Measurement of the bulk leakage current of silicon sensors of the CMS Preshower after an integrated luminosity of 6.17 fb−1, at √s = 7 TeV

The CMS Preshower is part of the CMS Electromagnetic Calorimeter (ECAL) system. It is a sampling calorimeter that was installed to improve π0 rejection in the forward direction. It is composed of two layers of lead, each followed by a layer of silicon sensors. The Preshower is installed at each end of CMS, at about ±310 cm from the CMS interaction point. The sensors are exposed to a wide spectrum of ionizing and non-ionizing radiation, causing an increase of the bulk leakage current and a change of the effective doping concentration, resulting in a change of the full depletion voltage. This paper presents the measurements of the bulk current increase from the 2010 and 2011 LHC runs, corresponding to an integrated luminosity of 6.17 fb−1, including luminosity taken outside stable beam conditions, at a centre-of-mass energy of 7 TeV. A computer program based on the Hamburg model was developed for calculating the effects of simultaneous irradiation and annealing on the bulk leakage currents. The calculated currents, as a function of time, were found to be in good agreement with the bulk leakage currents measured in 2011. The program will be used to estimate the long-term evolution of the bulk leakage currents. The absolute level of the bulk leakage current, and its radial dependence, is in good agreement with the predictions obtained with the Hamburg model folded with the FLUKA simulation of the CMS radiation field.