Development Of A Position-Sensitive Micromegas Detector With Four-Corner Readout

The Circular Electron Positron Collider (CEPC) is a long-term collider project proposed by the Chinese high energy physics community. It aims for precision measurements of the Higgs boson and some other particles predicted by the Standard Model (SM), and searches for new physics beyond the SM using high-luminosity electron positron collisions. Among the measurements targeted at the CEPC are heavy-flavor-physics measurements with Z bosons, which require good hadron-identification performance of detectors.The Ring Imaging CHerenkov detector (RICH) has been widely used in collider experiments as a hadron identification device for its large momentum range, which makes it an important option for hadron identification at the CEPC. The RICH identifies hadrons by detecting the Cherenkov photons emitted by the hadrons when passing through the RICH, therefore, photon detection plays a key role in the RICH. A RICH at the CEPC requires a photon detector that must have large active area, high single-photon sensitivity, high counting-rate capability and good spatial resolution. Development of a photon detector that meets all the above requirements is therefore the key in the R&D of the CEPC RICH. We propose a hybrid structure of the MicroMegas and the THGEM as the photon detector for the CEPC RICH, and have performed a variety of simulation and experimental studies following this technology path.To achieve the large momentum range of hadron identification required by the CEPC experiment, a combination of liquid/solid and gaseous Cherenkov radiators is considered for the CEPC RICH in its conceptual design. Simulation studies were performed on the gaseous part of the CEPC RICH using the GEANT4 toolkit. Simulation results show that a gaseous RICH with a proper Cherenkov radiator and a photon detector with spatial resolution better than 500?m is capable of distinguishing kaons and pions with momentum up to 40GeV/c. In view of the simulation results, a MicroMegas detector with four-corner readout based on charge diffusion on a resistive anode was built and its performance was studied. The gas amplification structure in this detector (about 100um thick) was fabricated by the thermal bonding method. The uniformity of the amplification gap is ensured using a soluble film and a laser cutting technique. The readout anode of the detector was divided into 1cm2 square pads with low-resistance strips at edges. Signals are read out from the four corners (nodes) of each pad. The pads were printed on the anode PCB with polymer resistor using the screen printing technique. By comparing the amplitudes of signals from the four nodes of each pad, the position of the gas avalanche can be inferred. With this four-corner readout scheme, the number of readout channels would be decreased by two orders of magnitude while preserving spatial resolution performance. The sheet resistivity of the pads and strips were studied and optimized by simulation. Results from the tests of the MicroMegas detector show a spatial resolution of about 250um. This validates the four-corner readout scheme. The impact of shaping time of the readout electronics on the spatial resolution was also studied. The spatial resolution becomes better with the increasing of shaping time from 2 ?s to 10?s.The MicroMegas detector with four-corner readout is suitable for application for the CEPC RICH given the extremely large detection area. It would also have potential for other applications, such as muon imaging and TPCs.