The Spectrometer onboard Integral (SPI) has the ability of detecting gamma-ray sky in the 20 keV - 8 keV energy range. SPI use 19 high-purity Germanium detectors in its camera to detect gamma-rays in the sky, and it also use Stirling cycle coolers to cool the Germanium detectors to a temperature of 85-90 K (spi-2) however these coolers are rather heavy and need to be operate in pairs to avoid the vibration problem they cause.(book) over roll design of SPI gives excellent opportunity for Integral observatory to reveal the mysteries behind the gamma-ray astronomy. Therefore it is worth to understand the design of the SPI
Description of the instrument
Fi-Pic of of spi (8)
Detectors and pre-amplifiers
SPI use 19 reverse-electrode n-type germanium detectors and Stirling cycle coolers are used to cool the detectors which help to reduce the leakage current and decrees the radiation effect, the germanium detectors mounted in a hexagonal shape to minimize the space which could take by the detector array, the first stage of the preamplifier circuitry fixed on the rear side of the cold plate house which only have passive components. And another set of preamplifiers are mounted on a second cold plate which connected to the cryostat that help to keep the low temperature, these second set of preamplifiers has 19 preamplifiers and these are mounted in a cold plate as aforesaid to reduce the electronic noise . (spi 8)
Cryostat is a cooling system which help the SPI detector to kept in there operation temperature, Cryostat is build from three systems; an active cooling system, a passive cooling system and a cold box. Four mechanical Stirling cycle coolers used in Cryostat to cool the detector system, the active cooling system brings the temperature of the cold plate down to 80 k where the detectors are mounted, the third system of the Cryostat which is the cold box (beryllium structure ) used to enclose the detector plane to keep an intermediate temperature of 210 K. Cryostat sub system temperatures are monitored regularly to provide information that could use for data processing and to give early notice in case of failures of the coolers. Because of these reasons design of SPI Cryostat keep germanium detectors at a constant cryogenic temperature which allow the detectors to give the best sensitivity and resolution.(spi 8)
Pulse shape discriminator (PSD)
Currently Pulse shape discriminator is not used in the mission. Main purpose to have the PSD is to reduce the -decay background in the Germanium detectors, the reason to stop getting use of PSD was past experience from the early mission disclose the fact that Pulse shape discriminator is not capable of considerably gain the signal to noise ratio.(spi 8)
Anti-Coincidence Subassembly (ACS)
Anti-Coincidence Subassembly prevent photons and particles which comes from sources outside the field of view reaching the detector, actually ACS shield the Germanium detectors which use in the SPI from the background, the SPI Anti-Coincidence Subassembly made of 91 bismuth-germanate (BGO) scintillator blocks, these scintillator blocks are position in four subunits, two units are use to surround the camera one assembly in the side-shield and other one shielding the rear of the camera, other two units are collimator rings that located between the
detector plane and the coded mask which are called the upper and lower collimator ring, if too great magnitude of bismuth-germanate used it can increase secondary neutron production which producing extra background in the germanium crystals therefore BGO crystal thickness is optimised to minmise the detector background.
Photo-multiplier tubes are used to detect the photons which convert by the BGO scintillator crystals, these crystals convert all incoming photons and particles into visible light and these photons convert into electrical pulses by the Photo-multiplier tubes which are combined into a veto signal.