The SPI detectors are the only elements of the INTEGRAL spacecraft that are actively cooled down. A low temperature is required for an optimum sensitivity and resolution, as well as for limiting the radiation damage. An active cooling system brings the temperature of the cold plate on which the detectors are mounted down to 85 K, using two pairs of cryocoolers. In normal operating mode all coolers work simultaneously. In case of failure of one of the cryocoolers the instrument can still operate with only one pair of cryocoolers in a degraded mode, with a detector temperature of about 100 K. The detector assembly is placed inside a cold box, which is kept at approximately 210 K by the passive cooling system. All temperatures of the cryostat subsystems are regularly monitored to provide temperature information that can be used for the data processing.
The detector plane is composed of 19 hexagonal-shaped Germanium detectors, encapsulated into beryllium capsules, and mounted in the most compact configuration on the cold plate. The size of a detector is 5.6 cm, flat-to-flat, with a height of 7 cm. The detectors are mounted with minimum space between them, such that the axes of two adjacent detectors are 6 cm apart.
At the time of writing, four of these 19 detectors have ceased to function properly and are disabled.
The death of a given detector results in a decrease of the effective area of the instrument, roughly by a factor 1/19. From revolution 929 on, the effective area is about 85% of the original area.
In order to recover from the radiation damage of the Germanium crystal structure, the detectors go through an annealing operation every 6 to 12 months. They are heated to 105 C for a few days. The instrument is not available for scientific observations during the time needed for the annealing operation and the cooling phase afterwards.
The signals from the detectors pre-amplifiers are sent to the amplification chain, which is made up of a Pulse Shape Amplifier and a Pulse Height Amplifier. The pulses are amplified in such a way that the performance of the spectrometer is optimized. This is done by making a compromise between getting the best signal to noise ratio for the pulses, operating in the full 20 keV - 8 MeV energy band of the instrument, and making the output pulses insensitive to the fluctuations in the detector signal rise time.