Thin silicon detectors for tracking in high radiation environments

The upgrade of the present Large Hadron Collider to high luminosity will impose the use of a factor of ten more radiation tolerant silicon sensors (microstrip and pixel) for the vertex and tracker system upgrades. The requirement for extreme radiation tolerance to hadron radiation is certainly the most stringent requirement for the HL-LHC sensors, but other parameters are also desirable, in particular a reduced mass of the whole systems. The use of thinner sensors than the standard 300 J.1m thick presently adopted could contribute to the reduction of the material in the acceptance volume of the experiments. It is now known that the thickness has also effects on the radiation tolerance of the sensors. The comparison of the signal generated by minimum ionising particles in 100, 140 and 300 J.1m thick sensors irradiated with reactor neutrons to various tluences up to the highest expected for the innermost pixel layers of the upgraded ATLAS experiment (2×10¹⁶ n_eq cm^-2) are presented. Conclusions about the fluences where a given thickness gives the best signal are drawn from the experimental results to provide guidance for the optimal choice of thickness for segmented silicon sensors to be deployed in harsh hadron radiation environments.