ENERGETIC PARTICLES INVESTIGATION (EPI) H. M. FISCHER^1, J. D.MIHALOV^2, L. J. LANZEROTTI^3, G. WIBBERENZ^1, K. RINNERT^4, F. 0. GLIEM^5, and J. BACH^5,6 Abstract. The Energetic Particles Investigation (EPI) instrument operates during the pre-entry phase of the Galileo Probe. The major science objective is to study the energetic particle population in the innermost regions of the Jovian magnetosphere - within 4 radii of the cloud tops - and into the upper atmosphere. To achieve these objectives the EPI instrument will make omnidirectional measurements of four different particle species - electrons, protons, alpha-particles, and heavy ions (Z > 2). Intensity profiles with a spatial resolution of about 0.02 Jupiter radii will be recorded. Three different energy range channels are allocated to both electrons and protons to provide a rough estimate of the spectral index of the energy spectra. In addition to the omnidirectional measurements, sectored data will be obtained for certain energy range electrons, protons, and alpha-particles to determine directional anisotropies and particle pitch angle distributions. The detector assembly is a two-element telescope using totally depleted, circular silicon surface-barrier detectors surrounded by a cylindrical tungsten shielding with a wall thickness of 4.86 g cm^-2. The telescope axis is oriented normal to the spherical surface of the Probe's rear heat shield which is needed for heat protection of the scientific payload during the Probe's entry into the Jovian atmosphere. The material thickness of the heat shield determines the lower energy threshold of the particle species investigated during the Probe's pre-entry phase. The EPI instrument is combined with the Lightning and Radio Emission Detector (LRD) such that the EPI sensor is connected to the LRD/EPI electronic box. In this way, both instruments together only have one interface of the Probe's power, command, and data unit. 1. Introduction The principal scientific objective of the Energetic Particles Investigation (EPI) is to measure the energetic particle population of the innermost region of the Jovian magnetosphere within 5 Jupiter radii (Rj). Both the Pioneer and the Voyager missions provided measurements in a portion of this inner magnetospheric region (Armstrong et al., 1981; Schardt and Goertz, 1983). The most prominent findings include a continuing increase of the particle fluxes with decreasing radial distance to the planet down to the closest distance measured (~1.6 Rj for Pioneer 11). However, the increase in fluxes is not simply monotonic, but rather shows a complex shell-like structure, which depends on energy and particle species (Fillius, 1976; Simpson and McKibben, 1976). These observations have been interpreted as indicating the absorption of the radially inward-diffusing particles by macroscopic objects orbiting the planet. There is also strong indication in the previous measurements for pitch angle scattering due to wave-particle interactions (Gurnett and Scarf, 1983; McDonald and Trainor, 1976; Simpson and McKibben, 1976; Van Allen, 1976). Closer to the planet than previous missions have gone (~ 1.6 Rj) other loss effects, such as radiation loss in the case of electrons and atmospheric absorption, will become more and more dominant. The measurements of EPI will be extended as close as possible to the planet's atmosphere (~ 1 Rj) in order to complete the knowledge about the energetic particle population at the inner edge of the Jovian magnetosphere and to make it possible to study the related magnetospheric transport processes. The operational period of the EPI is terminated by the beginning of the entry phase when the power supply for the sensor head is switched off. The Voyager 1 mission detected, by imaging observations, a ring of dust particles around Jupiter near 1.8 Rj as well as the small satellites Adrastea and Metis at the outer edge of this ring (Jewitt et al., 1979; Owen et al., 1979; Smith et al., 1979). The existence of absorbing material in this region was already suggested from particle observations made by Pioneer 11 (Schardt and Goertz, 1983). Because of the absence of measurements, little is known about particle loss, source processes, and fluxes inside the closest approach of Pioneer 11. Particle losses due to the existence of Jupiter's ring and the satellites mentioned above are likely to be significant and to be dependent upon energy and pitch angle. Particle losses analogous to terrestrial conditions, such as Coulomb energy degradation in the atmosphere, may or may not be important, depending upon the density of free and bound electrons in Jupiter's upper ionosphere. A candidate for internal particle sources at low altitudes is cosmic-ray produced albedo neutron decay. This especially interesting region will be fully covered by EPI measurements. In summary, electron and ion data acquired by the EPI instrument within 1.1-2 Rj will be used to: -test the validity of radial diffusion as a transport and acceleration mechanism in the deep inner magnetosphere; -infer the nature of field perturbations responsible for radial diffusion; -infer the size distribution and radial structure of Jupiter's ring; -identify possible additional inner magnetosphere source and loss mechanisms. Moreover, comparisons of Galileo Probe and Pioneer 11 measurements in the region ~5 Rj to ~1.6 Rj will provide information on any temporal variations of inner-zone particle fluxes. 