We measure and investigate the relationship between well potential perturbation and MCUs (Multiple Cell Upsets) by neutron irradiation. Area-efficient cell-based perturbation detectors are placed adjacent to FFs (Flip-Flops). They can measure duration time of perturbation with 5 µm spatial resolution at two voltage levels. The measurement results by neutron irradiation on a 65-nm bulk CMOS show that 95% of MCUs occur simultaneously with well-potential perturbation, while there is very weak relationship between SEUs (Single Event Upsets) and the perturbation.