he surface dark current is suppressed.) The average dark current increase is characterized by a damage factor defined as the change in dark current per unit proton fluence. As shown in figure 18, lower energy protons are more damaging than higher energy protons as expected based on the NIEL energy dependence. (Charge injection devices (CIDs) were used in this study because they have x,y addressable arrays of pixels that are not subject to CTE degradation. These CIDs also had hard oxides.) Although the increase in the mean dark current with proton irradiation is important, the dark current nonuniformity is generally the biggest concern for CCD applications in space. NIEL is an average quantity just as stopping power (or LET) is an average quantity. For ionization effects, the departure from the average dose delivered in a uniform medium is small down to dimensions measured in hundreds of cubic nanometers. This is not the case with NIEL. For Si detector arrays (such as CIDs or CCDs), with pixels measured in the tens or hundreds of cubic microns, the displacement damage sustained by adjacent pixels can vary considerably even though the identical pixels are exposed to the same environment [Dale89b, Dale90, Mars89b, Mars90]. This nonuniformity is inherent to the statistical nature of the collision kinematics producing the displacement damage and therefore cannot be hardened against. IV-85 DARK CURRENT (nA/cm2 2 ) 12 MeV PROTONS 4 X 101/cm2 000 EXPERIMENT N = 1967 CALCULATION N = 1.8 1 X 1011/cm2 N = 4918 N = 4.5 2 X 1011 1/cm2 N = 9835 Ni = 9.0 DAMAGE ENERGY (MeV) Figure 19 CID dark current histograms after exposure to increasing proton fluences. As the number of primary interactions per pixel, N, increases the distribution approaches a gaussian distribution. The high energy tail is produced by very infrequent but large nuclear reaction events. (N; is the average number of inelastic interactions per pixel.) After [Mars90 and Dale89b]. Figure 19 illustrates the spread in the dark current increases in individual pixels 