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Central Tracking System

Introduction     Central Tracking System   Calorimeter    Muon System 

The Central Tracking System

You can use the exhibit number (circled on the card next to the piece) and locate the same number on these web pages.

Exhibit numbers for the Central Tracking System are the following: 20, 25, 22, 23, 26, 33, 34, 10, 32, 31.

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The Central Tracking System consists of the silicon microstrip tracker (SMT) and the central fiber tracker (CFT) surrounded by a solenoidal magnet.

Both the SMT and CFT provide tracking information to the trigger.

You can find the information belonging to the tracker exhibit pieces further down on this page.

First, some pictures of the tracker installation, and a few overview drawings.

CFT on crane at D0    

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CFT just outside D0 Magnet bore

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CFT partly inside D0 Magnet bore

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Silicon Microstrip Tracker (SMT)

 The point where the beams collide is surrounded by "tracking detectors" to record the tracks (trajectories) of the high energy particles produced in the collision. The measurements closest to the collision are made using silicon detectors.

Silicon detectors are basically large thin diodes that are biased in the reverse (non-conducting) direction. A charged particle traversing the diode layer generates electron hole pairs which are separated by the electric field and collected as signal charge on thin metal film electrodes directly deposited onto the silicon. The signal charges are routed to readout chips adjacent to the silicon sensor, where they are stored, amplified and digitized.

The SMT is composed of three different geometries: Barrels, F Disks, and H Disks. There are four H Disks, two at each end; twelve F Disks and six Barrels intermixed in the center. Each one of these top level assemblies is then further broken down into smaller components. The H Disks and F Disks are reduced to a collection of twenty - four H Wedges and twelve F Wedges respectively. The Barrels are broken down into a number of differently sized rectangular detectors ('ladders') based on their radial distance from the beam line. This variety of components made the production of the detector challenging. Despite many difficulties, the SMT was successfully assembled and installed at the start of Tevatron Run II. The detector has worked well since then. However, due to radiation damage, the detector was aging and in order to compensate for this anticipated performance degradation and to improve the momentum resolution, a new inner barrel layer, called Layer 0 was installed in 2006. The SMT detector has been part of the D0 readout chain since the beginning of data taking in Run II, contributing to the almost three hundred interesting physics publications by the D0 collaboration.

 

 

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         H - Disk (20)

 

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- were designed to improve momentum resolution for tracks.

 

- provides tracking information;

- are made of 24 pairs of single - sided detectors, each detector is read out by 6 chips;

 

 Layer 0 (25) provides tracking information from two layers of sensors, which are mounted with center lines at a radial distance of 16.1 mm and 17.6 mm respectively from the beam axis.

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The sensors and readout electronics are mounted on a specially designed and fabricated carbon fiber structure that includes cooling for sensor and readout electronics. The structure has a thin polyimide circuit bonded to it so that the circuit couples electrically to the carbon fiber allowing the support structure to be used both for detector grounding and a low impedance connection between the remotely mounted hybrids and the sensors.

​                                                  Example of low mass flex cable (22, 23)

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Carbon Fiber (26) - a material that is very resilient against elongation. It is used to build very stiff support structures, stronger than any built from metal at the same weight.
Central Tracker Mockup (33)

      Forward Drift Chamber - a Part of the Previous ('Run I') Tracker (34)
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Central Fiber Tracker (CFT)

Just outside the silicon, D0 has an outer tracker made using scintillating fibers, which produce photons of light when a particle passes through. The whole tracker is immersed in a powerful magnetic field so the particle tracks are curved; from the curvature, the momentum can be deduced.

Central Fiber Tracker (CFT), Central and Forward Preshower (CPS, FPS) detectors utilize a similar readout: Particles crossing scintillating fibers or triangle scintillators generate light which propagates to solid state diodes, Visible Light Photon Counters (VLPCs). Long clear fibers (waveguides) transport light to the VLPCs. Analog Front End (AFE) boards amplify and digitize the signal. The raw data is buffered in the VME Readout buffer (VRB) and then send to the next level of processing.

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Long clear fibers (10) (waveguides) transport light to VLPCs. 

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A VLPC is a solid state photo-detector with a very high efficiency for converting photons into electric pulses ('quantum efficiency', QE). D0 uses VLPCs with eight input pixels 1 mm in diameter each. VLPCs provide high gain of 25,000 – 60,000 electrons per detected photon.

A solid state device, the VLPC relies on avalanche multiplication of charge carriers (holes, for the D0 devices) across a very small band gap of about 50mV. The QE is larger than 80 %, the devices have a large gain of typically 40,000, and very low dark noise (the electrical signal you see when there is no light) rates, all at a bias of less than 10V. But because of the very small band gap, it does take some special care - VLPCs have to operate at about 9K! (Otherwise too many charge carriers would be liberated by their thermal energy, and start 'dark noise' avalanches.)
In fact, the temperature has to be kept constant to about 50mK and the bias has to be kept within about 30mV of optimal.

The AFE (Analog Front End) (32) is the board that interfaces with the VLPCs. Here is how the basic system works: VLPCs convert the ~8 detected photons from a hit in the fiber tracker into charge- about 50fC. The AFE boards, each of which takes care of 512 VLPCs, take that signal, amplify it, send discriminator data (a simple yes/no) to the trigger system every crossing while keeping analog information about the hit in a pipeline for later digitization.

The digitization happens when the trigger system issues a L1 accept. A little while later the digitized data is available for use in the L3 trigger decision. To give you a scale of the whole system, there are about 100,000 channels in total, with 200 of these AFE boards.

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 Fiber tracker test cryostat

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      VLPC Cassette and Cryostat (31) - VLPC cassettes provide mechanical support, optical alignment, and appropriate operating services for proper operation and readout of the VLPCs. The lower portion of VLPC cassettes is immersed in cold helium gas, while the upper portion supports a pair of AFE boards. During the operational phase of the experiment the cryostat was never warmed up above 60 K.

 

The Analog Front End (AFE) is the ONLY electronics that interfaces to the VLPC, so it must handle not only the signals, but also the "care and feeding" of the VLPCs themselves. The VLPCs are grouped into "modules" of 64 units. These are specifically picked to have the same operating parameters – the same gain at the same bias voltage. Each group of 64 has mounted, in good thermal contact, one 100 Ohm carbon resistor and two metal film heater resistors. The carbon resistor is used as a temperature sensor- at 9K. Carbon resistors have a sensitivity of about 30 Ohm/K at this temperature. One of the metal film resistors is used as a heater (with the second being a backup). All this is run by a closed loop control system implemented in a small microcontroller on the AFE. Each module of 64 VLPCs is also supplied with a calibrated bias voltage by the AFE and the current drawn by this bias is also accurately monitored.

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Making a CFT cylindrical layer
​ ​Assembling the CFT cylinders
​ ​Inserting the tracker into the solenoid