 their interaction cross-sections are significant. Secondary protons produced in nuclear reactions within the shield do not contribute significantly to the device damage since they have short ranges and are stopped in the shield itself. Note that for low atomic number shields such as Al, the displacement damage caused by secondary particles is much less significant. In addition, figure 17 shows that per unit mass, a lower atomic number shield such as Al, minimizes the amount of displacement damage to a CCD [Dale93]. Nevertheless, per unit thickness Ta clearly surpasses Al in shielding efficacy due to it greater density. Since there is generally not the space (or weight budget) to accommodate bulky Al shielding in a satellite application, higher atomic number shields are typically utilized to protect very soft devices. SHIELD THICKNESS (g.cm 2 TANTULUM SHIELD TOTAL PROTONS NEUTRONS TRAPPED PROTONS SAGE ORBIT 705 km, 97.4° SHIELD THICKNESS (CM) Figure 16 Displacement damage dose behind a Ta shield in a trapped proton environment. Contributions from all protons and secondary neutrons are shown. (The secondary proton dose is negligible.) The secondary neutron dose dominates after about 2 cm Ta. In contrast, for low atomic number material such as Al, secondaries do not contribute significant displacement damage. After [Dale93]. IV-83 TRAPPED PROTONS SAGE ORBIT 705 km, 97.4° TANTALUM ALUMINUM SHIELD THICKNESS (G/CM²) Figure 17 Comparison of displacement damage dose behind Al and Ta shields in density independent units (g/cm2). Al is the superior displacement damage shield per unit mass. However, since Ta has about six times the density of Al, Ta is always the better shield per unit thickness. After [Dale93]. Perhaps the most difficult (and important) choice facing a radiation effects engineer is which measurement technique to employ in evaluating the proton induced CTE loss. Some of the more commonly used techniques are described in [Hopk96], and references therein. One measurement technique is to plot the intensity an