Evaluation of charged pions induced damage in the CMS silicon forward detectors

The bulk damage induced by charged pions in the Compact Muon Solenoid forward silicon tracker region has been numerically evaluated. Charged pion fluences have been calculated for a distance of 1 m from the interaction vertex and the forward disk, a disk radius between 20 and 40 cm and a magnetic field of 4 T. Times of continuous operation between 60 and 150 d have been considered. The calculated Fermi level and the bulk resistivity along the radius show the occurrence of a radiation induced type inversion of the bulk from n to p in the silicon microstrip after 120 d of operation at 293 K. The calculated effective impurity concentration Neff suffers changes above one order of magnitude along the disk radius, for operating times of 60 d or higher. The overall study evidences a strong radiation induced inhomogeneity in the bulk properties of the silicon microstrip detectors to be used in the CMS forward tracker region.