 previously described Monte Carlo calculation of collision cascades showing that the more dense subcascades do not begin to form until PKAs have energies over about 2 keV. Later measurements of the CTE degradation in Si CCDs (from 2 manufacturers) over a wide range of proton energies also reveal enhanced damage at lower proton energies [Dale93]. However, we note that such deviations were not apparent in a study by Luera et al. [Leur87]. 25 eV < E < 1 keV 1 keV < E < 10 keV 1 keV < E < 2 keV 2 keV < E < 5 keV DAMAGE ENHANCEMENT FACTOR Figure 8 The correlation between percent NIEL in Si due to recoils in the various energy ranges and the magnitude of the deviation from the ideal linear dependence is shown. The particles associated with a given deviation are labeled at the top of the figure. After [Dale88]. Evidence that lower energy protons are more effective at producing displacement damage in GaAs as compared to higher energy protons (than the NIEL correlation would indicate) has been reported by Luera et al. [Luer87, Grif91] and Barry et al. [Barr95]. These studies were based on measurements of carrier removal in Van der Pauw samples and minority carrier lifetime degradation in LEDs. Once again, the results were explained by variations in the recombination efficiency of the Frenkel pairs with PKA energy. In 1995, Barry et al. extended measurement of the minority carrier lifetime damage factors in GaAs LEDs to proton energies as high as ~500 MeV [Barr95]. IV-66 Figure 9 compares these results with the NIEL calculation by Burke [Burk87]. Other results in the literature also indicate departures of damage factors from the NIEL energy dependence [Zhao97, Summ88]. Although the 1993 paper by Summers et al. claims to have demonstrated a general linear correlation between device "proton damage coefficients" and NIEL for Si, GaAs and InP, using "solar cells as examples," it is important to note that the data presented do not cover the relevant range of proton energies for most satellite applications which are more heavily