 Some of these electrons (called delta rays) are liberated with sufficient energy to interact with other electrons at some distance from the incident proton's trajectory, thereby leading to an ionization track with some structure. This coulombic scattering process liberates electron-hole pairs at a rate that depends on the proton energy and also on the material it traverses. In Si, for example, the electron-hole pair creation requires (on average) 3.6 eV in energy from the incident proton. This empirically determined value is referred to as the ionization potential, and it depends on a number of factors including the material band-gap for the case of semiconductors. In insulators, ionization potentials are significantly larger, e.g., ~ 17 eV in SiO2. IV-16 LET and Range for Protons in Si 1.E+00 1.E+03 LET (MeV*cm2/mg) Range (cm Si) 1.E+01 1.E-01 1.E-01 1.E-02 1.E-03 1.E-03 1.E-05 Proton Energy (MeV) Figure 7. Lower energy protons with higher LETs are effectively stopped in satellite structural materials due to their short ranges. Higher energy protons are very penetrating, but fortunately transfer their energy at a lower rate. These relations do not address nuclear reactions occur for higher energy protons (>10 MeV) with a probability of about 10 -5 for a pathlength of a few microns in Si. The rate of energy loss for a proton (or any heavy ion) is termed the Linear Energy Transfer (LET) or stopping power, and the usual units are MeVcm2/mg though it can be converted to energy per unit pathlength by multiplying by the target material density. Again, this is an empirically determined relation, and its dependence on proton energy, known as the specific ionization curve, is shown in figure 7 along with the relation between proton range and energy. Note that the LET tends to decrease with increasing energy in the MeV regime encountered within satellites. Also, as the proton loses energy, the decrease in range is highly nonlinear. These energy loss kinematics are the basis for the situation in which the low energy proto