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Certain parts of the ROSAT orbit are contaminated by the very intense particle background: the belt regions and the South Atlantic Anomaly (SAA). The focal plane detectors are put into a detector specific safe mode during passage through these regions. Normally, time tagged commands turn down the HV of the operating detector (to 0 for the PSPC, to a low value for the HRI). As a backup, two charged particle detectors (the SAADs; one redundant) monitor the particle background, and if the rate exceeds an adjustable threshold, delivers a signal to the ZDE to reduce the detector high voltage.
The SAAD is an N-type silicon semi-conductor with an area of 10cm and a thickness of 700 m. Its corresponding electronics are partly integrated in the sensor box and one of the main electronic boxes. The sensor box contains the semiconductor, an HV-converter, and the front-end electronics (a charge sensitive pre-amplifier, followed by a pulse-shaping amplifier). For redundancy at high count rates, the detector current is also monitored.