Measurement of the charge collection in irradiated miniature sensors for the upgrade of the ATLAS phase-II strip tracker

Miniature sensors with external dimensions of 10 mm x 10 mm were produced together with full-size sensors for the innermost ring (R0) of the end-cap part in the upgraded ATLAS inner tracker (ITk). AC- and DC-coupled n-type strips with three different pitches (wide, default and narrow) were processed on high-resistivity p-type FZ silicon substrates by Hamamatsu Photonics. The miniature sensors were irradiated with 70-MeV protons at CYRIC, Tohoku University (Japan) and reactor neutrons at the Jožef Stefan Institute (Slovenia) to three different 1-MeV neutron equivalent fluences: 0.5, 1 and 2 x 10¹⁵ $n_{\mathrm{eq}}$cm⁻². The upper fluence range exceeds the highest anticipated in the innermost part of the ATLAS ITk-Strips over the HL-LHC lifetime ($\sim$1.25 × 10${}^{15}$n${}_{\mathrm{eq}}$cm²). The charge collection in the test sensors was evaluated systematically using a ⁹⁰Sr $\beta$-source and an Alibava analogue readout system at reverse-bias voltages up to 1000 V.