SPI: the spectrometer aboard integral

SPI is a high spectral resolution gamma-ray telescope on board the ESA (European spatial agency) mission INTEGRAL (International Gamma Ray Astrophysics Laboratory). It consists of an array of 19 closely packed germanium detectors surrounded by an active anticoincidence shield of BGO (bismuth germanate). The imaging capabilities of the instrument are obtained with a tungsten coded aperture mask located 1.7 m from the Ge array. The fully coded field-of-view is 16 degrees, the partially coded field of view amounts to 31 degrees, and the angular resolution is 2.5 degrees. The energy range extends from 20 keV to 8 MeV with a typical energy resolution of 2.5 keV at 1.3 MeV. Here we present the general concept of the instrument followed by a brief description of each of the main subsystems. INTEGRAL was successfully launched in October 2002 and SPI is functioning extremely well. (authors)