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Shield Material - g/cm2

Fig. 8-19. Dose from Trapped Electron Fiuence of 1014 Electrons/cm2 as a Function of Thickness of Shielding Material In g/cm2.

Polystyrene Fused Glass

4-:-^ Silicon Caibon

Teflon (FTFE) BakeBte

Rubber Qaskets A-►

Germanium Optical Materials Burta-N 4—:-^ ^—_-^

Neoprene Vycor Sealing Compounds

VltonA^ Sfficone Silicone Resin

Polyvtnytehlortde Propylene Mylar 4-^ ^¡^

Nylon CopperClad Conductive Adhesive

Polychloroprene <->

Insulation Natural Rubber Polyethylene

10s - 10® 107 10« 10s 1010 Radiation Dose [rads (SI)]

Fig. 8-20. Total Dose Capabilities of Satellite Materials.

Silicon electronic devices suffer decreases in operating parameters such as gain, gate voltage, or lifetime of the minority carrier. We can measure these operating parameter changes as a function of dose and develop curves of radiation deratings. Thus, during circuit design, these radiation deratings are used to ensure that the devices will continue to operate satisfactorily at the design exposure level.

Total dose also includes the ionization from prompt and delayed weapon radiation, as well as neutron-generated radiation. To the total dose from man-maide sources of hostile radiation, we must add radiation from the natural environments. The total dose depends on the amount of shielding, orbital parameters, and satellite life. In the absence of nuclear-weapon detonations, the total dose will normally increase linearly with time on orbit To harden a satellite against these effects, we would use silicon-based electronic devices which tolerate the effects and shield them to the appropriate level, depending on how long we want the satellite to last In the future, we will increase the use of electronic devices based on gallium arsenide, because gallium arsenide appears to be unusually immune to total dose effects.

Radiation hardened parts are required for all designs that must operate in nuclear weapon environments, but some commercial communications satellites can consider using radiation tolerant parts (<50 krads capability) or even commercial off the shelf (COTS) Class B type parts (10-15 krads capability), particularly if they will only operate in low-Earth orbits Qess than about 1,000 Ion) and have orbital design lifetimes of 2 to 3 years maximum. Table 8-7 shows a comparison of typical unhard-ened COTS parts and hardened parts capabilities.

TABLE 8-7. COTS and Rad Hard Parts Comparison. Rad hardening Increases radiation protection significantly, thus increasing spacecraft survivability.

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