same defects in n-GaAs, there are also different defects produced by each particle. The bottom line for the satellite designer working a mission in a proton environment is that devices that are highly sensitive to displacement damage should be radiation tested with protons. We will see other reasons for this recommendation later in the short course. 2.3.2 Limitations in Usage of NIEL The NIEL calculation is a useful tool to approximate the expected proton induced radiation response in a space environment, but it is necessary to appreciate the underlying assumptions and limitations in order to use it effectively. Deviations at very low proton energies (approaching the displacement energy thresholds) are expected [Dale 88, Summ93], but they are not generally of concern for proton applications in space because they contribute little to the total displacement damage behind typical shielding, as will be shown in section 2.4. However, indications of other systematic deviations from the NIEL correlation have been observed in Si device measurements (e.g., for several CCDs, a CID, a 2N2907 bipolar transistor [Dale88]), and also in GaAs measurements (e.g., an LED [Barr95], a laser diode [Zhao97], solar cells [Walt99], and a JFET [Summ88]). Depending on how the damage factor measurements were normalized to NIEL, the deviations have been reported either as the damage factors being over-estimated by NIEL at higher energies, or equivalently, being underestimated by NIEL at the lower energies. The choice of a damage function (i.e., the energy dependence given by the calculated NIEL or experimental damage factors) has been shown to be significant. For example, one study found a factor of two difference in the on-orbit predictions of the degradation in Si CCD performance depending on which damage function is employed [Dale91]. IV-64 PROTON ENERGY (MeV) 2N2970A (n-Si) 1 MeV NEUTRONS 2N2222A (p-Si) CID (n-Si) CCD (n-Si) NONIONIZING ENERGY DEPOSITION (keV-cm2/g) Figure 7 Transistor damage factors and dark current damage factors for 