Use of IV (current vs voltage) scans to track radiation damage in the LHCb VELO

This note describes the results of a study of radiation damage to the 88 silicon sensors of the VELO, using as input data the regular current vs voltage scans (IV scans). The current measured in the sensors can be broadly described as consisting of two components, bulk current and surface current, which evolve differently as a function of the fluence received by the sensor. This study categorises the sensors according to their IV characteristics, and identifies two categories where the behaviour is as expected, and a third category where the sensors who unexpected behaviour, and the evolution must be more carefully monitored. The average current increase for the sensors at the operational temperature of $\sim$-8°C is measured to be about 20 $\mu$A per fb$^{-1}$, with deviations of about 60% for the most and least irradiated sensors.