Contribution of the Light-Collection Non-Uniformity to the Energy Resolution for the Spaghetti-Type Calorimeter Modules

Spaghetti-type calorimeters (SpaCal) are being considered as a potential solution for experiments at the High-Luminosity Large Hadron Collider (HL-LHC), particularly for the LHCb ECAL Upgrade 2 project where the expected instantaneous intensity and radiation dose in the central area of the ECAL significantly exceed the limits tolerable by the current Shashlik-type modules. SpaCal modules consist of an absorber block containing a matrix of holes filled with scintillating fibres, offering flexible granularity. However, the total number of scintillating fibres exceeds the available photocathode surface area, necessitating the use of a light guide to efficiently collect and register the light from the scintillating fibres to a single photomultiplier. This introduces non-uniformity in the light collection, which adversely impacts energy resolution. In this study, we explored various geometries of light guides with the optical ray-tracing simulations in order to collect scintillating light from a 30× 30 mm^2 surface to the photocathode of photomultipliers with the following entrance window: 18× 18 mm^2 (e.g. R7600), 9× 9 mm^2 (multi-anode version, e.g. R7600-M4), and round photocathode ⊘ 8 mm (e.g. R9880). The light collection non-uniformity impact on the energy resolution is estimated.