Design and Construction of Radiation-Hard, Thermally Conductive Dees for the HL-LHC Upgrade to the CMS Inner Tracker*

During the upcoming High-Luminosity run of the Large Hadron Collider (HL-LHC), the silicon sensors on the innermost tracking layers of the Compact Muon Solenoid (CMS) experiment are expected to receive up to 1.2 × 10¹⁶ n_eq/cm² of accumulated fluence (1.5 Grad TID). Radiation-damaged sensors will generate larger leakage currents and require higher bias voltages for efficient operation, which increases the thermal load on the detector and heightens the risk of permanent damage from runaway heating. In this talk, we describe ongoing efforts to design sensor mounting structures (“Dees”) with radiation-hard, high thermal conductivity materials that can maintain robust thermal pathways to a liquid CO₂ cooling system for the full lifetime of the experiment. We focus on work performed at Cornell University, where we have developed a new thermal interface material (TIM) with exceptional radiation tolerance and are building up sophisticated laboratory infrastructure to automate Dee manufacturing and assess thermal performance. This work pertains to Dees in the forward region of the inner tracker, and is performed in collaboration with other universities as part of the Tracker Forward Pixel (TFPX) upgrade effort.