IM and SRIM family of codes, [18]). This figure shows the basic characteristics of ion-induced charge deposition as a function of depth. A peak in the charge deposition occurs as the particle nears its range, and then a precipitous drop in deposition as the particle reaches its range and comes to rest. The peak in charge deposition is referred to as the Bragg peak, and in general occurs as the particle reaches an energy near 1 MeV/nucleon. A more rigorous discussion of the Bragg curve and the Bragg peak is found in reference [19]. II-10 Bragg Peak Depth (um) Figure 3.1 Linear energy transfer (LET) VS. depth curve for 210-MeV chlorine ions in silicon. Whether or not the charge deposited through direct ionization is sufficient to cause an upset of course depends on the individual device and circuit that has been struck as well as the strike location and trajectory. Direct ionization is the primary charge deposition mechanism for upsets caused by heavy ions, where we rather loosely define a heavy ion as any ion with atomic number Z 2 (i.e., He and above, or put another way, particles other than protons, electrons, neutrons, or pions). Lighter particles such as protons do not usually produce enough charge by direct ionization to cause upsets in memory circuits, but recent research has suggested that as devices become ever more susceptible, upsets due to direct ionization by protons may occur [20,21]. This effect has not been experimentally confirmed to date. As discussed in references [21] and [22], these upsets will likely be difficult to observe and of limited importance to the overall upset rate because the upset rates will continue to be dominated by proton-silicon reactions for typical orbits and spacecraft configurations. In contrast to memories, upsets in photodiodes used in optocoupler applications have been observed and correlated to direct ionization by protons [23- 26]. Single-event current transients (SET) resulting from proton direct ionization are capable of causing upsets in these photodiodes because t