Speaker
Description
A new concept of tracking detector based on Scintillating Fibres (SciFi) read out with multichannel silicon photomultipliers (SiPMs) was installed during the upgrade I of the LHCb experiment at CERN. One of the main challenges that the SciFi tracker will face during its operation is the high radiation environment due to fast neutrons. In view of LHCb Upgrade II in 2033, the radiation levels will increase significantly and the SciFi tracker must undergo a major upgrade. By the end of lifetime, the expected radiation fluence reaches 3E12 neq/cm2 at the SiPMs location. To cope with the increase in radiation, cryogenic cooling with liquid nitrogen is being investigated as a possible solution to mitigate the performance degradation of the SiPMs induced by radiation damage.
Under this scenario, different layouts of SiPMs modules from two producers (FBK and Hamamatsu) are being investigated. Several modules were irradiated at Ljubljana at different neutron fluences and have been tested in a dedicated cryogenic setup down to 100 K, where the main key operational parameters were measured as a function of the temperature [1, 2].
In Figure 1 left, the breakdown voltage (Vbd) against temperature is shown for one of the irradiated FBK modules. It is reported a linear decrease of 31,5 mV/K till around 210 K, and below this temperature the linear decrease is not maintained as the Vbd decreases slower with temperature. In Figure 1 right, the dark count rate (DCR) against temperature is shown for the FBK SiPMs at four different irradiation fluences. At the selected overvoltage (8 V), it is reported a decrease of DCR of almost 5 orders of magnitude from RT down to 100 K. Also, above1E12 neq/cm2 it is observed an excess on the measured DCR as is not increasing proportional with the irradiation fluence anymore. The source of this excess DCR it is not clear yet and further studies are needed.
In parallel, a novel concept of integrating microlenses (uLens) at the wafer level, aligned with the SiPM pixel structure, has been developed to enhance photon collection and increase photon detection efficiency (PDE) [3]. The performance of the microlens depends on key geometrical and optical parameters such as uLens curvature, height, diameter, refractive index, pixel pitch, and alignment accuracy. These parameters were optimized through a validated simulation framework that includes realistic optical effects such as refraction, reflection, and angular acceptance, and the main implementation variables are summarized in Figure 2. Accurate fabrication and alignment are essential to achieve the expected performance gains; therefore, the residual layer thickness (RLT) and the alignment of the uLens relative to the SiPM pixel center were measured using mechanical profilometry and microscopy.
An example of these laboratory measurements is shown in Figure 3, demonstrating that the uLens geometry and placement meet the design tolerances and ensuring efficient light focusing with minimal crosstalk. The uLens integration significantly improves PDE by concentrating incoming photons onto the active microcells, effectively increasing the fill factor without changing pixel size. Both simulations and experimental measurements indicate a PDE gain of up to 15–30%, depending on the incident light angle and uLens geometry, and one example of the measured improvement is presented in Figure 4. This enhancement not only increases sensitivity but also benefits timing performance [4], making uLens a promising approach for the SciFi tracker upgrade.