ATLAS ITk pixel endcap demonstrator “Ring-0” construction and testing

The ATLAS upgrade programme for the High-Luminosity Large Hadron Collider (HL-LHC) includes the replacement of the inner tracking detector with an all-silicon Inner Tracker system (ITk). The outer layers of the new tracker consist of strip sensors and the inner region is made up of pixel sensors. This paper describes the development of the endcaps for the pixel system. The endcaps are built from half-rings, carrying the pixel modules and the services. A prototype half-ring populated with pixel modules, read out using FEI4 front-end ASICs in a serial powering chain, was constructed. The half-ring’s mechanical and electrical structure and the assembly process are described. The electrical measurements of the modules are presented, focusing on their performance in the serial powering chain.