The silicon-based tracking detector systems in the HL-LHC upgrade will be exposed to highly adverse radiation conditions for more than a decade, without access options for service or repair. It is therefore imperative to understand and predict the long-term evolution of sensor properties, including the signal collection efficiency and the leakage current. In this context, we present a summary of long term annealing studies performed on miniature p-type strip sensors from a leading manufacturer, irradiated up to a fluence of 2e15 n_eq/cm^2. Sensors were subjected to annealing at two temperature scales, room temperature and 60°C, and comparative measurements were undertaken. The measurements include the charge collection and leakage current behaviour.
Based on these measurements, we find that the charge multiplication effect at high bias voltages decreases with time, indicating that we should not blindly rely on charge multiplication as a means to achieve a higher signal level. We also estimate the scaling factor between the two temperatures and the behaviour of the effective doping concentration. The scaling factor commonly used does very significantly exceed the one we measure in this study. This indicates that the results from the accelerated annealing studies commonly undertaken give a far too optimistic picture of the long-term evolution of silicon detectors in HL-LHC conditions.