Experimental monoenergetic proton single-event upset (SEU) cross sections of a 65-nm low core-voltage static random access memory (SRAM) were found to be exceptionally high not only at low energies (< 3 MeV), but also at energies $>$ 3 MeV and extending up to tens of MeV. The SEU cross Section from 20-MeV protons exceeds the 200-MeV proton SEU cross Section by almost a factor of 3. Similarly, monoenergetic neutron cross sections at 14 MeV are about a factor of 3 lower than the 20-MeV proton cross section. Because of Monte Carlo (MC) simulations, it was determined that this strong enhancement is due to the proton direct ionization process as opposed to the elastic and inelastic scattering processes that dominate the SEU response above 3 MeV in other SRAMs. As shown by means of a detailed energy deposition scoring analysis, however, this does not appear to be caused by the critical charge of the SRAM being lower than the charge resulting from the average proton ionization through the linear energy transfer (LET). On the other hand, this is caused by high-energy $\delta $ -rays ( $>$ 1 keV) that can deposit their full kinetic energy within the sensitive volume (SV) of a cell despite their range being theoretically much longer than the characteristic size of the SV. Multiple Coulomb scattering events are responsible for increasing the trajectory path of the $\delta $ -rays within the SV, resulting in a six-fold increase in the probability of upset with respect to the sole electron ionization.
