 or from the proton upset saturation cross section. Once determined from either data set, the same FOM can be used to estimate IV-28 upset rates from either trapped protons or from heavy ions. The aggregate upset rate will then be the combination of the two contributions. The referenced paper shows good agreement for a variety of device types with varying levels of SEE sensitivity. In some instances it is of interest to assess neutron SEE sensitivity, such as in avionics applications. In [Norm-98] the Burst Generation Rate (BGR) technique for assessing proton upset sensitivity is compared with neutron BGR calculations and data. If neutron SEE data were available on a device, this approach could be taken to gain an idea of proton upset sensitivity, but as the reference indicates the correlation is not precise. Though it was not mentioned in the introduction where reasons for proton testing were listed, one possibility would be that the assessment with protons could be used with the BGR correlation to estimate neutron SEE sensitivity for avionics applications. 4.2 Proton Direct Ionization and SEEs The preceding section examined special cases of the conventional indirect proton SEE mechanism, which involves heavily ionizing nuclear reaction recoil products. Until just a few years ago, this was considered to be the only important mechanism for proton-induced single event effects. In several recent studies, SEEs due to direct ionization by protons have been reported. Though these may be "special" cases, their treatment in terms of mechanism identification, test issues, hardening solutions, and rate predictions are unique, and the remainder of this section will address these issues. At the outset, we note that for the indirect mechanism to occur there must be a reaction, and the reaction cross sections are SO low such that only about 1 proton in ten thousand undergoes such an event. Most protons traverse the region and leave only an ionization track, which often matters little. However, if the circuit is sensitive to t