haracteristics of devices are represented using the measured number of upset events, and the particle fluence used to produce the events, to calculate a cross section. Plotting of the measured cross section data and fitting the appropriate curve to the measured cross section values over a range of LET/energy produces the device single event upset characteristic. Both heavy ion and proton characteristics exhibit similar features of a threshold value in LET/energy and an increasing cross section which asymptotically approaches a maximum value. In characterizing the single event upset susceptibility of components, it is important to utilize sufficient particle fluence to produce a statistically significant result. The statistical uncertainty in the measured cross section for some measured upset counts is: Upset Events Uncertainty In o + 100% + 71% + 58% + 45% + 32% + 22% + 14% + 10% In addition to the statistical uncertainty in the measured cross section at various energy/LET values, consideration must be given to the changes in single event upset characteristics which will result from device operating voltage and temperature conditions in the application, distribution of component characteristics across manufactured population of devices, and the changes to device susceptibility which could result from total ionizing dose accumulation. Distribution of part and application conditions potentially impart a distribution about a mean value; total ionizing dose accumulation often causes the single event upset susceptibility of devices to worsen over mission life. In characterizing a device susceptibility to both heavy ions and protons, it is important to consider which segments of the cross section versus LET/energy curve have the dominant influence upon the rate calculation. For proton induced single event upset estimation, the portion of the energy spectrum between 40 MeV and 60 MeV results in the most significant contribution to the overall SEU rate, due to the population of protons of this energy. For this reason, it