PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "2006-03-29: Information entered by David Tarico with out a description.; 2006-07-14: Instrument description provided by Economou and DiDonna, entered by C. Neese.; 2007-01-24: Revisions to meet review liens" OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "CO" INSTRUMENT_ID = "HRD" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "HIGH RATE DETECTOR" INSTRUMENT_TYPE = "N/A" INSTRUMENT_DESC = " Instrument Overview =================== Overview ======== The High Rate Detector (HRD) is part of the Cosmic Dust Analyzer (CDA) on the Cassini mission payload. The overall objective of the HRD is to carry out quantitative measurements of particle flux and mass distribution throughout the Saturn ring system. The particle impact rate and particle mass distribution will be determined with respect to Saturnian distances, distance from the rings, and to magnetospheric coordinates. The particle mass range covered by the HRD (assuming a particle impact velocity of 15 km/s) ranges from 8x10^-13 to 8x10^-8 g for differential and cumulative flux measurements, and > 8x10^-8 g for cumulative flux measurements. For full information about the HRD instrument, see Srama et al. (2004). General Description =================== The HRD was designed, built and tested at the University of Chicago and measures differential and cumulative particle fluxes. The HRD has a high counting rate capability (up to 10^4 random impacts/s with <5% corrections) which will be particularly important during Saturn ring plane crossings, where fluxes are large enough to saturate the counting rate of DA (~1 c/s). The HRD has significant inheritance from the University of Chicago Dust Counter and Mass Analyzer instrument (DUCMA) flown earlier on the Vega-1 and Vega-2 spacecraft to Comet Halley (Perkins et al., 1985). The instrument employs the dust particle detection technique described by Simpson and Tuzzolino (1985) and consists of two polyvinylidene fluoride sensors with associated electronics. The PVDF sensors are round in shape with areas of 50 cm^2 and 10 cm^2 respectively, and are mounted together as a unit on the front of the HRD electronics box. For images showing the geometry of the sensors see the figures in Srama et al. (2004). The HRD detects individual particles impacting the PVDF sensors and provides continuous measurements of cumulative particle fluxes for particle masses greater than four mass thresholds for each of the two sensors (see Table I). The HRD is an independent instrument containing its own memory and processor. The only interface to the Dust Analyzer (DA) of the CDA is via the power and data cables. HRD power is supplied by the DA main electronics and data transfer responds by latching the appropriate data into the HRD data output register. The latching of the data generates an interrupt to DA indicating that the data is ready to be read by DA and stored into DA memory. The HRD is rigidly mounted to the DA so that as the CDA turntable is rotated, the HRD scans different particle arrival directions. The HRD pointing is exactly the same as the DA pointing. HRD Instrument Specifications ============================= Sensor #1: 50 cm^2, 28 microns thick PVDF detector; Sensor #2: 10 cm^2, 6 microns thick PVDF detector. Both sensors are sensitive to dust particles with velocity > 1 km/s. Each sensor has four mass thresholds (M1, M2, M3, M4 for sensor #1 and m1, m2, m3, m4 for sensor #2).These thresholds were set at values showing in the Table I for particles with a velocity of 15 km/sec. Discrete events: recording of impact time (1 s accuracy) and threshold firings for each impact Counting rates: up to 10^4/s with no corrections, while for rates 10^4 to 10^5 c/s the corrections have been determined and are known. We have not encountered such high rates during the entire Cassini mission Operating modes =============== 1. NORMAL MODE (CRUISE): Continuous recording by the HRD of individual particle impact time, threshold firings for each impact, and integral counts. This operating mode will be used for all interplanetary data collection. It may also be used during ring plane crossings up to particle impact rates of approximately few hundred impacts/s. 2. FAST MODE (ENCOUNTER, RING PLANE): Integral counts are recorded and stored by the HRD each At seconds during ring plane crossings. The time interval At is selectable at 0.1, 0.2, ..., 0.9, 1.0 s. At the highest time resolution (At = 0.1 s) the spatial resolution for the counting rates will be ~ 1 km. 3. CALIBRATE MODE: Periodic electronic calibration of the HRD with the HRD inflight calibrator. This mode allows assessment of the electronic stability of the HRD throughout the mission. PVDF Dust Sensors ================= The theory, fabrication