An optical sensor for the ATLAS muon spectrometer: The reference alignment system

The muon spectrometer of the ATLAS detector at the LHC has been designed to provide a precise track momentum measurement for muons up to 1 TeV. It is composed of several hundred monitored drift tube chambers. Their positions must be known with an accuracy of 20 μm in translation and 200 μrad in rotation. For this purpose, a precise alignment system is required. This system is based on several subsystems using optical sensors measuring the deformations, and the relative and absolute positions of muon chambers. In this article we will focus on one of them: the reference system.