PhD Thesis Chapter III

Chapter 3

Tracking Elements of the ZEUS Detector

3.1 Introduction

Both of the experiments planned for HERA, ZEUS and H1, have specialized tracking detectors in the forward direction because of the beam asymmetry mentioned previously. At ZEUS, there are four separate tracking chambers. The system as a whole can take measurements of varying degrees of accuracy for tracks with polar angles between 7.5° and 170°.

All of the tracking detectors are wire drift chambers in which the passage of a charged particle leaves a trail of ionization. Anode wires in each chamber are equipped with electronics to readout pulses due to the arrival of charge produced by this ionization.

All of the chambers are designed to operate in a high magnetic field. This causes particle trajectories to bend thus enabling momentum measurement. The tracking detectors differ in their geometry and in gas mixture, field shape and strength depending on their location. In particular, the magnetic field is highly non-uniform in some regions and this has had to be taken into account.

Of the four detectors, the CTD and the Vertex Detector (VXD) are cylindrical while the Forward Detector (FDET) and Rear Tracking Detector (RTD) are planar. The VXD is smaller than the CTD and occupies the space between it and the beam-pipe. The FDET is a composite of two detectors, the Forward Tracking Detector (FTD) and the Transition Radiation Tracker (TRD).

3.2 The Central Tracking Detector (CTD)

3.2.1 Introduction

The CTD has an overall length of 240 cm with inner radius of 16.2 cm and outer radius of 85 cm. However some space must be left inside the chamber to house readout electronics, and allow for cabling and cooling requirements. The sense wires are strung along the 205 cm active length of the chamber between two 20 mm thick aluminum end-plates.

The requirements which the CTD was designed to satisfy are:

  • Event triggering by vertex measurement (i.e. rejection of upstream gas), see section 5.2.
  • High resolution momentum measurement of tracks.
  • Identification of electrons by measurement of energy loss.
  • 3.2.2 Mechanical Construction

    The CTD[17] is radially subdivided into super-layers (SLs) which are numbered from one at the smallest radius to nine at the largest. Each SL thus forms an annular cylinder eight wires thick. There are two types of SL. In the axial SLs, the sense wires run parallel to the z-axis (see figure 3.1). In the stereo layers however a twist of 5° has been introduced corresponding to a two-cell displacement at the end-plates, in order to allow reconstruction of the z-coordinate of the tracks.

    Within a SL, groups of eight sense wires are termed cells. The line of eight wires is at an angle of 45° to a radial line from the center of the chamber. This angle matches the Lorentz angle so as to maximize use of drift space. This geometry is shown in figure 3.2. There are 32 cells in SL1 with more in the outer SLs so that the cell size is roughly constant.

    Cells consist of the eight sense wires plus a variety of other wires which are all designed to supply and shape the electric field within a cell such that electron drift trajectories are uniform. The maximum drift distance is 25.6 mm.

    The uniform electrical field within each cell means that the drift velocity is independent of trajectory within the cell, simplifying reconstruction. It would be helpful if the drift velocity could be fixed such that the maximum drift time was small compared to the beam-crossing interval. This would minimize difficulties in identifying which crossing a particular track is associated with. This would indicate a small cell size, but this would then require a larger number of wires to be readout. So the cell size has been fixed at ca. 2.5 cm which maintains a small probability that two events will overlap in the same cell, satisfies the requirement that a not unreasonably large number of wires must be readout, and produces a maximum drift time of 500 ns.

    3.2.3 Electronic Readout

    There are a total of 4,608 sense wires in the CTD. It is necessary to terminate the wires in the 390 Ω characteristic impedance of the chamber in order to prevent pulses being reflected back into it. At the FTD end, this is done by a resistor network (for those wires not equipped with z-by-timing readout). At the RTD end, it is done by pre-amp. cards[18] which are designed to increase the signal strength.

    The pre-amps are connected to 42 m long high quality coaxial cables which run through the drag-chain. This connects the chamber to the rucksack (section 2.4.4) which houses the majority of the electronics. Post-amp boards amplify the signals. At this point the readout chain splits into two with both parts simultaneously being fed data. Both parts are concerned with coordinate identification in different planes.

    3.2.3.1 R-φ Coordinates

    The pulses from the post-amps are sampled by 8 bit 104 MHz flash analogue-to-digital converters (FADCs). The results are continuously written to 512-location deep pipelines. On-board Digital Signal Processors (DSPs)[19] produce drift times and do pulse height/area analysis. [DSPs are microcomputers providing several MIPS of computing power and suited to high data through-puts.]

