The CMS detector at the CERN Large Hadron Collider is undergoing an extensive Phase II upgrade program to prepare for the challenging conditions of the High-Luminosity LHC starting in 2026. In particular, a new timing detector, the MTD, will measure minimum ionizing particles with a time resolution of 30-50 ps. The precision time information from the MTD will reduce the effects of the high levels of pile-up expected at HL-LHC and will bring new and unique capabilities to the CMS detector; it will allow the use of 4D reconstruction algorithms and will further discriminate interaction vertices within the same bunch crossing to recover the track purity of vertices in current LHC conditions.
The technology selected for the central part of the detector, the Barrel Timing Layer, consists of scintillating crystals of Lutetium Yttrium Orthosilicate doped with Cerium (LYSO:Ce) read out with Silicon PhotoMultipliers.
The first part of this project is aiming at characterize LYSO:Ce scintillating crystals from different vendors by measuring and comparing geometrical and optical properties of crystal bar samples. This activity will be performed at the Segrè laboratory in Sapienza. In addition a subset of the crystals will be irradiated with photons in Enea - Calliope facility to check the radiation tolerance by testing the optical properties before and after the irradiation.
In the second part of the project an optical bench dedicated to the test of the crystal arrays, coupled with SiPM arrays with final packaging and electronics, will be built. A pre-production of O(100) arrays from two or three selected vendors will be tested with the new bench to investigate more deeply the homogeneity in the properties of the crystals and the reliability of the final producer. Finally this project will be crucial in the identification of the vendor for the full detector production and the tuning of the procedure for the Quality Control of the crystals during the final production.
Several alternative scintillators were considered for use in the CMS BTL (plastic scintillators, crystal garnets, etc.), but LYSO:Ce was chosen as the optimum trade-off between performance, radiation tolerance, cost and mass production capability. The R&D studies proposed in this project will drive the choice of the crystal producer for the new BTL detector for CMS. The goal of the project is also to define the Quality Control procedure to be adopted by CMS during the MTD construction phase.
The photo-sensors of choice for the BTL are silicon photomultipliers (SiPMs). Rapid progress in the performance of SiPMs has led to their widespread use in accelerator and non-accelerator based particle and nuclear physics experiments, space-based telescopes, and medical imaging. SiPMs have a number of advantages over other photo-sensors, such as conventional photomultiplier tubes. SiPMs are compact, robust, and insensitive to magnetic fields. They can be exposed to room light without damage and operate at relatively low voltages with low power consumption. A photo-detection efficiency, PDE, of up to 40% is achievable in devices with small cell size (15 micron square pixels). Small cell sizes also extend the linear range of the SiPM and, combined with a fast cell recovery time, enhance its performance after irradiation.
We can certainly ensure the usefulness of the R&D on these sensors in several areas where this detection technique could find application, from detectors in future accelerators to medical and nuclear applications in which a medium to high-energy electron beam is utilized with precision event timing. For instance, important applications for these improved LYSO:SiPM detectors are envisaged in medical imaging, for the realization of RX panels or TOF-PET systems (PET scanners with time of flight resolution to suppress accidental coincidences).