An annealing study of charge collection efficiency on Float-Zone p-on-n ministrip sensors irradiated with 24 GeV/c protons and 20 MeV neutrons

Float-Zone n-bulk p-readout silicon sensors are currently operated in the tracking layers of many High Energy Physics experiments, where they are exposed to moderate to high fluences of hadrons. Though n-readout sensors, either with p or n bulk, are available and are offering an improved radiation hardness, p-on-n sensors are still widely used and are e.g. installed in the present ATLAS and CMS experiments at CERN. Their radiation hardness and long-term performance are therefore of high interest to the detector community.

We present here a study performed on these sensors after irradiation with 24 GeV/c protons and 20 MeV neutrons to fluences ranging from 1⋅10¹⁴ to 1⋅10¹⁵ n_eq/cm². The sensors were then investigated for charge collection efficiency after different isothermal annealing steps in order to understand the performance evolution of the sensor with annealing time. Additional measurements were performed for the highest neutron fluence by means of the Edge-TCT technique, to assess the electric field configuration within the sensor. The irradiation and the annealing scenarios were chosen to represent the radiation damage scenario over the expected lifetime of the LHC detectors (and even further) and to assess the effect of unplanned annealing due to potentially longer warm shutdowns or cooling problems.