and details of PVDF dust detector operation have been described in earlier reports (Simpson and Tuzzolino, 1985; Perkins et al., 1985; Simpson et al., 1989). A PVDF sensor consists of a thin film of permanently polarized material. A hypervelocity dust particle impacting the sensor produces rapid local destruction of dipoles (crater or penetration hole) which results in a large and fast current pulse at the input to the electronics (ns time range). The output pulse is sharp in time, with a maximum amplitude depending on impacting particle mass and velocity. Since the depolarization induced current pulse is fast, the output pulse shape is determined by the choice of electronic time constants for the pre-amplifiers and shapers. Electronic time constants (amplifier shaping time constants, discriminator width) in the few microsecond range permit a high counting rate capability for the HRD sensor-electronics combination (10^4 random impacts/s with <5% corrections). The high counting rate capability of the HRD is of particular importance for Saturn ring plane crossings, where high dust particle fluxes are encountered. Exposure Degradation of the PVDF Sensors ======================================== The HRD PVDF detector efficiency is proportional to the detector area. It is expected that with exposure to high dust flux there will be some degradation of the detectors due to the physical removal of some of the area by the impacting dust particles. However, for the small particles (most cosmic dust particles are less than 10 microns) the removed area is negligible compared to the total detector area of 10 or 50 cm^2. Acoustic Signal Suppression =========================== PVDF detectors have a secondary but minor mode of response due to their piezoelectric properties. Therefore, background acoustic disturbances of sufficient intensity could trigger the HRD PVDF detector thresholds, and these threshold firings could be mistakenly recorded as dust particle impacts. During the Cassini-Huygens mission, possible sources of acoustic disturbances include spacecraft gas jets, moving platforms, and large particle impacts on structures near the mounting positions of the HRD PVDF sensors. To minimize the effects of the possible acoustic backgrounds, the HRD PVDF sensors are mounted in sound absorbing pads. This design was highly effective in suppressing the acoustic response from mechanical shocks (Perkins et al., 1985). Thermal Aspects =============== Thermal Control. Although our studies have shown that PVDF sensors may be operated at temperatures up to ~+80 C for long periods of time (weeks) with small (<5%) degradation in dust particle response, our HRD sensor mounting technique, discussed above, acoustically and thermally insulates the sensor from its surroundings. Under these conditions, the effective sensor emissivity and absorbtivity are such that exposure of a PVDF sensor in space to direct solar illumination for a short period of time (~1 min) would result in sensor temperatures high enough to destroy the sensor. Since there have been the possibility of direct exposure of the HRD sensors to solar illumination at a radial distance of 0.68 AU from the Sun during the Cassini-Huygens mission (Jaffe and Herrell, 1997), we studied different sensor coating techniques which would restrict the sensor temperature to ~80 C during solar exposure at 0.68 AU. Temperature dependence of sensor output signal ============================================== Of the several PVDF material parameters which determine the magnitude of the PVDF detector depolarization charge signal resulting from an impacting dust particle (Simpson and Tuzzolino, 1985), the volume polarization magnitude, P, and its temperature dependence are the most important for determining the temperature dependence of the signal amplitude. Our laboratory measurements have shown that over the temperature range -50 to +80 C, the charge signal amplitude will vary by less than 6% from the values measured at room temperature. Thus, we expected an overall possible variation in output signal amplitude resulting from both detector capacitance and depolarization charge signal temperature effects of less than 10% over the expected temperature range for the detector. This maximum 10% effect will contribute a negligible uncertainty for the particle mass thresholds. HRD Electronics, Digital Data and Commands ========================================== The HRD linear electronics consists of charge sensitive pre-amplifiers (CSA), shaping amplifiers (SHAPER) and threshold discriminators. Each SHAPER provides single integration ~V single differentiation RC shaping with a shaping time constant of 2 microsec. A particle impact on either sensor #1 or sensor #2 will result in output signals from the shapers