Instrument Information |
|
IDENTIFIER | urn:nasa:pds:context:instrument:epi.gp::1.0 |
NAME |
ENERGETIC PARTICLES INVESTIGATION |
TYPE |
PARTICLE DETECTOR |
DESCRIPTION |
Instrument Overview =================== The Energetic Particles Investigation (EPI) instrument onboard the Galileo Probe was part of the Lightning and Radio Emission Detector (LRD). The complete instrument consisted of four sensors (RF antenna, 2 optical sensors, particle sensor) and a central electronic box. The EPI instrument was a two-element telescope using totally depleted silicon surface barrier detectors. The instrument operated during the pre-entry phase of the mission, when the Probe was in the configuration with the heat shield still protecting the descent module. Thus, the measured particles had to have high enough energies to penetrate the heat shield prior to their measurement by one of the 8 instrument channels. The instrument was designed to handle countrates up to 3 billion counts per second. The data coverage of the investigation consisted of three data samples acquired near the equatorial region at 5, 4.1, and 3.2 Rj, and a continuous series of measurements (12 data samples) from 2.3 to 1.25 Rj, which corresponded to a spatial resolution of about 0.02 Rj in the innermost region prior to Probe entry into the atmosphere. The EPI was manufactured in cooperation between the Max-Planck-Institut fuer Aeronomie Katlenburg-Lindau, Technische Universitaet Braunschweig, Bell Laboratories, University of Arizona, University of Florida, NASA-Ames Research Center and University of Kiel. The weight of the EPI part was 0.5 kg, the power consumption was 1 watt for 1 hour and the amount of bits for data transmission was 7680 bits. The EPI instrument was mounted on the descent module with its viewing direction towards the thinner rear heat shield and its telescope axis fixed with an inclination of 41 degrees with respect to the probe's figure axis. Scientific Objectives ===================== The principal objective of this investigation was to study the energetic particle populations in the innermost region of the Jovian magnetosphere. To achieve these scientific objectives the EPI made omnidirectional measurements to get intensity profiles with a spatial resolution of about 0.02 Rj. In addition to the omnidirectional measurement, sectored data were obtained for certain channels. For detailed description see Galileo Mission Description. Calibration =========== 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. During the whole time from start to end of the probe mission, the sensor box of the EPI flight spare unit was kept under vacuum conditions to check the behavior of the instrument. 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/n). The calibration measurements were performed in part with a layer of the original heat shield material in front of the aperture in order to determine the influence of the absorber on the instrument's response functions. Operational Considerations ========================== The EPI instrument was operated in the pre-entry phase of the mission where it was difficult to get reliable information about the magnetic field value from the LRD. The data acquisition scheme is based on the nominal probe spin frequency of 10.5 rpm, which was achieved very precisely according to the Mission Operations Final Report. Detectors ========= The EPI detectors were totally-depleted, circular silicon surface barrier detectors with a thickness of 0.5 mm. The detectors were shielded by a tungsten aperture of 0.27 cm thickness forming an opening angle of 73 degrees for detector A and of 44 degrees for the coincidence of both detectors. Having a sensitive area diameter of 2.8 mm and a waver diameter of 7.2 mm, the edge effect is of special interest for these detectors. Evidence for the influence of the edge effect was found in the energy loss distributions recorded during calibration measurements [FISCHERETAL1992]. Additional measurements with radioactive sources were carried out to determine the detection efficiency of the detector edges. In parallel a mathematical model of the detector assembly was developed for the investigation of the physical details by means of a Monte Carlo method using the computer code GEANT from CERN. A model of the detection efficiency was developed and validated