Abstract
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-μm-thick silicon (Si) pad sensors as the main active material and will sustain 1 MeV neutron equivalent fluences up to about 1016 neq 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 −30̂C 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 −30̂C. 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 1016 neq cm−2 exceeds 1 fC at voltages beyond 800 V.
Highlights
Samples and target fluencesThe irradiation campaigns were carried out in reactors in two facilities, Rhode Island Nuclear Science Center (RINSC) and UC Davis McClellan Nuclear Research Center (MNRC)
– The CCE performance of both polarity sensors improves by 20% and 40% by increasing the operating from 600 V to 800 V and 1 kV, respectively, at the fluence of (9.3 ± 1.1) × 1015 neq cm−2
CCE analysis suggests that the similar charge collection performance between 200P(LO) and 300P(LO) sensors at the fluence of (9.3 ± 1.1) × 1015 neq cm−2 results from close-to-equal depletion regions
Summary
The irradiation campaigns were carried out in reactors in two facilities, Rhode Island Nuclear Science Center (RINSC) and UC Davis McClellan Nuclear Research Center (MNRC). The two independent irradiation runs, 16 samples at RINSC and 14 samples at MNRC, were included in the study in order to cross-check dosimetries of the facilities as well as the method by which the effective fluences are extracted from the leakage current (Ileak) of the samples after the irradiation. ‘shd-FZ−LO−300P’ that refers to a 300-μm active thickness float-zone n-on-p type sensor with shallow diffused backplane implant and low oxygen bulk concentration (all varieties are presented in table 2). Neutron flux spectrum of the reactor is extracted by using the known cross section of these elements to be activated by neutrons as a function of neutron energy This measured neutron flux spectrum is convoluted with the damage function for silicon [10] and integrated to obtain the 1 MeV neutron equivalent flux of φ = (4.0 ± 0.8) × 1011 cm−2 s−1. The samples were shipped in thermally isolated containers packed with cold gel to Texas Tech University (TTU) to avoid annealing of the radiation induced defects, and were kept at −40◦C at all times between the measurements
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