Four of the CPD particle detectors look vertically upward along the 8-m boom while the remaining two look horizontally to the side. The orientation is seen in the photo to the left. The detectors look out through cut-aways in the top and side solar panels. The pitch angle for each detector can be determined from the vector magnetometer measurements combined with the rotation of the boom-mounted gondola with respect to the satellite body. In near-polar regions (above some 60 deg in latitude) with the high inclination of the geomagnetic field the 4 upward-looking detectors will all look into the loss cone which at the altitude of the satellite (around 700 km) is at 60 deg pitch angle. The 2 horizontally-looking detectors, correspondingly, will always look at the radiation outside the loss cone within polar regions.

Technical specifications

The overall main characteristics of the CPD instrument is shown in Table 1:

Table 1. Summary technical specifications for CPD experiment.

The CPD housing and the detector designations and view directions with respect to satellite axes (z1 is along the boom) are shown in the figure below

The CPD instrument uses two types of solid-state Silicon detectors. Their characteristics are shown in Table 2.

Table 2. Basic detector characteristics.

The energy thresholds for the detectors are determined by the entrance windows. The P1, P2 and E1, E2 detector units are shielded by 1.25 micron Nickel foils (in addition to the detectors own gold window). The P3 detector has a 1 mm thick Aluminium shield at its entrance window and the P4 detector a 1 mm Copper shield. The amplified pulses from detected particles penetrating into the depletion layer are fed to pulse height analyzer units with 8, resp. 4 threshold levels. The pulses for each detector unit and each energy level are counted. These counts are sampled by the instrument computer at the rates given in Table 1 and then temporally stored. Controlled by interrupt signals from the CDH main computer the data (and status information) are transferred to the CDH-controlled satellite data memory.
The properties of the detector units and their detection ranges are shown in Table 3.

Further details

The photo to the left shows the CPD box seen from below. In the large compartment one may observe the lowermost of 4 PC boards. This board holds the CPD experiment computer system based on an Intel 80186 processor. The other PC boards further up in the box hold the charge-sensitive preamplifiers and pulse amplifiers, the analyzer level detector circuits and the counter and read-out circuits. The uppermost of the smaller compartments to the right houses the two side-looking detector units. In the lower small compartment are housed the low-voltage regulators and the high-voltage converters used to provide the detector bias voltages.

This photo shows one of the six solid-state Si-detector elements. The detector is a reverse biased silicon diode with a depleted junction region only covered by thin gold layer which also provides the electrical contact for the diode.
High-energy particles may penetrate the gold layer and enter the reverse-biased depleted junction layer. Here they produce an ionized trail until they are stopped and come to rest. The electrons and holes drift to either side in the applied electric field to be collected by the electrodes at sides of the junction. The total charge collected is a measure of the energy deposited by the incoming high-energy particle. Until the primary particle penetrates the detector this energy amounts to the total energy of the incident particle less the losses in the gold window and the dead-layer just beneath it. For penetrating particles at still higher energies the energy deposited in the detector decrease. The detection process is sketched in the figure to the left.

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