First bulk and surface results for the ATLAS ITk Strip stereo annulus sensors

A novel microstrip sensor geometry, the stereo annulus, has been developed for use in the end-cap of the ATLAS experiment's strip tracker upgrade at the HL-LHC. Its first implementation is in the ATLAS12EC sensors, a large-area, radiation-hard, single-sided, AC-coupled, n${}^{+}$-in-p design produced by the ITk Strip Sensor Collaboration and fabricated by Hamamatsu Photonics in early 2017. The results of the initial testing of two ATLAS12EC batches are presented here with a comparison to specification. The potential of the new sensor shape to reinvigorate endcap strip tracking is explained and its effects on sensor performance are isolated by comparing the bulk mechanical and electrical properties of the new sensor to the previous iteration of prototype, the conventional barrel ATLAS12A sensor. The surface properties of the new sensor are evaluated for full-size unirradiated sensors as well as for mini sensors unirradiated and irradiated with protons up to a fluence of $2.2\times 10^{15}$ n_eq cm⁻². The results show that the new stereo annulus ATLAS12EC sensors exhibit excellent performance and the expected irradiation evolution.