An expression for SEU (single event upset) cross section for a unidirectional, monoenergetic ion beam is developed. The expression incorporates both the stochastics of the energy deposition process and the variability of the response of an array of memory cells. The energy deposition stochastics result from the distribution of path lengths the ion may take through a memory cell and the statistics of the charge production process. The variability in the memory response results from the dependence of the sensitive volume size on deposited charge and cell-to-cell variations due to processing. Thus the shape of a measured upset cross section curve is the result of the complex interplay of a number of physical processes whose relative contributions vary with experimental conditions, memory technology, and processing. A procedure for the separation of the basic memory response from the stochastics of the energy deposition process using heavy ion SEU measurements is given. This can be used to predict the SEU vulnerability in any radiation environment. Results of the model applied to data for bipolar and CMOS/SOS SRAMs (static random-access memories) are presented. >