ns are preferentially stopped by spacecraft materials. When the naturally occurring spectrum encounters satellite materials, protons at all energies lose energy, but the mean proton energy reaching the payload electronics actually increases with increasing shield thickness. Along the trajectory of an individual proton, the path of ionization produces more and more electron-hole pairs per unit length until the proton approaches the end of its range. The LET actually peaks at an energy of about 80 keV in Si. This maximum in energy loss is often (inappropriately) referred to as the Bragg peak. IV-17 The precise details of energy loss are beyond the scope of this discussion, but it should be noted that proton dose is deposited along ionization tracks. On a micro-dosimetry scale the presence of these tracks leads to dose deposition that is therefore highly nonuniform in nature. In the following sections, ionization effects to electronic materials will be discussed and the inherent non-uniformity of proton dose on the microdosimetric scale will be placed into perspective by making comparisons to electron (or Co-60) dose deposition, which is much more uniform in nature. Finally, it should be noted that the coulombic electronic scattering mechanism is by far the dominant mechanism for ionization purposes and for affecting proton energy loss, but other processes do occur. Nuclear elastic and inelastic processes lead to some ionizing dose deposition, but these are orders of magnitude down from electronic scattering. By far their most important role in electronic materials is in imparting non-ionizing energy leading to atomic displacements. These processes are discussed in detail in Part B of this Short Course section. In the previous section on environments, it was noted that protons are encountered in all orbits. However, protons are significant contributors of TID only in certain cases. As indicated in figure 3, from the ISSA example in low-Earth orbit, the relative contribution of proton to electron dose increases with 