We investigate the radiation effects on single-photon avalanche diodes (SPADs) fabricated in CMOS technology. For this investigation, a high-performance SPAD based on a vertical p-i-n construction with buried-N layer is proposed and used for the characterization. It retains good performance in terms of dark counts and photon detection probability upon irradiation doses from 10k Rad(Si) to 50k Rad(Si), with a slight increase of dark count rates and stable dark current, breakdown voltage, and sensitivity. This feature makes our device especially suitable to be integrated in SiPMs for applications in radiation-rich environments, where high radiation tolerance and low noise are essential.