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Billed as the most sensitive gamma ray observatory ever built, the European Space Agency's (ESA) INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) was launched into an eccentric, 72-hour, 9000 to 155,000 km orbit around the Earth on 17 October 2002.
Designed to provide new insights into celestial objects such as black holes, neutron stars, active galactic nuclei and supernovae, the 2 tonne INTEGRAL vehicle is equipped with a payload that includes a spectrometer (known as the SPectrometer on INTEGRAL - SPI), an imager (the Imager on Board the INTEGRAL Satellite - IBIS), an X-Ray monitor (the Joint European X-ray Monitor - JEM-X), an Optical Monitoring Camera (OMC) and an Integral Radiation Environment Monitor (IREM).
Of these, the SPI is designed to perform spectral analysis of gamma-ray point sources and extended regions in the 20 keV to 8 MeV energy range, with a resolution of 2.2 keV at 1.33 MeV. As such, the SPI device incorporates an array of 19 hexagonal, high purity, germanium detectors that are cooled to an operating temperature of 85 K by a Stirling engine. A hexagonal coded aperture mask is located 1.7 m above the detection plane in order to image a 16° section of the sky with 2° angular resolution. In order to reduce background radiation, the detector assembly is shielded by an anti-coincidence system that includes limiting the aperture to approximately 30° and the provision of a plastic veto below the mask to further reduce the 511 keV background. For its part, the IBIS provides fine imaging, source identification and spectral sensitivity to both continuum and broad lines over the 15 keV to 10 MeV energy range. Within the device, a tungsten coded-aperture mask is located 3.2 m above its detection plane and is optimised for high angular resolution. The IBIS detector uses two planes, namely a 2600 cm2 front layer (made up of 4 x 4 x 2 mm pixels) and a 3100 cm2 back layer (9 x 9 x 30 mm pixels) that are 90 mm apart. The use of a two-layer detector allows photon paths to be tracked in 3-D as they scatter and interact with more than one element of the array. Events can be categorised and the device's signal-to-noise ratio is improved by rejecting those responses (usually towards the high end of the energy range) that are unlikely to correspond to real celestial photons. The IBIS aperture is restricted by a lead shielding tube and is shielded in all other directions by an active scintillation veto system.
The JEM-X X-ray monitor supplements the SPI and IBIS instruments and is used to detect and identify gamma-ray sources and in the analysis and interpretation of acquired gamma-ray data. Accordingly, JEM-X takes readings simultaneously with those of the SPI and IBIS devices and provides imagery with arcminute angular resolutions within the 3 to 35 keV prime energy band. Its baseline photon detection system consists of two identical high pressure imaging microstrip gas chambers (1.5 bar pressure and made up of 90 percent xenon and 10 percent methane), at a nominal gas gain of 1500. Each detector views the sky through a coded-aperture mask that is located approximately 3.2 m above the detection plane.
JEM-X itself is supported by the OMC which is used to observe optical emissions from INTEGRAL's prime targets at the same time as those targets are being observed by the SPI, IBIS and JEM-X. The remaining instrument - the IREM - is described as performing a wide range of in-orbit radiation monitoring functions and downloading the acquired data via the INTEGRAL vehicle's telemetry system. Overall, the satellite's orbit is designed in such a way as to keep it at an altitude of 40,000 km or above for as long as possible in order to minimise background radiation effects from the Earth's own radiation belts. INTEGRAL's unit cost is put at 330 million in year 2000 values.
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