Abstract
The advent of thin-gap RPCs, with 1 mm gas gaps instead of 2 mm in the present RPCs, opened the possibility to instrument the inner barrel layer of the ATLAS muon spectrometer where there is very limited amount of space in radial direction from the beam line. The environment is particularly dense in the barrel end-cap transition region. A compact mechanical structure coping with the expected thickness variations of the assembled RPCs is needed to fit into the limited available space. At the same time the mechanical structure must be sufficiently rigid to keep the deformations of the RPC packages within the allowed envelopes. The tight space constraints make it impossible to achieve the required rigidity with thick paper honeycomb plates as used in the present ATLAS RPCs. For the phase I upgrade of the barrel end-cap transition region of the ATLAS muon spectrometer three 1 mm gap RPCs are put into an aluminium frame. In this frame the RPC triplet is compressed with pre-bent 2 mm thick aluminium plates. The rigidity of the frame is achieved by stiffening rods connecting the lateral structure of the frame. The same concept can be used for the phase II upgrade of the inner layer of the muon spectrometer.
Highlights
In the current ATLAS detector the inner layer of the barrel muon spectrometer is only equipped with MDT precision muon chambers
Like in the ATLAS experiment the triplet RPC and the accompanying small-diameter muon drift-tube (sMDT) chamber are mounted on rails with independent supports
The inner layer of the barrel part of the ATLAS muon spectrometer will be instrumented with a triplet of thin-gap RPCs to close acceptance gaps and recuperate the expected efficiency losses of the present RPC system at the HL-LHC
Summary
In the current ATLAS detector the inner layer of the barrel muon spectrometer is only equipped with MDT precision muon chambers. In half of the barrel, in the so-called “small sectors”, these leave no space for additional trigger chambers They will have to be replaced by sMDT precision muon chambers which occupy only half the space in radial direction than the MDT chambers. The liberated space will be equipped with a triplet of thin-gap RPCs. Figure 1 shows a technical drawing of the end of the inner layer of the ATLAS muon spectrometer after the upgrade with a new station of an sMDT chamber and a thin-gap RPC. It illustrates the tight space constraints which require a very compact mechanical structure with high mechanical rigidity in order to avoid conflicts with the sMDT chamber. A rigid mechanical frame for the three RPC singlets which stays within the tight 60 mm envelope including its deformations under gravity
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