Large size resistive Micromegas (MM) detectors will be employed for the first time in high-energy physics experiments for the Phase I upgrade of the ATLAS Muon Spectrometer at the LHC. The current innermost stations of the muon end-cap system, the Small Wheel, will be upgraded to retain the good precision tracking and trigger capabilities in the high background environment expected with the upcoming luminosity and energy increase of the LHC. Along with the small-strip Thin Gap Chambers (sTGC) the “New Small Wheel” will be equipped with eight layers of MM detectors arranged in multilayers of two quadruplets, for a total of about 1200 m2 detection planes. All quadruplets have trapezoidal shapes with surface areas between 2 and 3m2. Both MM and sTGC systems will independently provide trigger and tracking capabilities.
An individual gas distribution system has to be appropriately designed in order to ensure the necessary gas renewal rate among the MM layers. In this work we present and describe the methodology, the particular calculations and simulations to achieve the appropriate gas flow rates ensuring a uniform gas distribution among the same type of modules. The majority of the components used are in large multiplicity so space saving criteria is taking into account and simplicity on the performance with respect to the total cost as well.
An appropriate simulation program has been developed for studying the overall gas system determining the gauge pressure, flow rate in the crucial points and branches, respectively. Moreover, a number of particular extensive studies are implemented: a) A new designed manifold, “trident” type, for splitting the gas flow to the three-layer inlets of each module into fixed ratios. b) A novel-alternative method for measuring the gas leak rate of the modules, called Flow Rate Loss (FRL), has been developed and tested, for the mass production in the framework of quality checking-quality assurance. c) A calibration method is also proposed, based on the emulated leak branches for a variety of low cost medical hypodermic needles. An overall prototype configuration, implemented at the NTUA laboratory and based on the Lock-in Amplifier technique to be used in conjunction with the gas leak test via the FRL method is presented. The obtained performances, by means of sensitivity and S/N ratio improvement, are also discussed.