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A non-conventional upset mechanism is proposed to explain the proton test results for our 4M SOI SRAM for the first time in this paper. In a hardened SRAM cell where an active delay element is often used in the feedback loop, single particle hits to a single critical node are not likely to cause an SEU upset. However, when the secondary heavy ions, created by the interactions between high-energy protons and Si nuclei, travel through a critical node as well as the pass gate inside the delay element, the delay element will be shunted out by charge deposited and as a result the stored state can easily be disturbed. Simple calculations based on this assumption yield good correlation to test results in terms of upset cross-section. This upset mechanism will play a more important role as device geometries shrink.