Altitude and underground real-time soft error rate (SER) measurements on SRAM circuits have been analyzed in terms of independent multi-Poisson processes describing the occurrence of single events as a function of bit flip multiplicity. Applied for both neutron-induced and alpha particle-induced SERs, this detailed analysis highlights the respective contributions of atmospheric radiation and alpha contamination to multiple cell upset mechanisms. It also offers a simple way to predict by simulation the radiation response of a given technology for any terrestrial position, as illustrated here for bulk 65nm SRAMs.