Novel Pixel-Detector Developments for Upgrades of the ATLAS Central Tracking System at the LHC

High-energy physics is confronted with a number of questions that could change our understanding of nature fundamentally. The discovery of the Higgs boson has shown that the LHC at CERN is currently the most important tool for answering these questions. In order to sustain this position, the full potential of the LHC is exploited by regular upgrades. They pose challenges on the detectors and push technologies to their limits. Our research was conducted to tackle the problems that arise for ATLAS, the largest experiment at the LHC. As part of the Phase-0 Upgrade (2013 – 2015) the Insertable B-Layer (IBL) was installed in ATLAS. This additional silicon pixel detector layer was placed closer to the LHC beam pipe than the existing layers, which required complete reconstruction of the beam pipe. The thermal behavior of the new detector and the beam pipe had to be understood before installation. Therefore we constructed a 1:1 mock-up and conducted thermal tests. Based on these tests, the insulation and therefore material budget of the new beam pipe were reduced. We verified the heating system for the beam pipe bake-out, improved the operation of the new CO2 cooling system of the IBL and used the tests to tune computer simulations of the detector. This work contributed significantly to the safe installation and operation of the IBL. The Phase-II Upgrade of the LHC (2024 – 2026) will result in the High-Luminosity LHC (HL-LHC). It will increase the particle collision rate in the experiments by a factor of almost four. The current silicon detector system in ATLAS is not able to cope with this and new, extremely radiation-hard silicon sensors are needed. We try to meet the demands with a concept new to high-energy physics: high-voltage complementary metal- oxide-semiconductor (HV-CMOS) sensors used as semi-hybrid pixel detectors. This unique approach allows for electronics in the sensor chip and at the same time high voltage to increase the depletion zone and hence the signal. The capacitive coupling to the readout chip can be achieved by a thin glue layer, which spares complicated connection techniques like bump-bonding. We discuss the prospects and drawbacks of HV-CMOS sensors in high-energy physics and investigate the prototype family CCPD (Capacitively Coupled Pixel Detector) in detail. The gluing, the capacitive coupling and operation of the sensors after irradiation to more than 8.5 MGy (850 Mrad) and 1×10 16 neq cm–2 are shown. Efficiencies of 99.7 and 98 % are demonstrated for sensors irradiated to 1×10 15 and 5×10 15 neq cm–2, respectively. These results are important steps toward qualification of HV-CMOS sensors for HL-LHC detectors and may inspire other experiments to consider the new approach.