Development of the micro-RWELL discs for the CMD-3 detector

The CMD-3 is general purpose detector at the VEPP-2000 electron–positron collider at Budker Institute of Nuclear Physics. CMD-3 is intended to measure parameters of light vector mesons and main hadronic cross sections with accuracy better than 1% and study dynamics of multihadron production. In order to increase geometrical acceptance for charged particles and to improve precision of track polar angle measurements the end-cap discs based on micro-resistive-WELL (μ-RWELL) structures are proposed for the upgrade of the CMD-3 tracking system. Two μ-RWELL discs 50 cm in diameter were assembled and tested. The discs have two-layer readout structure with the top layer of quarter-rings with 2 mm pitch in radius and the bottom layer of sectors with average angular pitch of about 0.3 degrees. The quarter-rings occupy only 50% of the disc surface due to gaps between them, thus, allowing the signal to be induced on the bottom layer. The kapton foil with copper layer on top and high-resistivity DLC layer at the bottom, pierced with 80 micron diameter holes located at 140 micron from each other, is glued on top of the readout structure. With the first disc the manufacturer (CERN advanced PCB Workshop) could not reach proper insulation between the copper layer and the DLC and the copper was etched out leaving the disc with thin Ni–Cr adhesion layer on top. The second disc has copper layer on top of the μ-RWELL structure. The readout structure of the discs is connected to front-end electronics based on VMM3a ASICS. The results of study of the main parameters of the detectors with front-end electronics are reported in the paper.