ZEUS Calorimeter (1) At HERA high energetic electrons (e) collide with high energetic protons (P). The ZEUS detector measures the properties of the particles produced at this collision, by combining information from many subdetectors. One of these is the calorimeter, which measures the energy of the particles by total absorption. The calorimeter forms a substantial part of the whole detector. It is assembled of many similar parts, surrounding the interaction point. The ZEUS calorimeter is a sandwich struc- ture of many thin (~3mm) plates of high density material (depleted uranium) and scintillator material. A particle entering the calorimeter looses most of its energy via collisions in the high density material and produces a shower of particles in the sand- wich structure. The electrically charged particles produce a tiny light signal when traversing the scintillator plates. The strength of this light signal is directly proportional to the energy of the particle, thus if the energy of a particle is twice as big, the lightsignal doubles. In the ZEUS experiment, the interaction point is completely surrounded by the calorimeter. This con- struction is possible by stacking many calorimeter modules besides each other. Therefore the blue scintillator light is absorbed in wavelength shifting material and re-emitted as green light and eventually measured with the help of a photomultiplier. ForwardBarrelRear Calorimeters P e E (:) sum light output scintillator E Light signal electron anode vacuum dynodes Scheme photomultiplier photon amplified signal

ZEUS Calorimeter (2) The calorimeter surrounds the interaction point hermetically (apart from a small hole for beam passage). The forward and rear calorimeter consists of 23 modules, the barrel of 32 modules. The sandwich structure is the same for all modules, while the sizes show small variations. Forward module height 5m, width 0.2m, thickness 2m, weight 11 tonnes Barrel module length 3.2m, thickness 1.2m, weight 10 tonnes e P Isometric, cut-away view of a forward module, indicating the different parts. The readout of the scintillator is highly segmented to provide position information and particle identification. Particles are identified from the signal deposit in a calorimeter. Electrons and photons loose all their energy quickly after entering a calorimeter through the electromagnetic interactions, while a muon only looses a small fraction. Hadrons (protons and pions for example) loose their energy through the strong and electromagnetic interactions and require more material to be absorbed. These properties make it possible to identify particles from their signals in the calorimeter, provided that the read out of the light in the scintillator is properly segmented.

ZEUS Calorimeter (3) Assembly, tests and installation Depleted uranium plate packed in 0.4mm stainless steel Scintillator plates Plates are being stacked with a robot Module ready for transport First module in ZEUS First sign of life on a multi channel analyser To testbeam CERN