Silicon Tracker - Barrel part
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.