programs, and due to their analytic nature run very quickly. Analytic methods of computing track structure divide the target material into thin slabs perpendicular to the ion path. The incident ion and energy are specified, and the delta ray distribution and radial deposition of charge are calculated analytically in the first slab [237]. The particle energy is updated based on the energy deposition in the slab and the calculation proceeds to the next slab. In some codes, the LET in the slab is calculated by a call to a TRIM subroutine [18], and the energy is instead updated based on this LET and the thickness of the slab [118]. This method may be more accurate because the radial energy deposition tends to somewhat underestimate the energy loss in the slab, especially near the end of the ion path [237]. Analytical track structure codes have been extensively validated against available Monte Carlo results in the literature. Figure 5.1 compares the analytically-computed radial track structures of high- and low-energy ions that have the same incident LET of 11.4 MeV-cm2/mg: 210 MeV Cl and 5.04 GeV Kr [118]. The higher-energy ion strike is more nearly representative of a cosmic particle that might be encountered in the space environment, but such particles are difficult to achieve in typical laboratory particle accelerators. The Cl ion has an energy of 6 MeV/amu, and the Kr ion has an energy of 60 MeV/amu (ranges in silicon are 63.1 um and 1.1 mm, respectively). The generated charge density at the silicon surface is shown, as a function of radius from the center of the ion track. Note that both axes of this figure are plotted on a log scale, and the charge density is very 210 MeV Chlorine 5.04 GeV Krypton Gaussian Approximation 0.001 Radial Distance (um) Figure 5.1. High- and low-energy ion track radial charge distribution profiles [118]. Both ions have the same surface-incident LET. II-47 highly peaked about the center of the ion track. The maximum delta-ray radius at the surface for the 210 MeV Cl strike is about 1.