In the drawing below, a blue line is drawn at angle y=1, along the edge of
layers 2,3 and 4 of the barrel. The first two silicon endcaps are reduced to
start 1 cm
away from this line, in order to allow for support structures, cooling
services and power and signal cables. Also in blue are lines indicating the
central arm acceptance. On the bottom half, red lines indicate the muon arm
acceptance for an 'interaction diamond' of z=+-10 cm.