    The radius of a hit is defined by the wire number. The  coordinate may then be found from this and the drift time. There is, however, a left-right ambiguity. The drift time may be used to find the distance from the sense wire plane of the ionization causing it but not on which side of the wire it was produced. This has the effect that two sets of hits are found in each cell. Because of the 45° angle of the sense wire planes, one of these sets does not point to the interaction region and can be easily discarded. The design resolution is 100 μm.

    3.2.3.2 Z-coordinate

    There are two methods of measuring the remaining z-coordinate, with differing degrees of precision. All of the sense wires in SL1 and half of those in SL3 and SL5 are instrumented for z-by-timing.[20] Those wires which are instrumented have pre-amps at both ends of the chamber and corresponding post-amps in the rucksack. On a given wire, pulses arrive at different times at the two ends of the chamber depending on where along the wire the ionization was produced. Pre-amplifiers mounted on the chamber drive the signal along coaxial cable to the rucksack where post-amplifiers feed into constant fraction discriminator units. One end of the chamber has an extra 10 ns delay so that the pulses will always arrive in the same order. This enables time-to-amplitude conversion to take place based on charging a capacitor starting from the arrival of the start pulse and ending with the arrival of the stop pulse. The time difference is then proportional to the charge on the capacitor which is sampled by a FADC which has seven bits available to measure the z-coordinate. The design resolution is 3 cm.[21] The FADC data is sent to the pipeline (section 4.3.1), which is read out in the event of a trigger.

    Secondly, the wires in the stereo layers enable a three-dimensional track fitting. The drift times for hits in the axial layers for a given track are clearly independent of its location in z. However, moving in the z-direction, the cells in the corresponding stereo layers appear to rotate. Correlating the shift in r coordinates allows a measurement of the z-coordinate. At present it is likely that this data will only be used in the full event reconstruction, where it should provide a resolution in z of 1.2 mm.

    3.3 Forward Detector (FDET)

    The FDET consists of three FTD sub-chambers with two TRD modules in the gaps between them.

    3.3.1 The Forward Tracking Detector (FTD)

    A drawing of a sub-chamber is shown in figure 3.3.

    Figure 3.3: Sketch of an FTD sub-chamber.

    The FTD is intended to complement the angular coverage of the CTD and is crucial in providing data relating to tracks close to the beam-pipe. Each of the FTD sub-chambers contains three readout planes, each containing more than a thousand wires. The sense wires are parallel to each other within a plane but there is a 60° offset between the planes to permit three dimensional hit location.

    The Siegen group is designing 100 MHz FADC cards similar to those that have been produced for the CTD which will be used to readout the FTD. However, the design here will be simpler.

    3.3.2 The Transition Radiation Detector (TRD)

    Charged particles crossing an interface between materials having different dielectric properties will lose energy by emission of photons. These photons will in turn transfer energies to atomic electrons via excitation and ionization processes. In this way, if the original particle was sufficiently energetic, an electromagnetic shower may be built up.

    The TRD relies on this phenomenon. It has two parts. Firstly there is a radiator stack which consists of a nitrogen filled polypropylene fiber mass. Photons are produced here. A second stage is a drift chamber. The photons leave the radiator stack and enter a drift/amplification region. This part of the chamber is filled with xenon and the photons will excite atomic electrons which will cause an avalanche by further interactions. The shower results in an anode pulse which is read out by FADCs as in the CTD. The primary purpose of the TRD is to permit electron tagging. Transition radiation is not produced by π’s with momentum below 40 GeV/c[22] so while π’s and electrons have similar dE/dx characteristics the pulse shapes produced will differ. This should allow the TRD to distinguish between the two with a discrimination factor of 100 (for electrons with energies between 1 GeV and 30 GeV).

    3.4 The Rear Tracking Detector (RTD)

    The RTD is basically identical to one FTD sub-chamber but of slightly smaller size. Its sensitive volume extends down to 10° in the rear direction.

    3.5 The Vertex Detector (VXD)

    In order to obtain improved resolution, the design of the VXD progressed assuming the use of a ‘slow’ gas; dimethyl ether was chosen. This then meant that the cell size would be smaller than in the CTD in order to restrict drift times to a reasonable length. Constraining the number of readout channels led to a maximum drift time of 500 ns over a distance of no more than 3.6 mm. There are twelve sense wires at 3 mm intervals in a VXD cell.

    Taken in conjunction with the CTD, the VXD will improve the resolution with which tracks coming from the interaction region may be measured. The design goal is to improve the resolution on the impact parameter to 50 μm or better. This enhances the prospects of identifying particles with short lifetimes which decay before they leave the interaction region. If this occurs, the VXD may be able to separate tracks coming from the interaction in which the short-lived particle was created, and those coming from the point at which it decayed. There will be no z information from the VXD.

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