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Description
Radiation damage to a silicon photomultiplier (SiPM), as occurs during the lifetime of the planned HGCAL detector, increases the dark current and degrades the signal to noise (S/N) separation and thus the minimum ionizing particles (MIP) detection efficiency. To investigate this, a system consisting of a plastic scintillator tile directly coupled to a SiPM is used to detect the MIP from a $^{90}\mathrm{Sr}$ source. The design of the single channel is similar to the tiles for the CMS HGCAL calorimeter upgrade. A key novelty of this study lies in the comparative approach to emulate radiation damage. In particular, the effects of true radiation-induced damage were compared with a method that increases the dark count rate ($\mathit{DCR}$) exclusively through DC light illumination. Crucially, this second approach does not induce any physical structural damage or introduce trapping centers relevant for after pulse. These properties are inherently absent in the purely optical method. This allows the isolation of the effect of increased $\mathit{DCR}$ as the primary factor degrading the SiPM response. Our results show that an increase in the $\mathit{DCR}$, regardless of whether it was induced by irradiation or DC illumination, leads to an identical reduction in the MIP response and the S/N ratio. This confirms that the dominant factor for the performance degradation is the increased $\mathit{DCR}$ value itself and not additional damage or defects introduced in the silicon. The $\mathit{DCR}$ range investigated in this study extends from $\sim 10\,\text{kHz}$ before irradiation to $\sim 10\,\text{GHz}$ for the highest fluence of $5\times10^{13}\,\text{cm}^{-2}$ at an overvoltage of $2-4\,\text{V}$ and a temperature of $-20\,^\circ\text{C}$. This corresponds to a reduction of the signal down to $\sim 1-2\,\%$ of the initial response and an increase of the noise by a factor $\sim 20-25$ for the maximum fluence. The same results are obtained in SiPMs irradiated using reactor neutrons and fresh SiPMs illuminated by DC triggered LED light. This study highlights a significant insight: the primary consequence of radiation damage on SiPMs can be effectively mimicked under laboratory settings using optical illumination to increase the $\mathit{DCR}$. Such an approach enables accurate assessment of performance degradation and thus offers a powerful tool for the characterization of SiPMs and strategies for mitigating radiation damage.
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