LAUSR

In view of the high luminosity phase of the LHC (HL-LHC) to start operation around 2026, a major upgrade of the tracker system for the ATLAS experiment is in preparation. The expected neutron equivalent fluence of up to 2.4 * 1e16 1 MeV neq./cm2 at the innermost layer of the pixel detector poses the most severe challenge. Thanks to their low material budget and high charge collection efficiency after irradiation, modules made of thin planar pixel sensors are promising candidates to instrument these layers. To optimise the sensor layout for the decreased pixel cell size of 50 * 50 {\mu}m2, TCAD device simulations are being performed to investigate the charge collection efficiency before and after irradiation. In addition, sensors of 100-150 {\mu}m thickness, interconnected to FE-I4 read-out chips featuring the previous generation pixel cell size of 50 * 250 {\mu}m2, are characterised with testbeams at the CERN-SPS and DESY facilities. The performance of sensors with various designs, irradiated up to a fluence of 1 * 1e16 neq./cm2, is compared in terms of charge collection and hit efficiency. A replacement of the two innermost pixel layers is foreseen during the lifetime of HL-LHC. The replacement will require several months of intervention, during which the remaining detector modules cannot be cooled. They are kept at room temperature, thus inducing an annealing. The performance of irradiated modules will be investigated with testbeam campaigns and the method of accelerated annealing at higher temperatures.