Micromegas R&D for ATLAS MUON PHASE II Upgrade

In the framework of the ATLAS Phase II Upgrade, a proposal to extend the detector acceptance of the muon system to high η has been put forward (namely up to | η| ~ 4). Extension of the muon coverage has been demonstrated to enhance physics performance. The proposed location for the new detector is in between the end-cap calorimeter cryostat and the JD shielding; in this region there is no magnetic field applied, the aim of the new detector is therefore to only tag muons without performing any momentum measurement. The new η tagger should cope with extremely high particle rate, that has been calculated, by means of simulations, to be 9 MHz at R = 25 cm and 0.4 MHz at R = 60 cm for μ = 200, where μ stands for the number of pp collisions per bunch crossing. The required spatial resolution at the inner edge of the detector has been estimated in few hundreds micrometres. One of the most promising candidate technology for the new detector is the MicroMegaS one, which has already been adopted for the NSW upgrade, demonstrating to be suitable for very high rate environments and to provide good spatial resolution. We present here the development of the first MicroMegaS prototype for the new detector, based on small readout pads specifically aiming at matching all the muon tagger requirements. The prototype is composed of a read-out pads matrix with rectangular shape 0.8mm x 2.8mm (pitch of 1 and 3 mm in the two coordinates), for a total of 768 channels read-out by six 128-channels APV-25 chips. The characterization of the detector has been carried out by means of radioactive sources, X-Rays gun and cosmic rays. The results, as well as a description of the construction method, will be presented. Finally, performance studies of the detector, such as resolution and efficiency, have been carried out with testbeam data: the most important results will be therefore presented.