SPIinstrument on INTEGRAL, schematic (left) and as a cut-away showing components more clearly (right). Its key elements are the 19-element Ge detector camera mounted in a cryostat for operations at ~80 K, a mask with tungsten elements blocking parts of the field of view for about half of the detectors, and an anti-coincidence detector system (ACS) enclosing the entire instrument and vetoingevents from prompt cosmic-ray interactions, using BGO scintillators and a plastic scintillator plate (PSAC), each with photomultiplier tubes for measuring the scintillation light.