The following event display shows a b candidate reconstructed in the barrel part of the old vertex detector.
In the design of the barrel upgrade two decisions were made for practicality. The already double-sided outer and closer layer in the VD should be reused and a minimum number of new sensor designs adding mask costs to the upgrade should be needed. The geometrical design with three concentric cylinders were kept basically unchanged but the barrel length was doubled from 24 to 48 cm. The Closer layer lies at a radius of 63 mm, the Inner at 90 mm and the Outer at 109 mm. The layers involve modules which consist of two electrically independent half-modules joined together in the center.
The long barrel requires good stability from the modules and mechanics. The mechanical stability is improved by a carbon-fibre/honeycomb anti-twist cylinder between the inner and outer layers. The module stability is improved with kevlar omega-shaped beams with carbon-carbon backing. Kevlar was chosenbecause the beam had to be glued onto the active surface of the detectors. The new long barrel modules gain by a better performing front-end TRIPLEX chip which is based essentially on the same architecture as the MX6 chip.
The Outer layer is completely new. Since the amount of material in the outer layer hardly affects the vertexing capaciy of the barrel, the half modules are constructed of four pairs of 300 micron thick n-bulk sensors glued back to back and joined to a double sided hybrid at one end. The p-implanted strips on the sensors on the top side of the module are orthogonal to the ones on the bottom side. To avoid mechanical complication and degredation of the physics performance by having electronics in the active volume, the diodes orthogonal to the beam direction are routed out to the electronics placed on the mechanical support structure. A double metal layer structure processed on the sensors was chosen as the easiest and most elegant solution for routing the strips.
The Inner layer is recycled from the double sided outer layer of the old VD. The modules are extended with single sided sensors. The central sensors of the Inner layer are the only part of the silicon tracker which does not supply three dimensional information.
The only layer that has not undergone any changes is the double sided closer layer which was already built for a planned future upgrade.
The barrel part of the upgrade was completed in 1996. It is more than 99% functional.