First identification and modelling of SPI background lines

On Oct. 17, 2002, the ESA INTEGRAL observatory was launched into a highly elliptical orbit. SPI, a high resolution Ge spectrometer covering an energy range of 20–8000 keV, is one of its two main instruments. We use data recorded early in the mission (i.e. in March 2003) to characterize the instrumental background, in particular the many gamma-ray lines produced by cosmic-ray interactions in the instrument and spacecraft materials. More than 300 lines and spectral features are observed, for about 220 of which we provide identifications. An electronic version of this list, which will be updated continuously, is available for download at CESR. We also report first results from our efforts to model these lines by ab initio Monte Carlo simulation.