CERN Accelerating science

Projectile Spectator Detector (PSD)

The purpose of the Projectile Spectator Detector (PSD) is the measurement of projectile spectator energy in nucleus-nucleus collisions. The PSD is used to select central (with a small number of projectile spectators) collisions at the trigger level. Moreover, the precise event-by-event measurement of the energy carried by projectile spectators enables the extraction of the number of interacting nucleons from the projectile with the precision of one nucleon. The high energy resolution of the PSD is important for the study of fluctuations in nucleus-nucleus collisions which are expected to be sensitive to properties of the phase transition between the quark-gluon plasma and hadron-resonance matter. Namely, the PSD provides the precise control over fluctuations caused by the variation of the number of interacting nucleons and thus excludes the "trivial" fluctuations caused by variation of the collision geometry. The PSD calorimeter consists of 44 modules which cover a transverse area of 120x120cm2. The central part of the PSD consists of 16 small modules with transverse dimension of 10x10cm2 and weight of 120kg each. Such fine transverse segmentation decreases the spectator occupancy in one module and improves the reconstruction of the reaction plane. The outer part of the PSD consists of 28 large 20x20cm2 modules with a weight of 500kg each.

Each module, schematically shown in Figure (center), consists of 60 pairs of alternating lead plates and scintillator tiles with 16mm and 4mm thickness, respectively. The stack of plates is tied together with 0.5mm thick steel tape and placed in a box made of 0.5mm thick steel. Steel tape and box are spot-welded together providing appropriate mechanical rigidity. The full length of the modules corresponds to 5.7 nuclear interaction lengths.