The Passive Mask

Figure: Left: The passive mask of the SPI instrument. The bottom picture indicates the direction of the spacecraft Y and Z axes with respect to the mask. Right: Sketch of the detector plane and spacecraft orientation during observations.

The passive mask is located on top of the SPI instrument, above the plastic scintillator. The purpose of the mask is to code the incident gamma rays in the FoV, providing the instrument with imaging capabilities. The mask consists of a pattern made of hexagonal tungsten blocks, with a 120 rotation symmetry.

The mask is made of 127 elements arranged in a 78 cm diameter circle. Of these elements, 63 are opaque and 64 are transparent. Each opaque element is 30 mm thick and 60 mm flat-to-flat in size. The tungsten elements stop the gamma ray radiation in the range 20 keV to 8 MeV with an absorption efficiency greater than 95% at 1MeV. The holes in the mask have a gamma ray transparency of 60% at 20keV and 80% at 50 keV. The mask is located 171 cm from the detector plane. A sketch of the mask pattern is given in the left panel of Figure .