This volume consists of the array of 122 tiles making up the spherical part of the acrylic vessel:-
In the standard code the tiles are switched off. They can be turned on by setting the enable flag (word 1) of GEDS 201 to 1. With the tiles on then the standard, simple acrylic vessel volume IDRG_ACRV becomes the `glue' with the 122 tiles embedded in it. There is a command file GEOM_ACRYLIC_TILES.CMD which switches on the tiles and changes the media of the glue to poorer quality acrylic, thus ensuring different optical properties for the tiles and glue. The tiles only cover the spherical portion of the acrylic vessel, and are not implemented to describe the chimney.