Medium-Energy Particle Spectrometer (MEPS)
A diagram of a single MEPS sensor unit is shown in Figure 8. It shows the positions of the deflection plates relative to the entrance aperture, collimator, the particle sensors (Channel Electron Multipliers or CEM's), the light trap, and repeller screens. Incident particle energies are selected by stepping the voltage on the oppositely charged deflection plates. Electrons and positive ions passing through the collimator are deflected in the region between the plates, electrons toward the CEM on the right, ions toward the CEM on the left. Pulses from the CEM's (Galileo series 4800 channeltrons) are amplified by Amptek A101 amplifie r/discriminators and sent to the Central Electronics Package for accumulating and multiplexing into the MEPS data stream.
The analyzer constant k, sometimes called the deflection sensitivity, is the
constant of
proportionality relating the incident particle energy, E, and the potential
difference, 
V, between
the deflection plates. This constant and the energy resolution E/E are
dependent on the
electrical and mechanical characteristics and condition of the surfaces of the
collimation,
deflection, and sensing systems of Figure 8. They are determined by
laboratory calibration and
are shown in Table 3. To reduce UV reflection and secondary emission, the
surfaces of the
deflection plates, the light trap exterior, and the repeller screens were
coated with gold black.
A spectrum is measured by stepping the voltage on the deflection plates through 31 levels, equally spaced logarithmically, corresponding to energies between 0.5 eV to 32 keV. On each step, counts are accumulated for 54.5 ms. Simultaneous electron and ion spectral measurements are repeated every 2.048 ms.
MEPS operates in a single "spectral measurement" mode. When the instrument is "ON", electron and ion spectral measurements are made continuously. One modification to the spectral measurement is possible by ground command. It is a change in the voltages applied to the repeller screens in order to reduce the flux on the CEM's. The normal value applied to the screens is one volt (i.e., -1 V on the electron screen and +1 V on the ion screen) which can be changed to five volts (-5 V on the electron screen and +5 V on the ion screen) in order to achieve this flux reduction. The screens effectively serve as a cut-off at the low-end of the energy spectrum. Because of the sharp increase in number flux with decreasing energy of the spectrum in the few eV region, a change from lower to higher voltage will significantly reduce the flux striking the CEM's. During most of the mission the 5 volt setting has been used.
MEPS calibration was performed at SwRI with the same system that was used to calibrate the Dynamics Explorer plasma instruments (Burch et al., 1981; Winningham et al., 1981). In the calibration an electron beam was photoelectrically generated in a vacuum by irradiation of a gold surface with UV from a mercury-argon lamp source. For a full energy/angle calibration run, the deflection voltage is set to a designated value, and under computer control, the electron beam is stepped through a range of energies that covers the pass band as angular responses are measured at each step. This process maps the energy and angular responses of the particular MEPS detector. Another type of scan was made in which the electron beam energy was held constant and the deflection voltage stepped through values from 0 to 2200 V (the top step). This enabled a quick measurement of the deflection constant at several energies as well as a noise test at all step levels with and without the electron source activated. The detector responses of ion channels were calibrated using the electron source by reversing the voltages on the two deflection plates and on the two repeller screens.
Data obtained from the calibration runs yielded values of energy resolution (E/E) and deflection constant for specific angular orientations and for integrations over all angles. Likewise, the angular responses along both axes of the rectangular collimator apertures were determined at single energies and for counts integrated over the full energy pass bands. The measurements yielded values close to the theoretical values of each collimator system of 2.5 degrees x 10 degrees at the FWHM value. Results of the laboratory measurements for each MEPS detector are shown in Table 3.
Complete calibration required a determination of the detector throughput
function, which
includes the CEM efficiency as a function of energy. The count accumulated by
a MEPS
detector in time t when measuring the electron source of flux j (E) is
where G(E) is the energy dependent geometric factor of the analyzer, (E) is
the CEM
efficiency, and mu(E) is the energy band pass FWHM. An alternate form of equation
(1) is
where Gsubp is the (constant) physical geometric factor of the analyzer and F(E) is the energy dependent part of the throughput function, Gsubp F(E). Although the largest influence on the shape of F(E) is the dependence of electron efficiency on energy, F(E) will contain any variation with energy of the analyzer response not accounted for by the physical geometric factor.
The throughput function was determined for the MEPS detectors by exposure of each to a stable nickel-63 source. Detector response was measured as a function of deflection voltage or particle energy. All count vs. energy curves can be described with a single function F(E) by multiplication of each by a single factor. This factor was used to compute each physical geometric factor Gp, thus providing accurate relative geometric factor measurements for all MEPS analyzers. Computing the low-energy part of the throughput function required using calculations of CEM detection efficiency made by Bordoni (1971). At higher energies the shape of the Ni-63 emission spectrum was used to obtain the function shape (10-30 keV energy range). A smooth transition from the Bordoni function calculations to those resulting from the Ni-63 source takes place between 2500 and 10,000 eV. Since MEPS uses pre-acceleration (200 V) to shift particle energy to greater sensor efficiency, the electron transfer function is flat for low-energy electrons and has a value of 0.95. Above 1000 eV, the function decreases monotonically reaching a value of about 0.27 at 30000 eV. The transfer function for ion detection is constant over the energy range measured by MEPS. A value of 0.65, the same value obtained with the DE instrument, is used with the MEPS ion detectors.
Several internal coatings and arrangements of internal MEPS components were available before flight. To discriminate between them, tests were conducted with ultraviolet light (UV) as the sensors monitored the internally generated background of secondary emitted particles. These tests were conducted in the UV calibration system at Aerospace Corporation with the same high- voltage calibration supplies and monitoring equipment used in the calibrations at SwRI. Coatings for the flight units were chosen on the basis of the lowest UV generated secondary particle response. They were gold black over aquadag on the deflection surfaces and gold plate on the internal reflecting surfaces of the light trap. Some non-critical internal surfaces were coated with aquadag.
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