by comparison with calibration measurements [FISCHERETAL1992]. The investigation revealed that the area of the detector edge was of the same order of magnitude as the nominal sensitive area. This is important to know for a correct data evaluation. Electronics =========== Detector telescope and analog electronics, for 2 separate channels referring to detector A and B respectively, the detector bias supply converter, and three housekeeping channels for monitoring leakage currents in the two detectors and the ambient temperature were contained in the sensor box. The scaling, data processing, and data formatting were executed together with the Lightning and Radio Emission Detector (LRD) data in the central electronics box. The energy loss dE/dx in the detectors was used to set up the 8 channels as shown in the following table: channel dE/dx in detector A dE/dx in detector B ------------------------------------------------------ E1 >0.1 MeV N/A E2 >0.1 MeV >0.1 MeV E3 N/A >0.1 MeV P1 >0.6 MeV N/A P2 >0.6 MeV >0.6 MeV P3 >0.6 MeV >1.1 MeV HE >2.3 MeV >2.3 MeV HVY >2.3 MeV >24.5 MeV Channels E1, E3 and P1 were single counter channels, all others were coincidence channels. For more details see [FISCHERETAL1980]. Operational Modes ================= A total amount of 7680 bits was available for the EPI. The general approach was on the one hand to achieve for comparison some measurements in regions of the Jovian magnetosphere where already particle observations exist but on the other hand to concentrate the main investigation to the innermost edge below 2 Rj. The amount of bits was divided into 15 samples each with 64 words containing 8 bits, i. e., 60 words for scientific data and an additional 4 words for status information. Starting the first data sample at 5 Rj the subsequent samples were positioned at 4.1, 3.2 and 2.3 Rj. At 2.3 Rj the continuous sampling started. In case the phase locked loop circuit of the LRD was not able to detect the magnetic field zero crossing, the data acquisition was based on the nominal probe spin frequency of 10.5 rpm. One Major Frame Period (MFP) was 64 seconds. There were two different acquisition schemes for the first four samples down to 2.3 Rj and the one following. The two schemes are shown in the next two tables. Data acquisition scheme for EPI samples at 5, 4.1, 3.2 and 2.3 Rj: Chan MFP-1 MFP-2 ----------------------------------- E1 E11S E12 E2 E21S E22 E3 E31S E32 P1 P11S P12 P2 P21S P22S P3 P31 P32 HE HE1S HE2 HVY HVY E11S, E21S, E31S, P11S, P21S, P22S: Sectorized measurements in 8 sectors accumulated over 3 spin periods in such a way that always the contents of one sector are added to the equivalent sector of the next rotation. HE1S: For alpha particles the measurement is elongated over 8 spin periods but only divided into four sectors. E12, E22, E32, P12, P31, P32: Omnidirectional measurements accumulated over 3 spin periods. HE2: Omnidirectional measurement accumulated over 8 spin periods. HVY: Because of the uncertainty in finding heavy particles the investigation period is elongated to two MFP's without sectorization. Data acquisition scheme for EPI samples between 2.1 Rj and the innermost edge of the Jovian radiation belt: Chan MFP-1 MFP-2 MFP-3 MFP-4 ------------------------------------------------------------- E1 E11S E13 E2 E21S E22 E23 E24 E3 E31S E33 P1 P11S P13 P2 P21S P22 P23 P24 P3 P31 P33 HE HE1S HE3 HVY HVY1 HVY2 HVY3 HVY4 E11S, E21S, E31S, P11S, P21S: Sectorized measurements in 8 sectors accumulated over 3 spin periods in such a way that always the contents of one sector are added to the equivalent sector of the next rotation. HE1S: For alpha particles the measurement is elongated over 8 spin periods but only divided into four sectors. P31, E22, P22, E13, E23, E33, P13, P23, P33: Omnidirectional measurements accumulated over 3 spin periods. HE3: Omnidirectional measurement accumulated over 8 spin periods. HVY1-HVY4: Omnidirectional measurements accumulated over one MFP. Measured Parameters =================== The instrument counted charged particles using their energy loss in one or two silicon surface barrier detectors to attribute them to different channels. |
MODEL IDENTIFIER | |
NAIF INSTRUMENT IDENTIFIER |
not applicable |
SERIAL NUMBER |
not applicable |
REFERENCES |
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