ing a solar cell analysis because the measurements are fit to the NIEL calculation over a much smaller energy range. For the case of a 3 mil SiO2 coverglass, less than 10% of the total displacement damage dose calculated for the 1989 solar flare event is contributed by protons with energies greater than 12 MeV [Mess97b]. Clearly, proton test energies should be chosen accordingly, with care taken to ensure the NIEL is not varying significantly as the particle transverses the device under test (see section 3.2). IV-97 GaAs/Ge Anspaugh 1990 Proton Energy (MeV) Electron Energy (MeV) solid lines are the characteristic curves Displacement Damage Dose (MeV/g) (Electron data - Effective 1 MeV electron Displacement Damage Dose (MeV/g)) Figure 28 Although the maximum power output correlates well with NIEL over the proton energy range shown, the electron fit was obtained by defining the equivalent 1 MeV electron displacement damage dose as the product of the particle fluence with NIEL raised to the 1.7 power. After [Walt99]. The light shielding employed in solar cells applications has another important consequence. Displacement damage from electrons must be considered in addition to proton contribution. It has been long recognized that a linear relationship between electron damage factors and NIEL is not always observed in Si [Cart66]. In the case of GaAs solar cells, Summers et al. [Summ93] presented short circuit current data showing a "nearly" linear relationship (on a log-log plot) between both the electron and proton data and the calculated GaAs NIEL using data from [Ansp92]. (Many of the correlations of calculated NIEL with device performance discussed in section 2.3 were initially presented on log-log plots covering many orders of magnitude, which makes it difficult to assess the degree of linearity where factors of 2-3 are significant.) More recently, Walters et al. [Walt99] have performed a detailed review of the above-mentioned GaAs data set, and now recommend separate fits to the NIEL in order to describe proton 