Speaker
Description
Silicon pixel modules developed for the Phase2 CMS Inner Tracker were tested at HiRadMat facility, CERN, to evaluate potential damage in case of accidental beam loss at HL-LHC. Four pixel modules equipped with RD53 CROCv2 ASIC were exposed to fast and high intensity pulses of 440 GeV protons from SPS. The intensity of pulses was increased step-by-step: single bunch with increasing intensity up to 2e11 protons, then number of bunches increased up to 288 with intensity of 1.6e11 protons. Performance degradation for both sensor and ASIC will be presented in terms of leakage current, threshold and S-curve noise increase.
Summary (500 words)
Silicon pixel modules developed for the CMS Inner Tracker (Phase 2 upgrade) have been tested at the HiRadMat facility at CERN to evaluate potential damage in case of accidental beam loss at HL-LHC.
Pixel modules are the closest to the proton beams, so they would be the most affected by a catastrophic beam-loss event, an unlikely situation in which a high flux of secondary particles, generated by protons accidentally hitting collimators in the beam line, reaches detector components. Understanding how the pixel modules behave under such conditions is crucial for ensuring that the detector could safely operate at HL-LHC.
Four pixel modules equipped with CROCv2 readout chip (developed by RD53 collaboration) were exposed to fast and high intensity pulses of 440 GeV protons from SPS. The intensity of the pulses was increased step-by-step. In the first part, a single bunch with increasing intensity from 0.5e10 up to 2e11 protons was delivered. After that, the number of bunches was increased from 12 up to 288, with an intensity of 1.6e11 protons. This represents the highest beam intensity at which silicon pixel modules were ever tested, as HiRadMat is a unique facility in the world, providing proton beam directly extracted from SPS, in the same conditions as it is injected into LHC.
Two modules were equipped with 3D sensor and two with planar sensor technology. During beam transit, two modules (one 3D, one planar) were operated in Stand-by mode (readout electronics powered, sensor bias off), while the other two were in Stable beam mode (readout and bias on).
The performance of the modules was monitored with sensor IV and readout scans after each pulse. Minor performance degradation for both sensor and read-out chip was observed, in terms of leakage current, threshold and S-curve noise increase. Moreover, material activation was observed and studied.
Despite the extremely high beam intensity, all modules remained operational, with minor degradation in performance. No significant difference was observed between Stand-by and Stable beam operational modes and between 3D and planar sensor technology.
The increase in threshold (from 1000 up to 1500 electrons) was recovered by re-tuning. The increase in S-curve noise (from 100 up to 150 electrons) was also recovered afterwards.
The increase in sensor leakage current (from 1 uA to 50 uA) was found to be compatible with expectations (alpha factor in the order of 10^-17 A/cm).
Material activation was detected as hits recorded by the pixel matrix after the beam transit, with radioactive decay half-times in the order of several hours, compatible with the activation of copper layers present in the flexible circuit (powering and data transfer) glued over the silicon module.