M SHIELDS: 0.6x10 0.5 g/cm2 2 1 g/cm 0.4x10 2 g/cm2 8 g/cm2 0.2x10 PROTON ENERGY (MeV) Figure 11 The integral damage spectrum (integrated from the energy in question to the highest proton energy) is shown versus proton energy. The intercepts at zero energy give the yearly total damage for the entire proton spectrum. The values in order of increasing shield thickness are 9.2x106, 6.7x106 5.3x106, and 2.93x106 MeV g(Si)-1 year-1. The corresponding CTE losses per year given from the right ordinate are 3.6x10-4 2.6x10-4, 2.0x10-4 and 1.1x10-4 respectively. After [Dale93]. In some cases, the device parameter of interest does not degrade linearly with proton fluence (or displacement damage dose). This behavior has been observed for degradation of optocouplers [John96, Reed98] and solar cells [e.g., Ansp92, Yama96, Mess97, Walt99], and is discussed further in sections 3.3.5 and 3.3.6. In this case, we measure the device response as a function of displacement damage dose (or MeV- equivalent fluence at an agreed upon energy) to assess device performance in relation to the mission requirements. (Care should be exercised when the mission requirement falls in a regime where the device response is changing rapidly with increasing damage dose.) Note that the underlying assumption in this case is still that the device degradation correlates with the NIEL as a function of proton energy. To the extent that this is true, the curves measuring the device degradation as a function of fluence for various proton energies will fall on a common curve if they are plotted versus proton dose calculated using NIEL. IV-73 In summary, the NIEL-based methodology provides the satellite designer with an invaluable tool for estimating on-orbit degradation due to displacement damage and making use of the displacement damage literature, providing they are used with caution. This is particularly true since the radiation engineer must typically make assessments based on device radiation measurements at one (or at most a few) proton energies, which req