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Register a new userCharge Collection and Electrical Characterization of Neutron Irradiated Silicon Pad Detectors for the CMS High Granularity Calorimeter
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The replacement of the existing endcap calorimeter in the Compact Muon Solenoid (CMS) detector for the high-luminosity LHC (HL-LHC), scheduled for 2027, will be a high granularity calorimeter. It will provide detailed position, energy, and timing information on electromagnetic and hadronic showers in the immense pileup of the HL-LHC. The High Granularity Calorimeter (HGCAL) will use 120-, 200-, and 300-$mutextrm{m}$ thick silicon (Si) pad sensors as the main active material and will sustain 1-MeV neutron equivalent fluences up to about $10^{16}~textrm{n}_textrm{eq}textrm{cm}^{-2}$. In order to address the performance degradation of the Si detectors caused by the intense radiation environment, irradiation campaigns of test diode samples from 8-inch and 6-inch wafers were performed in two reactors. Characterization of the electrical and charge collection properties after irradiation involved both bulk polarities for the three sensor thicknesses. Since the Si sensors will be operated at -30 $^circ$C to reduce increasing bulk leakage current with fluence, the charge collection investigation of 30 irradiated samples was carried out with the infrared-TCT setup at -30 $^circ$C. TCAD simulation results at the lower fluences are in close agreement with the experimental results and provide predictions of sensor performance for the lower fluence regions not covered by the experimental study. All investigated sensors display 60$%$ or higher charge collection efficiency at their respective highest lifetime fluences when operated at 800 V, and display above 90$%$ at the lowest fluence, at 600 V. The collected charge close to the fluence of $10^{16}~textrm{n}_textrm{eq}textrm{cm}^{-2}$ exceeds 1 fC at voltages beyond 800 V.
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A silicon-based fine granularity calorimeter is a potential technology for the future International Linear Collider ILC, the future circular collider CEPC, and is also the chosen technology for the upgraded CMS experiment of the Large Hadron Collider. Active silicon sensing pads are used as MIP counters and the standard calibration of the calorimeter uses weights based on the average energy loss, $dEdx$. In this work, the limitations of the dEdx calibration method in terms of energy linearity, scale and resolution are explored. In the case of a calorimeter with varying passive layer thickness as the one planned for CMS, the $dEdx$ method leads to a significant constant term in the resolution function and a non-linearity of energy response. For these reasons, a method based on the calorimeter sampling fraction that exploits the per-event measured shower depth is presented and shown to deliver superior absolute energy scale, linearity and resolution. Calorimetric designs in which the back of the shower is sampled less, offer reduced cost without loss in performance. Therefore, a proper calibration as proposed here is crucial in obtaining the most cost- and performance-effective silicon-sampling calorimeter design.
The CMS experiment at the CERN LHC will be upgraded to accommodate the 5-fold increase in the instantaneous luminosity expected at the High-Luminosity LHC (HL-LHC). Concomitant with this increase will be an increase in the number of interactions in each bunch crossing and a significant increase in the total ionising dose and fluence. One part of this upgrade is the replacement of the current endcap calorimeters with a high granularity sampling calorimeter equipped with silicon sensors, designed to manage the high collision rates. As part of the development of this calorimeter, a series of beam tests have been conducted with different sampling configurations using prototype segmented silicon detectors. In the most recent of these tests, conducted in late 2018 at the CERN SPS, the performance of a prototype calorimeter equipped with ${approx}12,000rm{~channels}$ of silicon sensors was studied with beams of high-energy electrons, pions and muons. This paper describes the custom-built scalable data acquisition system that was built with readily available FPGA mezzanines and low-cost Raspberry PI computers.
A detailed study of charge collection efficiency has been performed on the Silicon Drift Detectors (SDD) of the ALICE experiment. Three different methods to study the collected charge as a function of the drift time have been implemented. The first approach consists in measuring the charge at different injection distances moving an infrared laser by means of micrometric step motors. The second method is based on the measurement of the charge injected by the laser at fixed drift distance and varying the drift field, thus changing the drift time. In the last method, the measurement of the charge deposited by atmospheric muons is used to study the charge collection efficiency as a function of the drift time. The three methods gave consistent results and indicated that no charge loss during the drift is observed for the sensor types used in 99% of the SDD modules mounted on the ALICE Inner Tracking System. The atmospheric muons have also been used to test the effect of the zero-suppression applied to reduce the data size by erasing the counts in cells not passing the thresholds for noise removal. As expected, the zero suppression introduces a dependence of the reconstructed charge as a function of drift time because it cuts the signal in the tails of the electron clouds enlarged by diffusion effects. These measurements allowed also to validate the correction for this effect extracted from detailed Monte Carlo simulations of the detector response and applied in the offline data reconstruction.
A pixel detector with a CVD diamond sensor has been studied in a 180 GeV/c pion beam. The charge collection properties of the diamond sensor were studied as a function of the track position, which was measured with a silicon microstrip telescope. Non-uniformities were observed on a length scale comparable to the diamond crystallites size. In some regions of the sensor, the charge drift appears to have a component parallel to the sensor surface (i.e., normal to the applied electric field) resulting in systematic residuals between the track position and the hits position as large as 40 $mu$m. A numerical simulation of the charge drift in polycrystalline diamond was developed to compute the signal induced on the electrodes by the electrons and holes released by the passing particles. The simulation takes into account the crystallite structure, non-uniform trapping across the sensor, diffusion and polarization effects. It is in qualitative agreement with the data. Additional lateral electric field components result from the non-uniform trapping of charges in the bulk. These provide a good explanation for the large residuals observed.
We show that doubly peaked electric fields are necessary to describe grazing-angle charge collection measurements of irradiated silicon pixel sensors. A model of irradiated silicon based upon two defect levels with opposite charge states and the trapping of charge carriers can be tuned to produce a good description of the measured charge collection profiles in the fluence range from 0.5x10^{14} Neq/cm^2 to 5.9x10^{14} Neq/cm^2. The model correctly predicts the variation in the profiles as the temperature is changed from -10C to -25C. The measured charge collection profiles are inconsistent with the linearly-varying electric fields predicted by the usual description based upon a uniform effective doping density. This observation calls into question the practice of using effective doping densities to characterize irradiated silicon. The model is now being used to calibrate pixel hit reconstruction algorithms for CMS.
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