«The main role of the Target Tracker of OPERA is to locate the lead/emulsion brick where a neutrino interaction occurred. It will also provide a neutrino interaction trigger for the readout of the whole detector and be used as a calorimeter for the event analysis.
The required high brick finding efficiency puts strong requirements on the Target Tracker spatial resolution. Taking also into account the fact that the replacement of faulty elements of the Target Tracker is extremely difficult, this detector must present a long term stability and reliability. In case of problems on a portion of the tracker, the brick finding efficiency not only of the bricks just in front of this dead surface but on several walls upstream will severely be affected. The surface to be covered being of the order of 3000 m2, a cost effective technology had to be chosen.
The electronic detectors chosen to be placed in the target section of the OPERA detector are scintillator strips, 6.86 m length, 10.6 mm thick, 26.3 mm wide, read using Wave Length Shifting (WLS) fibres and photodetectors placed at both ends of the fibers. The particle detection principle is depicted by Fig. 1 The scintillator strips are obtained by extrusion, with a TiO2 co-extruded reflective coating for better light collection. A 6.86 m long groove, 2.0 mm deep, 1.6 mm wide, in the centre of the scintillator strip, houses the WLS fibre which is glued in the groove using a high transparency glue.
Figure 1: Particle detection principle.
The chosen detector is very reliable due to the robustness of the elements used (plastic scintillator strips, optical fibres, PMT’s). Delicate elements, like electronics and PMT’s are placed in accessible places.»
Source: Technical Design Report (see links below)
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