Experiment Proposal - opera - Infn

Figure 56: Magnetic field distribution at the center of the iron plates (left) and at the connection with
the top flux return iron (right).
4.5.2 Inner Trackers
RPC’s, shown schematically in Fig. 57, are an optimal choice for the instrumentation of the muon
spectrometer given their high intrinsic and geometrical efficiency, the low cost, the robustness and the
ease of segmentation with the desired shape.
We plan to adopt the technology developed by R. Santonico et al. [61, 62], which is used in various
accelerator experiments [63], in cosmic rays physics [64] and which is foreseen for the first-level muon
trigger systems of the ATLAS and CMS experiments [65]. With a different technology, they are envisaged
for the MONOLITH calorimeter [33] and for the ALICE time of flight detector [66]. The knowledge
gathered in several years of R&D has already been transferred to industry and the production capabilities
are well-matched to the large detector area required by OPERA.
As shown in Fig. 57, the electrode plates are made of 2 mm thick Bakelite (phenolic polymer) with
a volume resitivity of 10 11 − 10 12 Ω − cm, painted on the external surfaces with graphite of high surface
resistivity (100 kΩ/square) and covered by two 300 µm mylar insulating films. The two graphite layers
are connected to high voltage (∼ 8 kV ) and ground, respectively. Gap surfaces are treated with linseed
oil, which strongly enhances the RPC performance (low noise, high efficiency). Planarity of the two
electrodes is assured by Lexan spacers located on a 10 cm-square grid in the sensitive volume. The
active area is filled with an argon-based gas mixture at atmospheric pressure.
A charged particle crossing a RPC produces a quenched spark which induces signals on external
pickup electrodes. The induced pulses are collected on two pickup planes, with a pitch of 30 mm and
2 mm of separation, made of copper strips glued to plastic foils located on either side of the chamber.
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