Mechanical challenges of the innermost layers of ePIC Silicon Vertex Tracker

Abstract At the BNL Electron Ion Collider, the electron Proton/Ion Collider (ePIC) experiment intends to primarily investigate a broad and variegated QCD and Nucleon physics plan, requiring novel and high-performing detectors. An example is its Silicon Vertex Tracker (SVT). Its innermost barrel layers consist of bent radiation-hardened 65 nm CMOS imaging Monolithic Active Pixel sensor chips, minimizing the material budget and multiple scattering effects in the region close to the beam interaction point. The chips are being realised using TPSCo 65 nm CMOS imaging technology and the stitching technique in flat 300 mm wafers. The mechanical bending occurs outside the company and during the assembly of the detector layers. Their design presents several mechanical and cooling challenges faced in synergy with the ALICE ITS3 project. Moreover, the SVT layers will cover cylindrical lateral surfaces with larger radii than ITS3 layers, which drives the need for dedicated R&D and solutions for large radii. For example, in SVT, the radius of the first layer is ∼38 mm, whereas the ITS3 design foresees a ∼19 mm radius. The bending procedure and the local mechanical structure have been validated and optimised on 50 μm-thick silicon dummies. The successive steps consist of assembling a multi-layer mock-up to finalise its construction procedure, and a mechanical and thermal mock-up to perform the thermal and wind tunnel tests. The last prototype will be equipped with temperature sensors and heaters, which mimic the temperature distribution when the power is supplied. The measurements will validate the design of the final detector, and they will be compared with the ongoing simulations.