Cracking under pressure --- investigating mitigation approaches for silicon fractures on ATLAS strip tracker petals at cold temperatures

Abstract

For the High-Luminosity upgrade of the Large Hadron Collider, the ATLAS experiment will replace its current Inner Detector with an all-silicon Inner Tracker (ITk), consisting of pixel and strip detectors. The strip detector will consist of a central region or "barrel" assembled with staves and forward regions or "end-caps" assembled with petals. The ITk will nominally operate with liquid $\textrm{CO}^2$ cooling at $-35\,^\circ\textrm{C}$; however, in the event of cooling system failures, it is possible that sensors will experience temperatures below $-35\,^\circ\textrm{C}$. At these low temperatures, it has been observed that the silicon sensors within modules --- the fundamental readout units of the detector --- can physically crack, rendering the modules inoperable. Understanding and resolving the issue of sensor cracking was one of the most important and urgent issues for the ITk project. This paper presents part of the mitigation strategies developed for petals. These mitigation strategies are based on modifications to the choice of adhesive and its deposition pattern for module assembly and petal loading. The most promising mitigation strategy presented here prevents cracking to temperatures as low as $-45\,^\circ\textrm{C}$, which can be expected in case of cooling system problems, with a small percentage of cracks observed after being cycled to $-55\,^\circ\textrm{C}$, which can be expected in case of catastrophic cooling system failures.