Study on saturation of SiPM for scintillator calorimeter using UV laser

The saturation of SiPMs is a potential issue for scintillator calorimeters with SiPM readout. When a large number of particles hit a scintillator, the SiPM output can be saturated due to the limited number of pixels. In order to convert the SiPM output into the number of incident photons correctly, it is necessary to understand the behavior of the SiPM saturation. The saturation curve is usually measured by directly injecting a fast laser pulse ($\sim$400 nm) to a SiPM . However, the effect of the time constant of the scintillation light emission, which is not negligible compared to the recovery time of the SiPM pixel, is not included in this method. We propose a new method to measure the SiPM saturation with scintillation light excited by an UV laser. The measured saturation curve can directly be used for the saturation correction of the calorimeter response, since the effect of the cell recovery during the scintillation emission is included. A fast fs UV pulse laser with a wavelength of 190 nm is used. The 190 nm laser causes scintillation excitation, whereas it is invisible to the SiPM . The laser is injected to a plastic scintillator coupled to a SiPM . For comparison, the same test is carried out also by injecting a fast laser pulse with a wavelength of 470 nm. The saturation curve is obtained for a wide range of $N_{pe}$, and a large over-saturation is observed for the 190 nm injection, which is significantly larger than the 470 nm injection. This difference is the effect of the time constant of the scintillation emission, and can have a big impact on the saturation correction.