Q

100 1,000 10,000 100,000 Circular Orbit Attitude (km)

B. Nuclear-enhanced electron dose In 30 days, 30 deg Inclination circular orbits, for one value of aluminum shielding (0.254 cm).

Fig. 8-15. Nuclear-enhanced Electron Environment, 30 Days Duration. Figure assumes 30 deg Inclination circular orbit and 0.1 inch aluminum shielding.

100 1,000 10,000 100,000 Circular Orbit Attitude (km)

Figure 8-1 SB gives the dose resulting from a 30-day exposure to a nuclear weapon-enhanced electron flux for one value of aluminum shielding and as a function of orbital altitude. The dose for shorter periods can be estimated by linear scaling; however, longer periods cannot be estimated by linear scaling since the saturated environment decays rapidly. As with the natural trapped electrons, the nuclear weapon-enhanced electron flux is practically nonexistent near the north and south poles. For satellites with higher inclination orbits, i.e., greater than 60 to 70 deg, the accumulated dose is greatly reduced, compared to satellites with inclinations of zero to 60 or 70 deg.

An example will show how these calculations work in practice. Consider a satellite which must operate for 1 year in the natural environment and then operate (survive) for 1 day following a high altitude nuclear explosion which creates an electron-enhanced Van Allen belt We assume a circular orbit, 30 deg inclination and an altitude of 6,(XX) km. For a wall thickness of 0.254 cm (corresponding to a shielding value of 0.69 g/cm2), Fig. 8-15A gives a dose of about 130 krads in 1 year. Figure 8-15B gives a dose of 21 Mrads for 30 days and 700 krads for 1 day. Adding the two, we get 830 krads. The electronics must be able to function properly after accumulating a total dose of this amount Solid-state electronics can be hardened to tolerate from a few krads to about 1 Mrad so our satellite, if hardened to 830 krads or more, would satisfy the survivability requirement of 1 year natural plus 1 day weapon enhanced.

Nuclear Weapon Effects on Materials. The X-radiation pulse lasts tens of nanoseconds, and its energy is absorbed almost instantaneously in solid material through the photoelectric effect and Compton scattering. In the photoelectric effect, bound electrons of the material are ejected from their atomic orbits and take on a kinetic energy equal to the difference between the energy of the incident photon and the atom's ionization energy. The incident photon disappears in the photoelectric effect, with its absorption per atom proportional to the 5th power of Z, the atomic number of the absorbing material, and inversely proportional to the 7/3 power of the incident photon's energy [Heitler, 1954]. Therefore, high-Z materials shield against X-rays more effectively, and the absorption cross section decreases dramatically for incident-photon energy from 1 to 20 keV (1 keV = 1.6 x 10"16 J).

Compton scattering is an elastic scattering event in which an electron receives some of the energy of the incident photon, and the incident photon changes direction. As a result, the photon's energy decreases and its wavelength increases [Heitler, 1954], The cross section per atom for Compton scattering is proportional to Z and, for the range of photon energies we are interested in, is inversely proportional to the incident photon's energy. Therefore, for Compton scattering, increasing Z only slightly increases the absorption coefficient, whereas the cross section decreases moderately as the photon energy increases.

The energetic, free electrons described above can cause electronic circuits to malfunction, and their energy ultimately appears as heat in the material. In fact, the material heats rapidly enough to create shock waves which develop tensile stresses that may cause spall at its unconstrained boundaries. If the deposited energy is high enough (usually not the case at typical satellite fluence levels), the material may vaporize or melt, creating direct damage in addition to the shock waves. For spacecraft, where flux or fluence levels are low, malfunction of electronic circuits is the most likely occurrence.

Gamma rays resulting from nuclear explosions range from a few hundred keV to several MeV. In preliminary designs, we can assume that gamma rays have a mean energy of approximately 1 MeV and interact with matter primarily through Compton scattering. Because gamma radiation is very penetrating, we cannot effectively shield against it. Thus, when we wish to protect against the less penetrating, but more highly ionizing, X-radiation, we need only provide enough shielding to reduce the prompt dose to levels approximately equal to that of gamma radiation. Figure 8-18 of

Sec. 8.23 gives the prompt dose induced by a unit fluence of X-radiation as a function of additional shielding (the abscissa of the figure can be converted to linear dimensions by dividing by the density of the shielding material). For higher or lower fluences, linear scaling is appropriate.

Neutrons interact with material by colliding with atomic nuclei. The collisions impart energy to the atoms of the material and displace the atoms from their normal positions in the lattice. Changing the lattice structure can seriously harm solid-state electronic devices because they depend on the characteristics of the lattice for their function. At fluences greater than about 1012 n/cm2, neutrons can cause solid-state devices to stop working, thus "electronically killing" a satellite.

Effects on Communications. A nuclear weapon detonated in space near the Earth interacts strongly with the atmosphere and the Earth's magnetic field. The electromagnetic energy radiated from the detonation creates large-scale ionization in the bomb material and in the atmosphere. Radioactive debris contributes beta particles (positrons and electrons) from radioactive decay. The ionized bomb debris and beta particles move along the lines of force of the geomagnetic field, as described in Sec. 8.1. As the magnetic field lines enter the atmosphere, the energetic particles interact with it, creating more ions and electrons. The free elections thus created absorb and reradiate RF energy and refract the electromagnetic waves of the radio communications links between ground and satellite, creating phase and amplitude changes. These in turn reduce the signal strength in radio receivers, thus interrupting communications.

Based on the theory of electromagnetic propagation, the attenuation, a, in dB per km, is given by a = 4.4 x \&Nevl{(2 itf)2 + v2) dB/km (8-13)

where Ne is the number of electrons per cm3, v is the frequency of collision of elections with ions, atoms or molecules in Hz, and /is the frequency of the electromagnetic radiation in Hz. The values of these parameters are difficult, if not impossible, to obtain. However, the U.S. Defense Threat Reduction Agency (formerly the Defense Special Weapons Agency) can provide computer programs for propagation analyses in nuclear environments, assuming appropriate clearances and need to know can be established.

For space-to-ground links, we can use the form of Eq. (8-13) and the fact that the density distribution of the atmosphere is approximately exponential to infer the absorptive behavior of RF signals as a function of frequency. The form of Eq. (8-13) indicates that the absorption passes through a maximum as a function of collision frequency, v, which is proportional to the density of air. Above approximately 80 km, the density is so low that v is essentially zero and absoiption does not occur. Below about 60 km, electrons rapidly reattach to atoms and molecules, so the low electron density again leads to small absorption. Therefore, the attenuation is at a maximum for any given radiation frequency between 60 and 80 km. In this region the attenuation varies with the inverse square of the radiation frequency. Thus, we should choose the highest communication frequency we can to minimize attenuation due to nuclear weapons environments. For a more complete treatment of nuclear effects on communications, see Mohanty [1991].

High data rate requirements for military satellites with surveillance sensor payloads and future commercial communications satellites have resulted in the use of optical/ laser links in modem systems. While having many advantages over RF links, such as weight and power, optical link components are also affected by nuclear environments. Table 8-6 contains general guidelines for the effects of nuclear radiation on optical link components.

TABLE 8-6. Radiation Effects on Optical Link Components.

Device Type

Total Dose

Neutron

Prompt Dose Rate

0 0

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