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1er3 io-a 10"1 Hardness Level (cal/cm2)

Fig. 8-17. Cost to Harden a Satellite as a Percent of Total Satellite Cost Costs include production cost plus proportional share of engineering costs.

Prompt dose results from penetrating X-radiation and, to a lesser-extent from prompt gamma. Typically, the X-ray prompt dose is 3 to 4 orders of magnitude larger than the dose from prompt gamma. At typical spacecraft levels, prompt dose can break the bonds of the leads on susceptible integrated circuits and can cause electronic circuits to experience burnout, latchup, and temporary upset

In most cases, shielding of the X-radiation can reduce the internal dose to manageable levels. Figure 8-18 gives the prompt dose in silicon shielded by the basic 0.040-inch aluminum enclosure plus a range of additional tantalum shielding in g/cm2. We can use this figure to estimate the extra tantalum weight required to shield against the prompt dose. The procedure is as follows:

Fig. 8-18. Prompt Dose as a Function of Additional Tantalum Shielding (Worst Case 1-15 keV Spectrum).

Fig. 8-18. Prompt Dose as a Function of Additional Tantalum Shielding (Worst Case 1-15 keV Spectrum).

• Using Fig. 8-18, scale the maximum allowable prompt dose by the fluence appropriate to the system under consideration and determine the surface mass density (g/cm2) required;

• Multiply the surface mass density required by the total area to be shielded on the satellite.

The total dose is the sum of the ionizing dose from all sources of radiation and is usually expressed in rads (Si). In almost all cases, the total dose is dominated by trapped electrons in die geomagnetic field. Figure 8-19 gives die dose in silicon as a function of thickness of shielding material in g/cm2, normalized to an incident 1 MeV election fluence of 1014 electrons/cm2. The asymptotic nature of the dose curve for large mass densities results from the bremsstrahlung electrons produce as they stop in the shielding material. Thus, we would shield interactively for total dose and prompt dose. The prompt dose shielding also attenuates the radiation from the Van Allen belts, and the extra aluminum needed to attenuate the Van Allen belt radiation also attenuates the prompt X-radiation. For example, as Fig. 8-18 shows, an aluminum box 0.102 cm thick can reduce an external prompt dose of 3 x 108 rads (Si)- cal-1 ■ cm2 to an internal dose of 4 x 105 rads (Si)* caH • cm2. We can reduce the prompt dose even further by adding more high-Z material, such as tantalum or tungsten as shown in Fig. 8-18. This high-Z material also reduces the dose caused by trapped electrons, as mentioned above.

Metals are relatively unaffected by total dose. However, total dose degrades certain properties of organic materials, beginning between 0.1 and 1 Mrad, and makes them unusable above 10 to 30 Mrad. For example, organic materials may soften, become brittle, or lose tensile strength. NASA [1980] and Bolt and Carroll [1963] give data on how the total dose affects organic materials. Figure 8-20 shows the "sure-safe" total dose capabilities for commonly used satellite materials.

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