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Vienna Conference on Instrumentation / Investigations on the radiation damage of the LHCb VELO: a full review

The LHCb Vertex Locator (VELO) is a silicon micro-strip detector operating extremely close to the LHC proton beams. During nominal data-taking the innermost active strips are as close as 8 mm to the beams. In addition to providing the vertex measurements that allow the LHCb experiment to select signal events containing b and c quarks, this proximity makes the LHCb VELO an ideal laboratory to study radiation damage effects in silicon detectors. Two particular features of the VELO make it even more interesting to closely study the effects of radiation damage in this detector: the second metal layer and the direct comparison of n+ -in-n and n+-in-p technologies. Several methods are employed to evaluate the radiation damage effects in the LHCb VELO that complement one another. Measurements of bias current versus voltage (IV scans) are performed frequently and fully automated starting from early 2016. The measurements of bias current versus temperature (IT scans) are performed less frequently, as they require expert interventions to the VELO cooling system. The CCE scans require colliding beams and therefore have to be well prepared and negotiated in order not to cut unnecessarily into the data-taking efficiency of the LHCb experiment. This results in about four CCE scans taken per year; the aim is to scale the frequency of these scans with integrated luminosity rather than time. The CCE scans provide the best handle on to project the bias voltages needed to operate the VELO efficiently throughout LHC Run 2. The analysis of charge collection efficiency (CCE) data showed that there is a correlation of cluster finding efficiency (CFE) with the distance of strip to a second metal layer routing line. The detectors are constructed with two metal layers to cover the R/φ strips and route the signal to the front-end chips. A loss of signal amplitude is observed with a dependency on the distance to the routing lines. A TCAD simulation was implemented with the detailed detector geometry. Using the Perugia n-type bulk model and the Peltola surface damage model it is shown that up to 60% of the charge is collected by routing lines. This is caused by trapping of the otherwise mobile electron accumulation layer at the oxide-silicon interface, causing the shielding effect on the routing lines to be reduced. The observed drop in CFE can be explained by the angular dependence of charge loss to the second metal layer. The efficiency drop as function of track radius and angle is reproduced combining 2D and 3D TCAD simulations. A complete review of the whole history of the LHCb VELO radiation damage studies will be presented with results of run 1 and 2, as well as comparisons to TCAD simulations.

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