pulsed, chaged-particle beam, needed within the atmosphere, will probably not propagate well in vacuum. We will consequently assume space-space encounters below. The most basic "kill” process for the exoatmospheric PBW beam is the volume deposition of the beam energy within the target. Typical values of the energy deposition density associated with various kill mechanisms are listed in Table C.2- 2. Structural damage by melting can evidently be accomplished for energy deposition densities of about 100 cal/g. Warhead damage might be accomplished either by structural effects or by thermal initiation of the high explosive (HE) content, at somewhat lower temperature rise requirements. Damage to electronic circuitry can also be accomplished structurally—with solder melt at the indicated temperatures as an effective but less demanding criterion than silicon melt—or by heating the components beyond acceptable operating tolerances. Finally, a variety of "microscopic" mechanisms such as lattice damage to semiconductor materials, can also disable electronic circuits. The severity of such damage mechanisms depends in a complicated way on the rate and duration of the exposure and on the temperature history of the components during and after exposure. Without detailed consideration of a particular threat description, the values shown in the table for "microscopic" mechanisms are to be regarded as order-or-magnitude estimates. C.2.3 Safeguards/Countermeasures The penetrating character of a particle beam makes it quite difficult, once the target has begun to be irradiated, to shield against it. Indeed, heating and melting damage to the shield may create serious local problems even if integrity of the shield could be maintained. Any adequate shield would be enormously bulky. A nominal beam (250 MeV protons) would penetrate a shield with surface density in the 50-100 g/cm^2 range (500-1000 kilograms/m^2 ), i.e., steel armor 7-14 cm thick. Other countermeasures suggested include artifices to deceive the target acquisition/tracking/fire-control components, such as decoys and chaff; defensive PBWs could be overcome by saturating the system with multiple warheads and decoys.
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