 InP [Summ93], and Si [Dale94]. Ionizing Non-lonizing 0.001 PROTON ENERGY (MeV) Figure 5 Comparison of the energy loss rate through ionization and excitation of the Si lattice (LET), and through atomic displacements (NIEL) over a wide range of proton energies. The LET was calculated as in [Zeig85], and NIEL as in [Dale94]. IV-60 The nature of displacement damage as a function of proton energy is governed by the interaction cross sections, and the non-ionizing energy of the PKAs as governed by the Lindhard function. For proton energies below about 10 MeV, Coulomb elastic scattering is by far dominant in Si, and produces atomic recoils with non-ionizing energies in the hundreds of eV. At higher energies, the bend in the curve occurs because nuclear elastic scattering becomes more important resulting in recoils with non-ionizing energies in the tenths of MeV range. As the incident proton energy increases the elastic cross section decreases athough it is still larger than the inelastic cross section. By about 100 MeV half of the non-ionizing energy imparted to the Si lattice is from nuclear inelastic reactions with a mean PKA non-ionizing energy that is still about 0.1 MeV (due to the Lindhard partition). NIEL has also been calculated by other means including Monte Carlo programs such as HETC [Alur91], CUPID [McNu81, McNu94] and TRIM [Zeig84]. A comparison between the most recent Burke and CUPID calculations of Si NIEL is discussed in [Dale94]. Although HETC, CUPID and Burke's calculations of the recoil distributions as a function of incident proton energy show similar trends, they differ in details [Dale94]. The TRIM program only includes the Coulombic interactions, SO it is not appropriate to use it directly for damage calculations for proton energies above about 8 MeV or so, depending on the target material. Note that all of the above calculations include a "fudge factor" that accounts for the fact the most of the initially produced vacancy-interstitial pairs recombine and therefore do not produce electrically act