The advent of thin-gap RPCs, with 1mm gas gaps instead of 2mm 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 2mm 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.

薄间隙 RPC 的出现，气隙为 1 毫米，而不是目前的 RPC 中的 2 毫米，开启了对 ATLAS 介子光谱仪的内筒层进行仪器检测的可能性，因为在束线径向方向上的空间非常有限。枪管端盖过渡区域的环境特别密集。需要一种紧凑的机械结构来应对组装 RPC 的预期厚度变化，以适应有限的可用空间。同时，机械结构必须足够坚固，以将 RPC 包装的变形保持在允许的范围内。狭小的空间限制使得无法使用当前 ATLAS RPC 中使用的厚纸蜂窝板实现所需的刚度。对于 ATLAS μ 子光谱仪的桶端帽过渡区域的 I 阶段升级，三个 1 毫米间隙 RPC 被放入铝框架中。在这个框架中，RPC 三元组被预先弯曲的 2 毫米厚的铝板压缩。框架的刚度是通过加强连接框架横向结构的杆来实现的。相同的概念可以用于μ子光谱仪内层的第二阶段升级。