Experimental and simulation study of irradiated silicon pad detectors for the CMS High Granularity Calorimeter

The foreseen upgrade of the LHC to its high luminosity phase (HL-LHC), will maximize the physics potential of the facility. The upgrade is expected to increase the instantaneous luminosity by a factor of 5 and deliver an integrated luminosity of 3000 fb$^{-1}$ after 10 years of operation. As a result of the corresponding increase in radiation and pileup, the electromagnetic calorimetry in the CMS endcaps will sustain maximum integrated doses of 1.5 MGy and neutron fluences above 10$^{16}$ n$_{\mathrm{eq}}$/cm$^2$, necessitating their replacement for HL-LHC operation. The CMS collaboration has decided to replace the existing endcap electromagnetic and hadronic calorimeters by a High Granularity Calorimeter (HGCAL) that will provide unprecedented information on electromagnetic and hadronic showers in the very high pileup of the HL-LHC. The HGCAL will be realized as a sampling calorimeter with 52 layers of active material. The electromagnetic section and the high-radiation region of the hadronic section will use hexagonal silicon sensors as active material. The low-radiation regions of the hadronic section will use plastic scintillator tiles with on-tile silicon photomultipliers (SiPM). The silicon sensors will be divided into cells of $\sim0.5-1.0~\textrm{cm}^2$ and will have active thicknesses from 100 to 300 $\mu\textrm{m}$ depending on their pseudorapidity (thinner sensors at higher $\eta$). In order to employ Si detectors in HGCAL and to address the challenges brought by the intense radiation environment, an extensive R\&D program has been initiated, comprising production of prototype sensors of various types, sizes and thicknesses, their qualification before and after irradiation to the expected levels, and accompanying simulation studies. The ongoing investigation presented here includes measurements of current-voltage and capacitance-voltage characteristics, along with predicted charge collection efficiences of the sensors irradiated to levels expected for the HGCAL at HL-LHC. The status of the study and the first results of the performance of neutron irradiated Si detectors, as well as their comparison with numerical simulations, are presented.