which may trigger the M1, M2, M3, M4 thresholds for sensor #1, and m1, m2, m3, and m4 for sensor #2. As a contingency either M or m, or both thresholds may be increased by a factor of 10 by ground command. Every time a dust particle hits one of the sensors, the event will increment one or more 16-bit counters associated with this particular sensor and set the 8-bit event latch, according to what thresholds are triggered. For a higher threshold to be fired, the lower threshold associated with the same sensor will also be fired. There are 4 thresholds for each detector and each threshold is connected to one of the counters. The thresholds for the large detector are indicated by M1, M2, M3, M4 and the thresholds for the smaller detector are indicated by m1, m2, m3, m4. The 8-bit latch indicates which threshold fired when an event occurs. In normal mode if a discrete event occurs or data is requested by the Dust Analyzer instrument and the storage buffer is empty, the 21-bit clock and the 8-bit latch are stored into HRD memory. When 256 discrete events have been stored, HRD stores the 64-byte header. The header consists of a 24-bit sync word, 8-bit HRD status, 32-bit HRD clock, 32-bit spacecraft clock, temperature, and the eight 16-bit counters. In fast mode, HRD will store the Fast Mode header at an interval form .1 sec to 1 sec, depending on which time interval is selected. The header is stored at the selected interval until the memory buffer is full. Once the buffer is full HRD will automatically switch back to normal mode and will only store data when room is available in the buffer. HRD Normal (Cruise) Mode Format (Raw binary data Format) --------------------------------------------------------- *** H R D D a t a F r a m e 1 *** |-----------------------------------|----------------------------| | HRD Sync (24 Bits) | HRD Status (8 Bits) | |----------------------------------------------------------------| | (MSB) HRD Clock (32 Bits) (LSB) | |----------------------------------------------------------------| | (MSB) Spacecraft Clock from DA (32 Bits) (LSB) | |--------------------|-------------------|-----------|-----------| | Sector (8 bits) | Sector (8 Bits) | Temp #1 | Spare | | Position (MSB) | Position (LSB) | (8 Bits) | (8 Bits) | |--------------------------------|-------------------------------| | M1 (16 Bits) | M2 (16 Bits) | |--------------------------------|-------------------------------| | M3 (16 Bits) | M4 (16 Bits) | |--------------------------------|-------------------------------| | m1 (16 Bits) | m2 (16 Bits) | |--------------------------------|-------------------------------| | m3 (16 Bits) | m4 (16 Bits) | |--------|-----------------------|---|---|---|---|---|---|---|---| | 3 Bit | Discrete #1 Clock | M | M | M | M | m | m | m | m | | Sector | (21 Low Order Bits) | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | |--------|-----------------------|---|---|---|---|---|---|---|---| | 3 Bit | Discrete #2 Clock | M | M | M | M | m | m | m | m | | Sector | (21 Low Order Bits) | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | |--------|-----------------------|---|---|---|---|---|---|---|---| | 3 Bit | Discrete #3 Clock | M | M | M | M | m | m | m | m | | Sector | (21 Low Order Bits) | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | |--------|-----------------------|---|---|---|---|---|---|---|---| | 3 Bit | Discrete #4 Clock | M | M | M | M | m | m | m | m | | Sector | (21 Low Order Bits) | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | ---------------------------------------------------------------- * * Discrete Data 5 to 254 * * |--------|-----------------------|---|---|---|---|---|---|---|---| | 3 Bit | Discrete #255 Clock | M | M | M | M | m | m | m | m | | Sector | (21 Low Order Bits) | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | |--------|-----------------------|---|---|---|---|---|---|---|---| | 3 Bit | Discrete #256 Clock | M | M | M | M | m | m | m | m | | Sector | (21 Low Order Bits) | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | ---------------------------------------------------------------- *** H R D D a t a F r a m e 2 *** |-----------------------------------|----------------------------| | HRD Sync (24 Bits) | HRD Status (8 Bits) | |----------------------------------------------------------------| | (MSB) HRD Clock (32 Bits) (LSB) | |----------------------------------------------------------------| | (MSB) Spacecraft Clock from DA (32 Bits) (LSB) | |--------------------|-------------------|-----------|-----------| | Sector (8 bits) | Sector (8 Bits) | Temp #1 | Spare | | Position (MSB) | Position (LSB) | (8 Bits) | (8 Bits) | |--------------------------------|-------------------------------| | M1 (16 Bits) | M2 (16 