

Chapter 2

Non-Tracking Elements of the ZEUS Detector

2.1 Introduction

The ZEUS detector consists of three main types of detector: those which are sensitive to charged tracks, those that measure energy deposition and those which identify muons. The UK’s major responsibility on ZEUS is the Central tracking Detector (CTD). Because of this and the fact that a large part of the work outlined in this thesis has been connected with tracking components, their description deserves a separate chapter, which follows.

The tracking detectors reside in the inner region close to the interaction point. The remaining major components fall into two functional classes: calorimetry and muon detection. The most significant characteristic common to all groups of components results from the substantial asymmetry in the beam energies at HERA. The forward direction is defined to be the one in which the proton moves. Clearly it is therefore to be expected that the forward hemisphere will be the more active. For this reason components here are more sophisticated than their near region counterparts. An overview of the detector is shown in figure 2.1.

Figure 2.1: Section through the ZEUS detector along the beam-line.

2.2 Calorimetry

2.2.1 Introduction

The purpose of the calorimeter is to investigate jet properties by measuring their energy deposition. The design aims to cover the full solid angle so far as is consistent with the presence of the beam-hole. It allows for the discrimination of jet angles with resolution of better than 10 mrad. Discrimination between hadrons and electrons is foreseen. The resolution on the jet energy should be

The calorimeter at ZEUS has been designed to be ‘compensating’ i.e. it will give the same response per unit energy irrespective of whether the depositing particle is electromagnetic or hadronic. This reduces the systematic error in the energy measurement, as can be seen from the following example. In a given event π0 decay leads predominantly to electromagnetic showers via π0 → 2γ whereas charged π’s will give a hadronic deposition. So in an uncompensated calorimeter the measurement of a set of events with the same energy would depend on the ratio of charged to uncharged π’s in the events.

ZEUS has adopted a compensating calorimeter of depleted uranium/scintillator design. Here the high-Z absorber plates are interleaved with plastic scintillator tiles which are read out by photomultiplier tubes. Achievement of compensation requires careful consideration of the layer thicknesses because of the different cross-sections for the various processes via which hadronic and electromagnetic particles lose energy.

The calorimeter is made up of a large number of cells which are of two types: electromagnetic or hadronic. These are referred to as EMCs or HACs. Figure 2.2 shows the arrangement of cells. Electrons are less penetrating than hadrons and will thus predominantly interact in the EMCs which are the first part of the calorimeter to be encountered by particles emanating from the interaction region. There are two layers of HACs behind these in the FCAL and the BCAL and one layer in the RCAL.

Figure 2.2: Arrangement of cells in the calorimeter.

2.2.2 Forward, Rear, Barrel Calorimeter (F/R/BCAL)

The calorimeter has three major sections. Their coverage in terms of polar angle and depth is shown in table 2.1. Depth is measured in radiation lengths, X 0 , over which distance the energy of an electron will be reduced by a factor of e. It can be seen that there is some angular overlap between sections.

Each section has one interaction length of EMC at its face closest to the interaction point. Behind that, the layer of HACs varies from ca. 6 λ in the forward direction to ca. 3 λ in the rear. For readout purposes, cells are grouped into ‘towers’. In the FCAL and RCAL these are non-projectives as are the BHAC towers. Tower sizes are shown in table 2.2.

2.2.3 Backing Calorimeter (BAC)

The BAC, together with the iron return yoke, is between the inner and outer muon chambers. It is designed to be complementary to the main calorimeter and the muon chambers. It will allow measurement of late-showering particles and it will provide a muon trigger in the bottom yoke where there will be no muon chambers.

There will be around nine layers of BAC modules depending on the exact location. The layers are made up of either seven or eight tubes which each contain one gold/tungsten wire and use an argon/CO 2 gas mixture. Four modules will be grouped on readout into towers of around 50 cm x 50 cm base and summed in depth. The final position resolution should be 1.3 mm and the design energy resolution for hadrons is σ(E)/E approximately = 100%/√E.

2.2.4 Hadron Electron Separator

Silicon pad detectors will be mounted on ceramic cards which lie a few radiation lengths inside the EMC parts of the calorimeter. The separator is based on diodes with a small (3 cm x 3 cm) active area. This improves segmentation and thus position resolution.

The diodes are operated in depleted mode. The passage of a charged particle creates many charge carriers. The resulting pulse is readout and is of different heights for electrons and hadrons even if they are of the same energy. The ability of the calorimeter as a whole to distinguish electrons is therefore improved. Using only HES data, electron identification efficiency of 90% should be obtainable with only 4% hadronic contamination.

2.3 Muon Detectors

2.3.1 The Forward Muon Detector (FMUON)

This component is based on a toroidal magnet. Its detectors comprise streamer tubes and drift chambers, both of which measure ionization, and a time-of-flight (TOF) plane between the two toroids. It comprises in addition to two ‘wall’ sections (LW1,2) a ‘spectrometer’ section with five detector planes. These five planes are labeled LT1 to LT5. Particles from the interaction region encounter the wall sections first and then the spectrometer. The walls provide overlap of angular coverage with the BMUON. The planar sections are divided into eight sectors in φ. The TOF plane consists of sixteen elements, each are made up of a pair of scintillation counters separated by 10 cm.

