Produce a multi-slit collimator

Hello everyone,
I’m quite new to Geant4.
Could someone please help me fix this problem?
I am simulating a multi-slit collimator. The collimator was a brass cylinder (radius 4.5 cm, thickness 6.5 cm) with five vertical slits (400 micrometer * 2 cm) spaced apart at a center to center(c-t-c) distance of 4 mm. The slits were parallel, i.e. the c-t-c distance was the same at the entrance and exit.
But unfortunately, it doesn’t work.
Thanks for any help!