Contribution of passive materials to the background lines of the spectrometer of INTEGRAL (SPI)

The passive materials inside the SPI BGO shield represent a non negligible fraction of the SPI detection unit total mass, containing many iron-rich materials, if we except the berrylium. The bombardment of the passive materials by the cosmic rays and by their secondaries in the BGO shield produce radioactive unstable nuclei which in turn decay and possibly emit a γ-ray. In this paper we will compute the contribution of various passive materials inside the shield to the SPI background lines. This study will able us then to determinate for each line of astrophysical interest which passive materials should be avoided while conceiving a γ-ray spectrometer, as a potential emitter of this line. It has also enabled us to deduce the less noisy material to be choosen when technological trade off occur.