Spectrometer SPI of the INTEGRAL mission

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Abstract
INTEGRAL is ESA's high-energy astrophysics mission to be launched into a high eccentric orbit early in the next decade. One of the two mission's main telescopes is the gamma-ray spectrometer SPI. This instrument features a compact array of 19 high-purity germanium detectors shielded by a massive anticoincidence system. A coded aperture of the HURA type modulates the astrophysical signal. We present the spectrometer system and its characteristics and discuss the choices that led to the present design. The instrument properties like imaging capability, energy resolution and sensitivity have been evaluated by extensive Monte-Carlo simulations. The expected performance for narrow-line spectroscopy is characterized by an energy resolution of approximately 1.6 keV at 1 MeV, an angular resolution of approximately 2 degrees within a totally coded field of view of approximately 15 degrees, and a sensitivity of (2 - 5) multiplied by 10-6 gamma/(cm2 s) for 4 multiplied by 106 s observation time in the nominal energy range from approximately 20 keV and approximately 8 MeV. With these characteristic features it will be possible for the first time to explore the gamma-ray sky in greater depth and detail than it was possible with previous gamma- ray telescopes like SIGMA, OSSE and COMPTEL. In particular the field of nuclear astrophysics will be addressed with an unprecedented combination of sensitivity and energy. Especially the high-energy resolution allows for the first time measuring gamma-ray line profiles. Such lines are emitted by the debris of nucleosynthesis processes, by the annihilation process near compact objects and by the nuclear interaction between cosmic rays and interstellar matter. Lines of all these processes have been measured so far, but, owing to the relatively poor energy resolution, details of the emission processes in the source regions could not be studied. With the high-resolution spectroscopy of SPI such detailed investigations will be possible opening a wealth of astrophysical investigations.

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