elf, namely models for simulating ion track structure in semiconductors, and device/circuit models that predict the response of devices to the ion track. Models for predicting error rates have been covered elsewhere [19,22,227]. Interaction Models As discussed earlier, energetic particles can interact with an IC through either direct ionization or nuclear interactions. Computer codes have been developed that simulate spallation reactions and/or upset rates for protons, neutrons, and other cosmic ray components [36,228- 232]. Since spallation reactions eventually cause upset through the direct ionization process also, we will limit our discussion here to models that predict the direct ionization-induced ion track structure in the semiconductor. The reader is directed to the above references if they desire further information on the simulation of nuclear reactions. 5.1.1 Track Structure Models An aspect of SEU and charge-collection simulation that has received considerable attention recently is that of the ion-strike track structure used as input to physical device simulation. Most work until the last few years has employed a simple cylinder of uniform charge generation to represent the ion strike. Detailed calculations of ion strike track structure have been performed using Monte Carlo methods [128,233]. As mentioned in Section 3.7, differences in the track structure between typical particles used in SEU experiments and high-energy particles which might be encountered in a real space environment have raised concerns over the fidelity of the operating environment simulated by accelerator tests [118,128]. Charge production around the path of an incident particle is accomplished by the release of energetic electrons (also referred to as delta rays) along the track, which subsequently travel away from the path and produce further electron-hole pairs. The higher the energy of the incident particle, the higher the energy of the delta rays and the larger the radial extent of the induced charge distribution. As it passes 