NUCLEAR INSTRUMENTS Nuclear Instruments and Methods in Physics Research A309 (1991) 77-100 & METHODS North-Holland IN PHYSICS RESEARCH Section A Design and construction of the ZEUS barrel calorimeter M. Derrick, D. Gacek, N. Hill, B . Musgrave, R . Noland, E. Petereit, J. Repond, R. Stanek and K. Sugano ' Argonne National Laboratory, Argonne, IL 60439, USA Received 6 June 1991 We present the mechanical design and construction techniques used in building the barrel calorimeter for the ZEUS detector . The latter is currently under construction for use at the HERA electron-proton colliding beam facility . The calorimeter consists of 32 wedge shaped modules with approximate dimensions of 3 x 0.5 X 1 .7 m3. The modules use alternate layers of depleted uranium and scintillator with one radiation length sampling . The light is collected via wavelength shifter plates placed on the sloping sides of the modules and read out by photomultiplier tubes located at the outer radius . The unit cell dimensions result in a ratio elh = 1 which yields an optimal energy resolution for hadronic jets . The placing of the structural components and the arrangement of the cracks between modules were chosen to maximize the uniformity of the response . Details of the construction and assembly effort needed to realize the total calorimeter are given. We finally describe the procedures used for transporting the completed modules to DESY and to install them on ZEUS . 1. Introduction detection. To achieve these goals the following major components are provided : a vertex chamber, a central The HERA electron-proton colliding beam facility tracking system with capability of measuring dEldx, a under construction at the DESY laboratory in Ham- forward tracking system including transition radiation burg, Germany, will start operation in the fall of 1991 . detectors, a superconducting solenoid, a high resolu- -2 The design luminosity, -"= 16 X 10 3° cm s-t , is tion calorimeter, an iron backing calorimeter and a expected to be achieved after one or two years of muon detection system that utilizes the toroidally mag- running. The facility will extend deep inelastic scatter- netized return yoke in the central region and addi- ing studies into the momentum transfer region above tional toroids in the forward direction. 10 4 GeV2 and give rise to events with maximum labo- The design emphasizes precision energy measure- ratory energies of the current jet and of the electron ments for hadronic jets, since accurate measurement of from 30 to 800 GeV, depending on the scattering such jets is crucial to the determination of the proton angle. Two general purpose detectors are currently structure functions extracted from both charged and under construction to study electron-proton collisions neutral current events . Over almost the entire kinemat- at HERA : H1 and ZEUS. The ZEUS detector [11 has ical range the precision of measurement of the struc- been built by an international collaboration involving ture functions is dictated by the resolution of the 46 institutions from 10 different countries. The techni- energy measurement of the hadronic jet. cal proposal was submitted in March 1986 and ap- In this article we describe in detail the design and proved in November of the same year . construction of the central part of the high resolution Fig. 1 shows an overview of the ZEUS detector . The calorimeter, called the barrel calorimeter (BCAL). We asymmetry in the beam energies leads to an asymmet- also discuss transportation of the modules between the ric design with additional tracking and a deeper construction sites and DESY and the installation on calorimeter in the direction of the proton beam . The the ZEUS detector. main features of the detector include charged particle tracking over almost the full solid angle, hermetic, high-resolution calorimetry, excellent lepton identifica- 2. The high resolution calorimeter tion, leading proton and forward electron and photon The calorimeter for the ZEUS detector was de- t Now at Institute for Particle Physics, University of Califor- signed to give the best possible resolution in the meas- nia, Santa Cruz, CA 95064, USA. urement of hadronic jets, to present a homogeneous 0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . All rights reserved 78 M Derrick et al. / The ZEUS barrel calorimeter response over most of the solid angle and to have a hadronic response can be enhanced by efficient detec- good resolution for electromagnetic showers. The tion of the neutron component of hadronic showers as calorimeter consists of two parts: a high resolution well as detection of -y-rays coming from nuclear deexci- inner section and a lower resolution backing calorime- tation, while the electromagnetic component can be ter. The latter is provided by the iron return