On-chamber readout system for the ATLAS MDT muon spectrometer

The ATLAS MDT Muon Spectrometer is a system of approximately 380 000 pressurized cylindrical drift tubes of 3 cm diameter and up to 6 m in length. These monitored drift tubes (MDTs) are precision glued to form superlayers, which in turn are assembled into precision chambers of up to 432 tubes each. Each chamber is equipped with a set of mezzanine cards containing analog and digital readout circuitry sufficient to read out 24 MDTs per card. Up to 18 of these cards are connected to an on-chamber DAQ element referred to as a chamber service module (CSM). The CSM multiplexes data from the mezzanine cards and outputs this data on an optical fiber which is received by the off-chamber DAQ system. Thus, the chamber forms a highly self-contained unit with DC power in and a single optical fiber out. The MDTs, due to their length, require a terminating resistor at their far end to prevent reflections. The readout system has been designed so that thermal noise from this resistor remains the dominant noise source of the system. This level of noise performance has been achieved and maintained in large scale on-chamber tests.