The ZEUS Detector

Artist's view of the ZEUS detector showing its internal structure.

The goal of the ZEUS detector is to determine with high precision the energies, directions and nature of single particlesand particle jetscreated in the interactions.

The ZEUS detector is located in the South hall of HERA Its dimensions are 12 m x 10 m x 19 m and Its total welght is 3600 tons.

The heart of the ZEUS detector is the uranium scintillator calorimeter (CAL) which measures energies and directjons of particles and particle jets with high precision. This hermetically encloses the tracking detectors which measure the tracks of charged particles using wire chambers and which consist of: a vertex detector (VXD), the central drift chamber (CTD), forward (FTD) and backward (RTD) drift chambers and in the forward direction a transition radiation detector (TRD) to identify high energy electrons These chambers are surrounded by a thin superconducting solenoid coll producing an axial magnetc field of 1.8 Tesla for determining the momenta of charged partlcles from track curvature. Energy not fully absorbed in the uranlum calorl- meter is measured in the backing calorimeter (BAC) which uses the 7.3 cm thick iron plates of the return yoke as absorber and proportional tube chambers for observing penetrating particles.

Particles which are not absorbed in the substantial material of the uranium scintillator and backing calorimeter are typically identified as muons. Their tracks are measured before and after the iron yoke by limited streamer tube chambers (MUON). The muon momenta are determined by the deflections of their paths by the solenoid and by the iron yoke which is magnetized toroidally up to 1.6Tesla by coppercoils. In the forward direction magnetized iron toroids instrumented with limited streamer tube and drift chambers measure very energetic muons (up to 150 GeV/c).

An iron wall equipped with two layers of scintillation counters (VETOWALL) is placed near the tunnel exit for detection of background particles produced upstream by the proton beam.

In the very forward direction the leading proton spectrometer is installed in the beam line to measure forward scattered protons.

In the direction of the electron beam photons and electrons are detected in the luminosity monitor.

The uranium scintillator calorimeter (CAL)

View of ZEUS with the iron yokes retracted; in the foreground the forward calorimeter halves in open position.

The momenta of charged particles are measured by the central tracking detector in the magnetic field of the solenoid. The solenoid is 280 cm long and has an inner radius of 86 cm . Its total material thickness was minimized to be about 0.9 radiation lengths X0 (equivalent to 8 cm of aluminum) in order to keep the loss of energy of electrons, photons and hadrons small. The coil is supported by two "wheels" 6 min diameter and 5 cm thick aluminum together with the central calorimeter.

The influence of the magnetic field of the thin solenoid on the HERA beam dynamics is compensated by a super conducting solenoid (compensator) with a length of 120 cm and an inner radius of 18 cm creating a field of 5 Tesla.

The fully equipped central tracking detector (CTD) hanging at the crane before installation in the ZEUS detector.

The central tracking detector (CTD) measures the trajectories of charged particles. It is built as a cylindrical drift chamber with a length of 241 cm and an inner/outer radius of 16 cm / 85 cm.

The principle of operation of the drift chamber is explained in the following figure. The signal wires (+) and potential wires (-) have opposite electrical potentials. A charged particle traversing the drift cell ionizes the gas of the chamber releasing atomic electrons. These d rift at known speed to the positively charged signal wire; the positively charged ions move more slowly to the field wires. By measuring the arrival time of the electrons, the track position in the cell is obtained.

The inner structure of the central tracking detector consists of 9 superlayers, each cell with 8 sense wires, and a total of 576 drift cells. This gives altogether 4608 sense wires and 19584 field wires. The chamber is presently operated with a gas mixture of Ar:CO2:C2H6 (90:8:2). Its spatial resolution is 120 - 130 um in the plane perpendicular to the beam.

The vertex detector (VXD) is positioned inside the central tracking detector in order to detect short-lived particles by their decays. It is designed to provide spatial resolution of 35 - 70 um.

The fully equipped forward tracking (FTD) andtransition radiation detector (TRD) before installation in the ZEUS detector.

The forward tracking detector (FTD) extends the tracking region in the forward direction where particle rates are particularly large. It consists of 3 planar drift chambers each with 3 layers of drift cells. The spatial resolution is 120 - 130 um. The 3 chambers are sepa- rated by two gaps of 21cm, leaving space for the transition radiation detector (TRD) which identifies electrons especially in jets. The probability for misidentifying pions as electrons P (Pi -> e) is about 2 x 10E-2 at 2 GeV/c.

The rear tracking detector (RTD) consists of a planar drift chamber of similar construction as the FTD's covering an angle of 160 < Theta < 170 in the backward direction.

Installation of the forward muon spectrometer.

The forward muon detector (FMUON) located in front of the magnet yoke Idenbfles muons In the forward hemlsphere. It conslsts of two magnebzed Iron torolds surrounded by planar tracking chambers. The two colourful external iron toroids 6 m in diameter with a total depth of 0.9 m and a weight of about 200 tons are magnetized by 8 normal conducting coils per toroid to an internal field of 1.7 Tesla. Together with the magnetized iron yoke (1.6 T) they provide good muon identification and the bending power for a precise momentum measurement.

The barrel and rear muon detectors (BMUON, RMUON) detect muons and measure their positions and directions in layers of streamer tube chambers inside and outside of the iron yoke. Some of these chambers with sizes of up to 4 m x 11 m are amongst the largest chambers ever built.

Some of the readout components for the tracking detectors are still being installed. The instrumentation at startup was adequate to unambiguously identify events of interest. This capability will, however, continue to improve as the luminosity of HERA improves.

The leading proton spectrometer measures forward scattered protons. It consists of 6 detector stations in the proton ring located over a distance of 90 m. These stations have high resolution silicon strip detectors with a spatial resolution of +/-25 um leading to a momentum resolution for scattered protons of less than 1%. The total number of readout channels is about 50 000.

The luminosity L is determined in the luminosity monitor by observing the bremsstrahlung reaction e p -> e p gammma for which the cross section is well known. L is a characteristic parameter of HERA and determines the rate of events n per second produced for a specific reaction with cross section d, n = L d. The luminosity monitor measures the rate of bremsstrahlung by detecting in coincidence the scattered electron and photon in lead scintillator calorimeters positioned at distances of 35 m and 105 m from the interaction point downstream in electron beam direction.

The ZEUS detector was moved into the interaction region in February 1992. By April 1'st all interlocks were set and operation started. The first ep collisions were produced and detected on May 31'st.

A photoproduction event tagged by the luminosity monitor.