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This paper reports the development of a ZnS:TiO2/ n-Si heterojunction photodiode structure by deposition of colloidal ZnS:TiO2 quantum dots (QDs) on the n-Si substrate. To study the diode performance under harsh radiation atmospheres, in situ dark I- V characteristics of the ZnS:TiO2/n-Si photodiode have been studied under the irradiation of 120 MeV Au9+ ions with an incremental increase in fluences from 3 ×1011 to 1 ×1013 ions/cm2. It shows the increase in rectification behavior with the increase in irradiation fluences caused by interface smoothening and interface defect annealing at higher fluences. X-ray diffraction patterns and TEM analysis also show the increased crystallinity of ZnS:TiO2 QDs of size approximately 2-5 nm, whereas photoluminescence spectra show the reduction of defects. These results are in support of the irradiation-induced effects in in situ I- V measurements. The studies are also made for ultraviolet light of 376 nm wavelength (laser power 50 mW) and visible light of power 60 W for pristine and irradiated (at fluence 1 ×1013 ions/cm2) ZnS:TiO2/n-Si heterojunction photodiodes.