2. EPI Instrument Sensor. Fundamental to the design of the detector configuration was the requirement to maintain expected counting rates below about 3 million counts per second under the most intense anticipated flux conditions. Another design constraint was the lower threshold for the particle species to be measured. This is defined by the material thickness of the Probe's aft heat shield. The heat shield consists of different layers of aluminum, silicon adhesive, kapton, mylar, dacron, and phenolic nylon. For perpendicularly-penetrating particles it has a resulting thickness of 1.34 cm, equivalent to about 1.87 g cm^-2 'shielding' (determined from a piece of heat shield material used in the calibrations; see last section). Fig. 1. Orientation of the EPI look axis with respect to the Probe spin axis, and detector and shielding configuration of the EPI sensor head. The detector assembly is a two-element telescope using totally-depleted, circular silicon surface barrier detectors. Both detectors have a radius of 1.4 mm with sensitive areas of 6.2 mm^2 and thicknesses of 0.5 mm. A 3 mm thick (equivalent to 2.55 g cm^-2) brass absorber is inserted between the two detectors in order to expand the energy range of particles to be investigated. The total length of the telescope (upper surface front detector to lower surface back detector) is 6 mm. The detector assembly is surrounded by tungsten shielding with a cylindrical wall thickness of 2.7 mm (equivalent to 4.86 g cm^-2) The entire Probe and its contents, estimated to be equivalent to at least 80 g cm^-2, forms the shielding at the rear of the telescope. A reduced background rate is thus expected from the rear. The tungsten shielding cylinder is elongated beyond the front detector surface and forms an open aperture cone of length 6.7 mm with an opening diameter of 9 mm. The aperture opening angle of 44 degrees and the detector area define an effective opening angle of 73 degrees and a geometrical factor of 0.045 cm^2 sr, for single particle counting events in the front detector. The corresponding values for coincidence events are 44 degrees and 0.01 cm^2 sr, respectively. The axis of the telescope is mounted at an inclination angle of 41 degrees with respect to the Probe's spin axis (Figure 1). The orientation of the Probe's spin axis in space and the local magnetic field direction define the range in which pitch angle distributions can be determined by the instrument. Telescope and analog electronics, the detector bias supply converter, and three housekeeping channels for monitoring leakage currents in the two detectors and the ambient temperature are contained in the sensor box (Figure 2). The scaling, Fig. 2. Photograph of the EPI sensor box. data processing, and data formatting are executed with the Lightning and Radio Emission Detector (LRD) data, in the central electronics box (see Lanzerotti et al., 1992). A summary of the energy channels for the different particle species is given in Table 1. Electronics. The limits on the available Probe power prohibited the design and development of extremely fast circuitry for the EPI. As a compromise between the demands of power consumption and high-frequency response, a pulse rate of 3 million counts per second can be handled by the EPI analog electronics. An overview of the EPI analog electronics is given in Figure 3. TABLE I EPI energy channels. The channels are valid for each particle type but they are associated with the given 'particle species' and the 'energy range' related to this species. ------------------------------------------------------------------------------ Channel Particle Detector Energy range No. species (MeV nucl^-1) ------------------------------------------------------------------------------ E1 electron A > 3.2 E2 electron AB > 8 E3 electron B > 8 P1 proton A 42-131 P2 proton AB 62-131 P3 proton AB 62-92 HE alpha-particle AB 62-136 HVY heavy particle AB 12C: 110_910 32S: >210 ------------------------------------------------------------------------------ The detectors must be maintained in operational condition during the ~6 years cruise phase from Earth to Jupiter. There is no power available from the Probe's main battery supply, which will only be activated shortly before the pre-entry part of the cruise phase. Therefore, in order to supply the detectors with bias voltage, a compact battery package with a capacity of about 250 mA-h is contained in a special fixture on the front side of the electronic box. The battery consists of Li/CrOx cells having an open circuit voltage of approximately 3.6 V. The assembly is configured in two stacks with ten cells each. The overall voltage output is ~75 V, which has been monitored in different test assemblies for more than 4 years. The batteries have been carefully tested and selected from an especially fabricated series with respect to the operational requirements under vacuum conditions. 3. Modes of Operation A limited data rate is available for the Energetic Particles Investigation during the pre-entry phase. The general mission design approach was to make (for comparison purposes) some measurements in regions of the Jovian magnetosphere where particle observations from previous missions already exist and to concentrate the EPI investigation zone, below ~2 Rj. Therefore, the Fig. 3. Block diagram of the analog electronics of the EPI sensor system. As indicated, the electronics are contained partly within the sensor box shown in the photograph in Figure 2 and in the electronics box, respectively. (CSA - charge sensitive pre-amplifier, BLR-baseline restorer, Disc. - discriminator, Coinc. - coincidence circuit.) measurements must be distributed with respect to detected particle species and with respect to spatial locations in an appropriate manner, in order to obtain as much information as possible. The data coverage of the investigation consists of three data samples made near the equatorial region at 5, 4, and 3 Rj, and in a continuous series of measurements between 2 