 shielded. For example, both the GaAs data (from [Ansp92]) and the InP data (from [Yama84]) are for protons below 20 MeV, and are indeed most relevant to lightly shielded solar cell applications. It is interesting to note that a recent paper based on the same solar cell data set [Ansp92], shows damage coefficients falling below the calculated GaAs NIEL at higher proton energies [Walt99], consistent with figure 9. (The authors did not discuss this trend which was not relevant to their recent solar cell study.) Clearly, further efforts are required to better understand the nature of these deviations. Experimental Data Burke et al., 1987 Summers et al., 1988 Barry et al., 1995 NIEL Calculation Summers et al., 1988 PROTON ENERGY (MeV) Figure 9 Experimental damage factor from several studies are normalized to the GaAs NIEL calculation at 10 MeV. A significant deviation between the observed damage factors and NIEL is apparent for proton energies above about 40 MeV. Adapted from [Barr95]. During semiconductor research efforts in 1950s, it was noticed that NIEL calculations (which compute that portion of the total energy deposited via non-ionizing interactions) significantly over-estimated defect production. Analytic expressions were developed with energy dependent damage efficiency coefficients that represented the likelihood that the initial Frenkel pairs would survive recombination, and experimental efforts confirmed this behavior in metals [Aver83, Hein83]. If the space radiation effects community plans to use calculated displacement damage functions to describe the energy IV-67 dependence of device response for more than rough approximations, it needs to move beyond NIEL calculations and investigate the time evolution of the initial damage to a variety of electrically active defects. It is not clear to what degree the physical processes need to be modeled in order to derive a sufficiently accurate damage function for use by the radiation effects community. In the meantime, the satellite designer must typically make 