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Protons that produce significant direct ionization in a sensitive volume also slow down, so the LET is not constant in large sensitive volumes. A single-event cross section for energy deposition within the volume that takes this into account is calculated analytically. It is up to the user to select a critical energy and a sensitive volume that gives the best representation of the charge-collection physics for the purpose of calculating SEU rates from protons by direct ionization for the device of interest. A complete set of equations is given only for an ellipsoidal sensitive volume, but other geometries could be treated once the required analytical functions have been provided. An example that utilizes the ellipsoidal sensitive volume is considered and is compared to measured cross sections.