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[Silicon Detectors] [Mechanical Support] [Assembly and Testing] [Future Milestones]
The tracking system was designed to meet several goals: momentum measurement with the introduction of a solenoidal field; good electron identification and $e/\pi$ rejection (to compensate for the loss of the TRD); tracking over a large range in pseudorapidity ($\eta$~$\approx$~$\pm$3); secondary vertex measurement for identification of b-jets from top and for b physics; second level tracking trigger; fast detector response to enable operation with a bunch crossing time of 132 ns; and radiation hardness. The silicon tracker is the high resolution part of the tracking system and is the first set of detectors encoutered by particles from the collision.
Several of the Run II Collider machine parameters have an effect on the silicon design. The luminosity sets a scale for the radiation damage expected over the life of the detector, which in turn dictates the operating temperature (<10 degrees centigrade). The long luminous region length sets the length scale, and motivates our hybrid disk and barrel design. The crossing interval sets the design parameters for the electronics and readout.
Since the Collider interaction point is extended, with a $\sigma_z$ of 25cm, it is difficult to deploy detectors such that the tracks are generally perpendicular to detector surfaces for all $\eta$. This forced us to a hybrid system, with barrel detectors measuring primarily the r-$\phi$ coordinate and disk detectors which measure r-z as well as r-$\phi$. Thus vertices for high $\eta$ particles are reconstructed in three dimensions by the disks, and vertices of particles at small $\eta$ are determined by the barrels.
The interspersed disk and barrel design is shown in Fig. 2. In such a system, the disk separation must be kept small to minimize extrapolation errors. However, each plane of disks also introduces a dead region between the barrels which lowers the overall efficiency of the detector. Thus there is a compromise between vertex resolution at large $\eta$ (~1/disk spacing) and efficiency at small values of $\eta$. This design clearly puts a premium on minimizing the gap between barrel sections.
We have chosen the following overall geometry:
Table 1 shows the numbers of detectors and the geometric parameters of the tracker. The 12cm long barrel segments are separated by 8mm gaps containing F~disks at z=6.4cm, 19.2cm and 32.0cm. Three more F disks are located at each end of the barrel at |z|=44.8cm, 49.8cm and 54.8cm. The H disks are located at |z|=110cm and 120cm.
The barrels and the F disks are based on 50 $\mu$m pitch silicon microstrip detectors, 300$\mu$m thick, providing a spatial resolution of approximately 10 $\mu$m. The small angle stereo design provides good pattern recognition with a resolution in r-z at the vertex of 0.5--1.0~mm, allowing separation of primary vertices due to multiple interactions. The detectors are AC coupled; each strip has an integrated coupling capacitor and a polysilicon bias resistor. This technology has been shown to be sufficiently radiation hard.[2] F disks are made from 12 double-sided detectors which have $\pm15^\circ$ stereo strips.
Results from tests of prototype F disks fabricated at Micron Semiconductor (UK) have shown that these detectors are suitable for use in D0.[3] Leakage currents were <120 nA/cm$^2$; coupling capacitors had a capacitance of >20 pF/cm and a breakdown of >80 V; and the polysilicon resistance was >3.5 M$\Omega$ with strip-to-strip non-uniformity of <3%.
Orders for all barrel detectors have been placed with Micron Semiconductor. The first pilot run (10 detectors) was completed on April 1, 1995 and we are currently evaluating these detectors. In a test beam run planned for April 1995 at TRIUMF we expect to re-verify radiation damage properties of the prototype F disk and barrel detectors.
The SVX II front end readout chips are mounted on a kapton high density circuit (HDI) which is laminated onto a 300 $\mu$m thick beryllium plate and glued to the surface of the detector. The end of the HDI consists of a kapton strip cable which carries signals and bias voltages to the outer radius of the detector (~18 cm) where a connection to a long (~8 m) low mass microstrip cable is made. These cables carry the signals to the port cards located on the D0 support platform.
A prototype HDI has been designed and is in fabrication. However, due to delays in the manufacturing process we are actively seeking an alternate vendor and expect to procure and test the prototype by August 1995.
The mechanical structure must provide a precise and stable support for the individual barrel and disk detectors, provide cooling for the heat generated in the SVX II chips and allow for the necessary cable paths for external connections. In the barrels, the basic mechanical unit is the ladder. Each ladder supports two detectors wire-bonded together, forming a 12 cm long unit with the SVX II readout at one end. A rohacell-carbon fiber support provides extra rigidity to the ladder. The ladders are mounted on beryllium bulkheads, which serve as a support at both ends of the ladder and provide cooling at the readout end by means of an integrated coolant channel.
The F disks are mounted in the 8 mm gap between the barrel segments. In analagy to ladders, disk modules consist of a single F disk detector with SVX II readout at the outer radius. Water cooling is via a beryllium cooling channel which also supports the modules at the outer radius.
The barrels and disks are mounted in a double-walled carbon half-cylinder which acts as a support with zero thermal expansion. The half-cylinder has a length of 2.2 m and an outer radius of 15.3 cm. As detector elements are installed, compensation will be made for the predicted 100 $\mu$m half-cylinder gravitational deflection.
Designs completed in the mechanical systems include: the beryllium bulkhead and fabrication of prototype straight sections to test the cooling channel for leaks; the single-sided ladders, and the cooling system, including finite element analysis. Fabrication of a 10-inch long prototype half-cylinder support is complete.
Barrel and disk modules will undergo thorough testing to avoid using modules with unacceptible fractions of dead channels. This will be done in stages, starting with probe testing of the individual silicon detectors and SVX II chips. Due to the large numbers involved, three sites have been set up with probe station facilities: Fermilab, UC Riverside and Oklahoma University. Further testing of HDI's instrumented with SVX II chips will be performed and, finally, the fully assembled ladders and disk modules will be checked with a 1064 nm wavelength laser, which simulates a charged particle track traversing the silicon.
Design and implementation of all assembly sequences and fixtures has been started. A detailed ladder fabrication sequence has been defined and a prototype ladder assembly fixture has been designed and fabricated.
Some important milestones for the silicon tracker R&D and construction are given below.
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[Silicon Detectors] [Mechanical Support] [Assembly and Testing] [Future Milestones]