n techniques may be found in [Hopk96] and references therein. 0.0006 EEV CCD02 after 7.2x10 9 10 MeV p/cm2 2 0.0005 Dark charge background 0.0004 = 150 electrons = 500 electrons 0.0003 = 2 000 electrons = 16 000 electrons 0.0002 0.0001 50000 100000 150000 Signal Size (electrons) Figure 15 The charge transfer inefficiency (CTI = 1-CTE) for a CCD exposed to a proton fluence of 7.2x109 cm-2, corresponding to TID of 4 krad(Si). Both the CTI and the efficacy of a dark charge background in CTI reduction are a function of signal size. After [Hopk94b]. For most satellite programs these mitigation techniques are not sufficient, and one must resort to shielding. Recall from part A of this segment that protons are not easily shielded against. In fact, quite thick (a cm or more) of high atomic weight shielding (e.g., Ta or W alloys) may be used to minimize displacement damage to the CCD. Recall that in section 2.4, we used the Si NIEL correlation together with measured CTE damage factors to predict the on-orbit performance of a CCD. The results were displayed figure 11 for several shield thicknesses, and confirmed the need for substantial shielding to maintain reasonable CTE performance in space. However, for heavily shielded IV-82 devices, the displacement damage caused by secondary particles produced in the shielding itself is significant, and in some cases dominant [Dale93]. (Incoming protons may interact with the atoms in the shield material causing nuclear reactions that produce secondary particles such as neutrons and protons.) The NIEL prediction methodology presented in section 2.4 is readily extended to include the displacement damage effects of secondary particles on CTE degradation. Figure 16 shows the relative contributions to the total displacement damage from the primary (incoming) protons and the secondary neutrons behind Ta shielding for a particular orbit. Of the secondary particles produced, neutrons are the greatest concern because they penetrate beyond shield depths that stop most primary protons, and yet