Photodetectors used in future high-energy physics experiments will need to keep sufficient performance during a few years of data-taking despite radiation load, which, for example, in the planned upgrade of the Ring Imaging Cherenkov detectors in Large Hadron Collider beauty (LHCb) experiment is estimated at about 10 $$^{13}$$ $_{}^{13}$ 1-MeV neutron equivalent per cm $$^{2}$$ $_{}^2$ (neq/cm $$^{2}$$ $_{}^2$ ). Silicon photomultipliers (SiPMs) are considered as candidates for photodetectors for this application. However, their sensitivity to neutron irradiation may seriously compromise their operation, with the increase in dark count rates being the primary limitation after the irradiation. In this work, 1 $$\times $$ $\times$ 1 mm $$^{2}$$ $_{}^2$ 15 $$\mu $$ $\mu$ m pitch NUV-HD-RH silicon photomultipliers developed by the Fondazione Bruno Kessler were characterized before and after the irradiation. In total, 5 SiPMs were irradiated at the Jožef Stefan Institute TRIGA nuclear reactor with different fluences from 10 $$^{9}$$ $_{}^9$ neq/cm $$^{2}$$ $_{}^2$ up to 10 $$^{13}$$ $_{}^{13}$ neq/cm $$^{2}$$ $_{}^2$ . The SiPMs were also annealed at high temperatures and re-characterized after the annealing. For the SiPM characterization in all the cases, current–voltage (I-V curve) measurements, dark count rate measurements, and waveform analysis, including single photon time resolution, were carried out at different controlled temperature steps from room temperature down to the liquid nitrogen temperature. While cooling to − 20 $$^{\circ }$$ $_{}^{\circ }$ C was enough for the SiPM irradiated at 10 $$^{9}$$ $_{}^9$ neq/cm $$^{2}$$ $_{}^2$ to recover the ability to resolve single photons, by 10 $$^{13}$$ $_{}^{13}$ neq/cm $$^{2}$$ $_{}^2$ , cooling to liquid nitrogen temperature was necessary. With sufficient cooling, the measured single photon time resolution was around 90 ps FWHM for all irradiation levels.