9 um, SO beyond this point the generated charge density falls to zero. For the 5.04 GeV Kr strike the maximum delta-ray radius is about 100 um, SO a low density of carriers (with respect to the central core of the track) exists out to this point. For the high- energy Kr ion, the charge deposited beyond 1.9 um amounts to less than 15% of the total. Since the curves have the same integral charge (i.e., the ions have the same surface-incident LET), this means the high-energy ion has 15% less charge in the central region of the track (evidenced in Figure 5.1 by the fact that the Kr curve is slightly underneath the Cl curve at small radii). If there were a difference in the SEU response caused by the two ions, one would expect the low-energy ion to be more upsetting since the track is slightly more concentrated and can deposit a greater amount of charge in a small sensitive volume [118]. For the high-energy ion, more charge is deposited at large distances where it may not be collected by the sensitive node [239]. Still, this is expected to be a small effect, since SO little of the high-energy ion's charge is deposited past the boundary of the low-energy ion's path. For a given ion strike, there is also the more fundamental issue of variation of charge density along the path (i.e., LET is not constant as a function of depth, as seen in Fig. 3.1). Detailed studies of the importance of faithfully representing the track structure within device simulations have shown that it is important to include this effect [240,241]. Neglecting this effect can change the transient current response by as much as 20%, while various fits to the radial track structure typically produce less than a 5% change in the total charge collected for ion strikes on simple p/n diodes [241]. In any event, since the analytic track structures are relatively easy to compute, simulation track structures including at least the variation of LET along the path and reasonable estimates of radial charge distribution are recommended [240]. Physics-Based Device 