Currently, one of the near detectors of the T2K experiment (Japan), ND280, is under upgrade. The new detector called the SuperFine-Grained Detector (SFGD) will consist of nearly 2 million 1x1x1 cm3 scintillating cubes made of polystyrene with 1.5% paraterphenyl (PTP) and 0.01% POPOP coated with a diffuse reflective layer.

Due to the fact that this reflective layer is not completely opaque, light leakage from the cubes is inevitable. Measuring these leaks is highly important for the 3D reconstruction of particle tracks, as well as for the correct description of the detector model in simulation programs. It is also necessary to investigate the effect of temperature on light leaks and light collection from the cubes since they can be heated during transportation or storage before final installation in the near detector of the T2K experiment.
The report presents the results of measurements of light leakage through the faces of scintillation cubes under the excitation of scintillation using a UV LED. The influence of heating of cubes on light collection from them and light leaks are investigated.