Experiment Proposal - opera - Infn

4 The detector
4.1 Detector configuration and choice of baseline options
The Progress Report [6] submitted in 1999 to the Scientific Committees outlined the design of an experiment
sensitive to ν τ appearance in the parameter region indicated by the atmospheric neutrino signal.
Different detector options fulfilling the physics requirements were considered. Our main efforts have since
then been devoted to the definition of a baseline experiment design with cost effective solutions and a
reduced need for R&D studies.
In particular, two different kinds of brick structures were envisaged in [6]: the so-called spacer and
compact bricks. The main features of spacer bricks can be found in Section 9 and in [3–6]. Their cell
structure includes a low density spacer of 3 mm thickness. The low density material minimises the
probability of reinteractions allowing to reduce the corresponding backgrounds.
On the other hand, compact bricks have the advantage of a denser brick structure, optimal for the
identification of the τ → e decay mode and for the measurement of charged particle momentum by the
multiple scattering method. Because of this capability, the hadron reinteraction background in compact
bricks can be suppressed by kinematical cuts. For a given emulsion surface, compact bricks also provide
a larger target mass and their mechanical rigidity ensures a better parallelism of the emulsion films.
The above arguments, together with the existing experience with similar detectors (DONUT), leads
us to propose as a baseline option a setup made up of three compact brick supermodules. A total lead
mass of about 2 kton can be obtained with a surface of emulsion films similar to what foreseen in [6].
Baseline options have also been identified for the magnet (dipole), for the spectrometer Inner Trackers
(RPC detectors), for the spectrometer Precision Trackers (drift tubes) and for the Target Trackers (scintillator
strips). In all cases, the decision has been driven by the physics requirements, by consideration
of cost effectiveness and by the needs of large scale production and reliable long term operation.
4.2 The ECC bricks
4.2.1 ECC brick structure
About 235000 ECC bricks compose the OPERA detector. A schematic drawing of a brick is shown in
Fig. 21. Its structure is obtained by stacking 56 1 mm thick lead plates interleaved with thin emulsion
films. Each film has 50 µm thick emulsion layers on both sides of a 200 µm thick plastic base. An extra
emulsion film in front of the first lead plate helps in connecting tracks from the upstream wall. Another
film, called Special Sheet (SS) is placed at the most downstream part, separated from the 56 th film by a
2 mm thick plastic plate. This allows to preserve the τ decay detection efficiency for events occurring in
the last lead plate.
The brick-stack is kept together by the so-called origami packing method, a technique used in
previous emulsion experiments. A photograph of a prototype brick packed with origami is shown in
Fig. 22. The packing material is an aluminum-coated paper closed under vacuum by thermo-sealing.
The paper quality is selected in order to avoid chemical contamination which could affect the emulsion
sensitivity.
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