The mosaic of the HCAL layers exhibits a hundred 3 × 3 cm2 tiles in the center, surrounded by a large area covered with 6 × 6 cm2 tiles and finally enclosed by a strip of 12 × 12 cm2 tiles. These nearly 8000 tiles in total are read out individually by wavelength-shifting fibers which illuminate small silicon photomultipliers mounted on each tile and insensitive to large magnetic fields.
