Inner TPC upgrade at RHIC-STAR

Abstract

The second phase of the Beam Energy Scan (BES-II), has been scheduled at RHIC at Brookhaven National Laboratory during 2019–2020 and the major scientific task is to study the QCD phase diagram and search for the possible critical point in relativistic heavy ion collisions. The physics measurements rely on key upgrades on beam luminosity and the detector upgrades, for example the inner Time Projection Chamber (iTPC) upgrade at the STAR experiment. With the proposed iTPC upgrade at STAR, the pseudo-rapidity coverage will be extended and the precision of energy loss for particle identification will be improved significantly, resulting in a big improvement in the acceptance for the measurement. The iTPC upgrade will rebuild all 24 inner multi-wire proportional chamber (MWPC) readout sectors including new electronics. The STAR-China group is responsible for the MWPC R&D and mass production. The MWPC includes three layers of wires: anode, shield, and gated wire grids, above the PCB pad plane board. High precision of wire location and wire tension of the wire grids on Aluminum strong back are required during the assembly in order to meet the desired detector performance. A laser based wire tension measurement system has been designed and implemented to inspect the wire tension and pitch. A strict control of the mechanical tolerance of the components has also been realized with position precision of 10€μm for wire pitch, and the distance between pad plane and wire planes using wire comb. The anode wire tension has been kept within 6% of desired value. With these quality control enforced, the gain uniformity and the energy resolution are around 1.5% (RMS/Mean) and 25% (FWHM/Mean) respectively, for all 28 produced sectors, tested using a 55Fe X-ray source. Comparing to the required values of 2% for gain uniformity and 30% for energy resolution by STAR, all sectors are qualified with good performances. The required 24 sectors have been installed at STAR experiment successfully.