Impact of beam-pipe geometry on radiation background in CMS

The importance of the inner experimental beam pipe ( z<10,m) for the radiation background in the CMS endcap muon system is demonstrated. Several simulation studies of idealized and realistic beam pipe designs are presented in order to underline the importance of an optimized geometry. The new results verify the known fact that a conical structure has significant advantages. Furthermore it is shown that, unless alignment errors become really excessive, aluminium can be replaced by stainless steel in a fully conical design. An optimum design is argued to be one with a beryllium central section, followed by an aluminium -- or possibly a tainless steel -- cone along eta=4.9 up to a minimized flange. There after the cone, incorporating the necessary bellows, could continue as stainless steel.