and 1.1 Rj. Limited radio-frequency data will also be obtained at these locations by the Lightning and Radio Emission Detector on the Probe (see Lanzerotti et al., 1992). During the arrival at Jupiter it will be of particular advantage for data analysis and interpretation if particle measurements are made simultaneously on the Orbiter and on the Probe in the region 5 to 4 Rj. The measurements from ~4 Rj down to the innermost edge of the trapping region will be continued by the Probe instrument. The distribution of the EPI samples, the related position of the Orbiter, and the trajectories of both spacecraft at Jupiter arrival are presented in Figure 4. The instrument will make omnidirectional measurements of the particle species Fig. 4. Orbiter and probe trajectories at Jupiter arrival, showing locations of the EPI data acquisition and corresponding locations of the Orbiter and its science instruments. listed in Table 1. Time-intensity profiles of these species, with a spatial resolution of about 0.02 Rj during the period of continuous measurement between 2 Rj and 1 Rj, will be recorded. Three different energy range channels are allocated to both electrons and protons to provide a rough estimate of the spectral index of the energy of each species in the energy measured. In addition to the omnidirectional measurements, angular sectored data will be obtained for electrons, protons, and alpha particles in certain energy ranges in order to determine directional anisotropies and particle pitch angle distributions. Fig. 5. Size, sensitive area, and efficiency of the EPI detectors. Because of the expected low statistics, the heavy particle measurements are accumulated over longer time periods. The LRD-determined main magnetic field direction will be used for these sectored measurements (see Lanzerotti et al., 1992). Fig. 6. Energy dependence of the count rates in selected EPI energy channels (cf. Table I). The calibration measurements and the simulation calculations both were performed without the heat shield and for protons only. Fig. 7. Comparison of the EPI calibration measurements with Monte-Carlo simulations for 20 MeV electrons including the heat shield material. Presented are the energy loss distributions in detectors A and B from calibration and simulation, respectively. The response of the EPI counting channels is demonstrated for comparison. 4.Tests and Calibrations There has been a long series of detector calibration measurements with radioactive sources over a period of 10 years to determine the long-term stability of the detectors under normal and thermal-vacuum conditions. For the higher energies, the instrument was calibrated with protons at the Harvard Cyclotron (30-150 MeV), with electrons at the Lawrence Livermore accelerator (20-30 MeV), and for heavier particles (He, O, Si, Ar) at the Berkeley Bevatron (70-340 MeV nucl^-1). The calibration measurements were performed in part with a layer of original heat shield material in front of the aperture in order to determine the influence of the absorber on the instrument's response functions. Subsequent to the calibration measurements, a model of the detector assembly was developed for the investigation of more physical details by means of a Monte-Carlo simulation method (Hoop, 1989; Pehlke, 1988). In a first attempt, the calibration measurements without the heat shield were simulated in order to compare the experimental and calculated data. By this procedure the influence of the detector edges on the counting efficiency could be determined. This is of special importance in case of such tiny detectors having a sensitive area diameter as small as 2.8 mm (which has to be considered in comparison to the diameter of the original silicon disc (7.2 mm) mounted in the detector housing). In the case of an ideal detector, the efficiency for particle detection should steeply drop from 100% to zero at the edges of the active area. A model of the real efficiency decrease in the edge regions of the detectors is presented in Figure 5. This model was a necessary assumption in order to explain in detail the distribution of the energy losses of particles which were obtained from the accelerator measurements. Figure 6 shows the energy dependence of the count rates in selected EPI energy channels (cf. Table I) resulting from calibration measurements, and simulation calculations performed without the heat shield and for protons only. Protons in the energy range below 30 MeV could not be provided by the accelerator. In this range, data were only obtained by means of simulation. A comparison of the calibration and simulation data yields a reasonable correspondence. Therefore, further simulations incorporating the heat shield were done. Figure 7 shows a comparison between the 20 MeV electron calibration results and the simulation data, both with the heat shield. In the lower part of this figure the response of the EPI counting channels is added. The final aim of the simulation method is to determine a response function of the instrument operating behind the heat shield in a particle population with a spectral and spatial distribution as might be expected in the innermost part of the Jovian radiation belt. These simulation capabilities, together with the calibrations, will be invaluable for interpreting the data to be acquired in the harsh inner Jovian magnetosphere. Acknowledgements Several of the authors (H. M. F., L. J. L., and J. D. M.) would like to thank A. Koehler and the staff of the Harvard Cyclotron Lab as well as I. Proctor and the staff of the LLNL rf Linac for their generous support during the calibration measurements with protons and electrons, respectively. 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