Bits) | |--------------------------------|-------------------------------| | M3 (16 Bits) | M4 (16 Bits) | |--------------------------------|-------------------------------| | m1 (16 Bits) | m2 (16 Bits) | |--------------------------------|------------------------------ | | m3 (16 Bits) | m4 (16 Bits) | ---------------------------------------------------------------- HRD Fast (Encounter) Mode Format (Raw binary data Format) ----------------------------------------------------------- |-----------------------------------|----------------------------| | HRD Sync (24 Bits) | HRD Status (8 Bits) | |--------------|-------------------------------------------------| | 5 Bit Sector | (MSB) HRD Clock (27Bits) (LSB) | |--------------------------------|-------------------------------| | M1 (16 Bits) | M2 (16 Bits) | |--------------------------------|-------------------------------| | M3 (16 Bits) | M4 (16 Bits) | |--------------------------------|-------------------------------| | m1 (16 Bits) | m2 (16 Bits) | |--------------------------------|-------------------------------| | m3 (16 Bits) | m4 (16 Bits) | ---------------------------------------------------------------- HRD Commands ============ When the CDA main electronics supplies power to the HRD, the HRD turn on in NORMAL (CRUISE) MODE, with a sensor mass threshold relays in the ~SLOW MASS~T positions. HRD commands include: (a) ENCOUNTER MODE: Selects delta t at one value in the range delta t = 0.1, 0.2, ...,0.9, 1.0 s and runs the HRD in ENCOUNTER MODE. (b) SET RELAY: Selects the threshold relay switches for each sensor to LOW MASS RANGE or HIGH MASS RANGE (see Table I). (c) IN-FLIGHT CALIBRATE (IFC): Initiates a sequence of electronic pulses of varying amplitude within the HRD electronics which permits assessment of the stability of the HRD electronics. PVDF Sensor Calibrations ======================== Particle calibrations of HRD-type PVDF sensors (28 and 6 microns thick) were carried out at the Heidelberg and Munich dust accelerator facilities, as summarized in Table 10 of Tuzzolino (1996). The calibration data show that the signal amplitude N may be fit by a power-law dependence on particle mass and velocity of the form m^av^b, with values for the mass index a and velocity index b, as given in the expressions included. From these data, and an assumed ring plane crossing particle impact velocity of 15 km/s the electronic thresholds set for the HRD flight unit will result in the particle mass thresholds given in Table I. The upper half of Table I lists the electronic and particle mass thresholds for the case where the sensor mass threshold switches are set for LOW MASS, and the lower half of Table I for the case where the switches are set for HIGH MASS (by ground command). Table I ======= HRD PVDF sensors - electronic thresholds and corresponding particle mass thresholds and particle diamenter for 15 km/s impact speed (upper half: Low Mass, lower half: High Mass). PVDF Sensor #1: area= 50 cm^2, thickness = 28 microns Electronic Mass Particle Threshold (electrons) Threshold (g) Diameter (microns) -------------- --------------- -------------- LM1 = 2.1 x 10^6 4.0 x 10^-12 1.5 LM2 = 1.9 x 10^7 2.2 x 10^-11 2.6 LM3 = 4.1 x 10^8 5.1 x 10^-9 15.7 LM4 = 5.2 x 10^9 8.3 x 10^-8 40 HM1 = 1.8 x 10^7 2.1 x 10^-11 2.5 HM2 = 1.6 x 10^8 1.7 x 10^-9 10.9 HM3 = 3.5 x 10^9 5.5 x 10^-8 35 HM4 = 4.4 x 10^10 9.3 x 10^-7 89 PVDF Sensor #2: area = 10 cm^2, thickness = 6 microns Electronic Mass Particle Threshold (electrons) Threshold (g) Diameter (microns) -------------- --------------- -------------- m1 = 2.0 x 10^6 7.5 x 10^-13 0.8 m2 = 1.9 x 10^7 4.0 x 10^-12 1.5 m3 = 4.1 x 10^8 2.3 x 10^-9 12.1 m4 = 5.2 x 10^9 8.5 x 10^-8 40.2 m1 = 2.1 x 10^7 4.4 x 10^-12 1.5 m2 = 2.0 x 10^8 9.0 x 10^-10 8.8 m3 = 4.3 x 10^9 6.5 x 10^-8 36.7 m4 = 5.4 x 10^10 2.5 x 10^-6 124 assuming impacting particles with density 2.5 g/cm^3. References ========== Jaffe, L.D. and Herrell, L.M., Cassini/Huygens Science Instruments, Spacecraft, and Mission, J. Spacecraft Rockets 34(4), 509, 1997. Perkins, M.A., Simpson, J.A., and Tuzzolino, A.J., A Cometary and Interplanetary Dust Experiment on the VEGA Spacecraft Missions to Halley's Comet, Nucl. Instrum. Method A239, 310, 1985. Ratcliff, P., Gogu, F., Gruen, E., and Srama, R., Adv. Space Res. 17(12), 111-115, 1996. Tuzzolino, A. J., Applications of PVDF Dust Sensor Systems in Space, Adv. Space Res. 17(12), 123-132, 1996. Simpson, J.A. and Tuzzolino, A.J., Polarized Polymer Films as Electronic Pulse Detectors of Cosmic Dust Particles, Nucl. Instrum. Methods A236, 187-202, 1985. Simpson, J.A. and Tuzzolino, A.J., Cosmic Dust Investigations, II: PVDF Detector Signal Dependence on Mass and Velocity for Penetrating Particles, Nucl. Instrum. Methods A279, 625, 1989." 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