This component will provide small angle muon momentum resolution of 20%. This information is essential to complement tracking detector data. The purpose of the TOF plane is to ensure that particles are not associated with an incorrect beam crossing.

2.3.2 Barrel/Rear Muon Detectors (B/RMUO)

There are two sets of eight chambers in the barrel section, which thus has an octagonal cross-section looking down the beam-line in which the inner and outer octagons are separated by the magnetized iron yoke and and the backing calorimeter. The RMUO has two parts, inner and outer, each of which has a depth of one chamber. A chamber consists of two doublets of streamer tubes which are parallel to the beam-line in the barrel and are placed horizontally in the RMUO. These are readout by time-to-digital converters. Analog readout of strips in the orthogonal dimension is available in both cases.

2.4 Other Elements

2.4.1 The Veto-wall (VETO)

This is a large iron wall seven meters in front of the interaction region which has a hole in it through which the proton beam passes. It has hodoscopes on each side consisting of forty-eight individual scintillation counters. There is a ‘halo’ of protons not following the nominal beam trajectory and these may produce highly penetrative muons by collision with machine elements. The main purpose of the veto-wall is to veto these events and thus reduce the rate of spurious triggers in the detector.

2.4.2 The Luminosity Monitor

The measurement of cross-sections is of primary importance at ZEUS. This requires monitoring to arrive at the figure for time integrated luminosity. It is essential also to have the information online so as to be able to optimize the luminosity in the interaction region. In order to do this, the LUMI uses the process of photon emission from the interaction of the two beams ep → epγ. Also, the LUMI will identify photo-production processes by tagging small-angle electrons.

The detector itself has two parts, an electron detector near the electron beam at z = -36 m and a photon (γ) detector around the proton beam at z = – 108 m. The electron detector is a shielded lead/scintillator sandwich. The γ-detector is based on a γ-calorimeter which consists of layers of 1 cm2 silicon pads. A photon causes a shower of electrons through them permitting precise position measurement. A Čerenkov counter to veto electrons is included in the design; a prototype has been built using a 150 cm thickness of polyurethane foam.

2.4.3 Leading Proton Spectrometer (LPS)

It is expected[16] that in 25% of DIS events, the proton will interact diffractively, retaining its identity and losing momentum. If this happens, it may then leave the beam-pipe and be measured by the LPS with efficiency ca. 0.6 at the most favorable momentum.

The LPS will have six detector stations along the proton line between z = +24 m and z = +90 m as shown in figure 2.3. These will be based on turret extensions into which ‘pots’ can be inserted. Pots contain detector elements: their purpose is to allow for precise control of location of the detectors over the last few centimeters close to the beam-pipe.

The first three stations will have a single pot and the last three will be double pot assemblies. The single pots will be horizontal with respect to the beam-line and the double pots will be vertical. The exact location of the detectors is important because it determines the proton phase space acceptance of the LPS.

Each pot will contain a detector element comprising seven planes of silicon micro-strips with differing orientations to provide two-dimensional measurement and will be shaped to fit closely to the beam-pipe. The total area covered by the strips will be 1,560 cm2.

Figure 2.3: The LPS stations along the straight section of the beam-line.

2.4.4 Rucksack

The immediate environment of the detector is hostile to electronics owing to radiation from the uranium in the calorimeter and to proximity to the beam. Also, space is at a premium. Large systems of electronics are required however in order to read out the detector components and implement triggers. These factors have resulted in the inclusion in the design of a ‘rucksack’ which is simply a mobile construction of three floors each of which contains racks and space for the associated requirements in terms of cooling and safety.

The rucksack is connected to the components via a drag-chain which is designed to carry cables for readout. The rucksack must be mobile. This is because the yoke – the large iron clam-shells and the BMUO/BAC – retracts to allow access to the inner detectors. The rucksack moves in the same rails in which the yoke runs.

2.4.5 Solenoid

A magnetic field must be supplied in the region of the tracking detectors so that charged tracks will bend in it and thus their momentum may be measured. A 1.9 m diameter superconducting solenoid is located between the calorimeter and the CTD in order to supply this. It is required to supply a field of 1.8 T within two major design constraints. Firstly, the electron beam trajectory is very sensitive to variation in the B-field and so non-uniformities must be as small as possible. For example, the axis of the field must be centered to within +/- 1 mm.

Secondly, the structure of the solenoid must not present a large amount of material to the passage of electrons and photons as this would introduce an unacceptable systematic error in calorimeter measurements. Therefore the design goal states that at an angle of 90° the solenoid should have a thickness of less than 0.9 radiation lengths.

Next Chapter: http://timlshort.com/2011/03/02/tracking-elements-of-the-zeus-detector