only electrons and over a more limited range of energies. An additional complement of two low-energy proton (LEP) detectors, each using a single surfac e-barrier detector covering the energy range of 0.1 MeV to 0.5 MeV, have their fields of view near the zenith at -15 degrees and +15 degrees. The characteristics of the HEPS elements are given in Table 2.
In the HEPS 1 and 2 telescopes, the D detector (200 microns thick) measures low-energy protons and electrons. High-energy electrons and medium-energy protons are stopped in the E detector which is comprised of two lithium-drifted detectors totaling 1 cm thickness. The E detector (0.3 cm thick) differentiates the medium-energy protons from the highest proton energy range and rejects particles penetrating from the back direction. Around the stacked E detectors is a system of annular detectors ( 0.1 cm radial thickness), the purpose of which is to reject high-energy particles entering from the sides. The LEP uses a 75 micron thick detector, and electrons of energy less than 1 MeV are deflected away from the detector using a permanent magnet. The HEPS 3 telescope consists of a 200 micron thick D detector, a single 0.3 cm thick E detector to detect higher-energy electrons, and a 0.3 cm thick E detector to reject high-energy particles penetrating both E and E detectors.
Signals from the D and E detectors are pulse-height analyzed simultaneously by fast analog-to-digital converters. Logic analysis of the pulses by the coincidence/ anticoincidence circuitry distinguishes electrons from protons and accumulates pulse height distributions in memory. The coincidence/anticoincidence logic is similar to that in multi- particle spectrometers flown in the past (Reagan, et al.,1981; Gaines, et al., 1986); however, with its fast parallel processing, the HEPS is capable of pulse analysis rates in each telescope of approximately 100,000 events per second.
HEPS operations are controlled by microcomputers. The microcomputer controls the accumulation of signals to form a 32-step logarithmic energy spectrum at each of the mounting angles every 4 seconds. The software in each HEPS microcomputer controls how HEPS collects, accumulates, and compresses its science and engineering data. HEPS microcomputers control data from the various instrument buffers and shift these data into the telemetry structure.
HEPS operates continuously throughout the UARS orbit. To monitor instrument performance, two types of in-flight instrument calibration mode are executed periodically. The first of these is an electronic pulser calibration mode which applies fixed amplitude pulses to the input of the amplifier of each detector in a timed pattern that tests the logic functions. It also tests the gain and resolution in the energy detectors. The second mode utilizes onboard radio-active sources.
In the radioactive source calibration mode, weak (<40 Bq) Americium 241 alpha sources mounted within the HEPS 1 and HEPS 2 units, provide absolute gain calibration of the D proton detector and E electron detector elements of these units. The energy of the most intense alpha line is 5.48 MeV, which is above the range of the normal operating modes, thus minimizing background interference from the sources. When the instrument is commanded into the radioactive source calibration mode, the pulse-height analyzer channels for the detectors described above are reselected to the uppermost channels (~5 to 6 MeV). These in-flight calibrations allow compensation for system drifts over the time length of the UARS mission.
Preflight calibration of the HEPS instrument was performed after final assembly and environmental testing. In initial calibrations at Lockheed in Palo Alto, Ba-133, Bi-207, Co-57 and Cs-137 conversion electron sources were used to determine channel energies over the range 129 keV to 976 keV. Live time as a function of counting rate was measured with sets of Sr-90 and Tl-204 beta sources spanning 3.7x10sup4 to 3.7x10sup7 Bq in activity.
Final pre-launch energy and detection efficiency calibration with both electrons and protons from 200 keV to 1500 keV was done at the Goddard Space Flight Ceneter van de Graaff accelerator. High-energy proton calibration of HEPS 1 and 2 was done on the 88-inch cyclotron of the Lawrence Berkeley Laboratories. The energy range was 6 MeV to 55 MeV, the maximum attainable at that facility .
Comparisons of electron calibration data with Monte Carlo calculations using accurate modeling of the collimator and detector geometries yielded good agrement on detector responses to monoenergetic electrons.
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