With greater use of commercial electronic parts in space, there are often cases when the shielding provided by the spacecraft structure is insufficient, and alternative radiation-resistant parts are not available. In such cases, spot shielding of the part is appropriate, including integrating shield material (high-Z elements like tantalum or tungsten) into the part package itself, a technique often referred to as “RadPak.” As noted above, the implementation of these various shielding schemes to reduce the expected total dose is aided by the fact that shielding calculations used to determine the effect of a material on the flux and energy spectra of various types of radiation are well established. These calculations also take into account the production of secondary adiation.While shielding can facilitate the use of “soft” parts in many cases, it is important to realize that shielding is not always effective, and can even make the situation worse.

As a rule of thumb, shielding is most effective in reducing the low to moderate energy component of ionizing radiation, that is, electrons and protons. For very-high-energy radiation, such as gamma rays and GCR ions, shielding is not particularly effective, and can even be detrimental. This is partly due to the production of secondary radiation in the shield material, and also because the energy loss per unit length in the electronic part can increase with decreasing particle energy. The production of secondary radiation results in the asymptotic behavior of shielding effectiveness. In other words, the first thin layer is much more effective in reducing radiation than the additional thicknesses of shielding, as shown in Figure 10-6. Thus, beyond a few tens of mils of Al-equivalent shielding, the weight penalty is often more important than the added benefit of the radiation shielding.

The effectiveness of shielding in reducing the overall amount of radiation impinging on spacecraft electronics causes a sharp distinction between the natural space environment and the nuclear weapon-enhanced environment [1]. As noted earlier, weapon detonation results in a burst of prompt, high-energy gamma rays and, somewhat later in time, a significant fluence of 14-MeV neutrons. Shielding is of little use in moderating either of these radiation threats. Similarly, it is difficult to shield against the neutrons and high-energy gamma rays continuously emitted by RTGs as a byproduct of the power generation process. For this reason, the RTGs on Cassini have proven to be a challenging radiation threat to nearby microelectronics on the spacecraft.

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