yoke for suppressed by choosing a high-Z material as radiator. the central magnetic field segmented into 73 mm thick At the time of the design of the ZEUS detector, a plates and instrumented with proportional tubes, see sampling calorimeter using depleted uranium (DU) as fig. 1. The purpose of the backing calorimeter is pri- absorber and plastic scintillator as active material was marily to tag jets with substantial leakage out of the the only technology known to both achieve elh = 1, high resolution calorimeter. The energy resolution is within a few percent and to have an acceptable resolu- QF , - 100%/ E[GeV] and the 10 layers of iron in tion for electromagnetic showers. the barrel region correspond to a thickness of 4.3A . The unit cell is about 8 mm and contains 3 .3 mm In order to achieve the best energy resolution in a DU, which corresponds to one radiation length (1X,,), calorimeter, the response to the electromagnetic and and 2.6 mm of scintillator . An air gap is provided to the hadronic parts of jets must be equal, expressed as ensure that the scintillator is not subject to pressure elh = 1 . Typically, calorimeters of existing collider de- caused by irregularities in the manufacture of the DU tectors are undercompensating, i.e. the electromag- plates. This particular choice of parameters yields a jet netic response is substantially larger than the hadronic energy resolution o,,,,,, = 35%/ E[GeV] and a reso- response, e.g. e/h > 1 .5 . Compensation is achieved lution for electromagnetic showers o-E , = 18%/ both by enhancement of the hadronic response and by E[GeV] . These resolutions were demonstrated in suppression of the electromagnetic response [2]. The test beam measurements at CERN [3] and corrobo- FMUON SPECTROMETER Fig. 1 . Cross section in the plane parallel to the beam axis through the ZEUS detector showing its major components. M. Derrick et al. / The ZEUS barrel calorimeter 79 36 .7° FCAL BCAL LI ME I OLENOID CENTRAL ~ RACKING _e - P 30 GeV 820 GeV _~ FORWARD o TRACKING REAR TRACKING _ ~í////%%///////%//N//llllllllllllll\\\\\\\\\\\~\ I 1 v Fig. 2. Schematic of the high resolution calorimeter. rated by Monte Carlo calculations based on the HER- calorimeter, thus ensuring a homogeneous response MES [41 and EGS4 [51 shower simulation codes. The over a large solid angle and for the entire depth of the cell structure is common to the entire high resolution calorimeter. Fig. 3. Cross section through the barrel calorimeter. The electron beam direction is into the figure. 80 M. Derrick et al. / The ZEUS barrel calorimeter The light produced in the scintillator is read out by 3. Design of the barrel calorimeter the standard technique of wavelength shifting and guiding to the back of the calorimeter using wavelength 3. I. General description shifter plates. The latter are located on the two sloping sides of a module. The BCAL covers the region between 0 = 36.7 ° Fig. 2 shows a schematic of the high resolution and 129.1' in polar angle and 360' in azimuth. It calorimeter. It is divided into three parts: consists of 32 wedge shaped modules each spanning - the forward calorimeter (FCAL) covering polar an- 11 .25' in azimuth, as shown in fig. 3. The modules gles from 0 = 2.2 ° to 39.9 ° ; extend from an inner radius R i = 1232 mm to an outer - the barrel calorimeter (BCAL) covering polar angles radius Ro = 2912 mm from the beam axis. Each mod- from 9 = 36.7 ° to 129.1 ° ; ule is rotated by 2.5 ° clockwise in the azimuthal plane - the rear calorimeter (RCAL) covering polar angles around an axis parallel to the beam axis and located at from 6 = 128 .1 ° to 176.5 ° . a radius of 2309 mm. This rotation ensures that the The asymmetry along the beam axis reflects the kine- wavelength shifter plates do not point to the beam axis, matics of the electron-proton collisions at HERA. The thus preventing photons from escaping undetected in depth is chosen to contain at least 95% of the energy the gap between modules and so compromising the for 90% of the hadronic jets with the highest energy measurements of jet energies and missing transverse kinematically allowed. This energy varies from 800 energy. GeV in the forward direction to 30 GeV in the back- Each module weighs approximatively 10 tons for a ward direction relative to the proton beam. In the total weight of 320 tons for the barrel. In the overall BCAL the maximum jet energies vary from 200 GeV in BCAL structure the modules are attached at their the forward direction to 40 GeV at the backward front and rear to two large aluminum discs, referred to angles . These depths correspond to 7 interaction as the spokeswheels, whose thickness varies between lengths, A, for the FCAL and 5A (4A) for the BCAL 50 and 130 mm, depending on the radius from the (RCAL) at normal incidence. beam axis, as seen in fig .