itial pairs to the formation of stable defects. The annealing of Frenkel defects occurs in less than 1 millisecond and stable defects are formed on the time scale of seconds. Displacement Damage Processes in Si PROTON ENERGY 6-10 MeV > 20 MeV Log N SINGLE MANY SUB DEFECTS, CASCADE, CASCADES, Coulomb Nuclear Elastic Nuclear Reactions RECOIL ENERGY 1-2 keV 12-20 keV Figure 3 Pictorial relating the initial defect configuration to the primary knock-on atom (PKA) energy in Si material. Note from the plot of the number of interactions (N) versus incident proton energy that most interactions are Coulomb events producing isolated defects. For recoil energies IV-55 above a couple of keV, the overall damage structure is relatively unchanged due to the formation of cascades and subcascades. After [Wood81]. The final configuration of electrically active defects formed by particle irradiation has been a topic of much research, but is still not well understood. As we will see this issue is at the heart of understanding the use and limitations of calculated non-ionizing energy loss rate (NIEL) damage functions to predict the displacement damage response of a device in a proton environment. Figure 3 is a pictorial of the spatial distribution of the initial vacancy-interstitial pairs in Si investigated using the Monte Carlo code MARLOWE [More82]. As can be seen from the plot of the log of the number of interactions (Log N) versus the incident proton energy, most events are Coulomb interactions which produce PKAs with Ethreshold < E < ~2 keV, and result in isolated defects. Although there are many fewer of the nuclear elastic and inelastic reaction events that produce cascades, these events are far more damaging, and can contribute a significant fraction of the total displacement damage at higher proton energies. As indicated in the figure, recoils with energies between about 2-10 keV produce single subcascades, whereas those with energies in excess of 12-20 keV form a tree-like structure with branches containing multiple subcasca