CAD view of an OSM without cover. The Aperture of S1_F is shown (AP). The coloured outlined volume includes the seven surfaces from which the particles are generated to estimate the amount of additional channel counts coming from particles that are incident on the shielding box but outside the aperture. The bottom and top of the OSM were covered by a representative 8 mm thick Al cover for the simulation. No particles were simulated from the rectangular surface behind the sensor stack.