THE HIGH RATE DETECTOR (HRD)
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 8 × 10-13 to 8 × 10-8 g for differential and cumulative flux measurements, and > 8 × 10-8 g for cumulative flux measurements.
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 104 random impacts s-1 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 s-1).
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
Figure 1
the dust particle detection technique described by Simpson and Tuzzolino (1985) and consists of two polyvinylidene fluoride sensors with associated electronics (Figure 1). The sensors are mounted on the front of the HRD electronics box and 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.
The HRD is an independent instrument containing its own memory and processor. The only interface to the DA 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