A dedicated single-cell SiPM structure is designed and measured to investigate the radiation damage effects on the gain and turn-off voltage of SiPMs exposed to a reactor neutron fluence up to Φ = 5e13 cm−2. The cell has a pitch of 15μm. The fluence dependence of gain and turn-off voltage are reported. A reduction of the gain by 19% and an increase of Voff by ≈0.5 V is observed after Φ = 5e13 cm−2.