The intensity of neutrons generated by the weapon also falls off as 1/R^2 from the burst point. Neutron energies are distributed over the range from 10 keV to 14 MeV, with peaks around IMeV and 14 MeV. Since neutrons have positive rest mass, they travel slower than light, so the time of arrival for neutrons is later than for x-ray or gamma rays, and the pulse width increases with range. Neutron fluence is measured in units of neutrons/cm^2 . Neutrons collide with atomic nuclei, causing them to be displaced. (Some ionization also occurs, but this can usually be ignored.) This displacement damage can severly degrade the electrical properties of semiconductor devices, especially for large power devices such as power transistors. This material degradation can result in device and circuit malfunctions. There is no practical way to shield against neutrons, but circuit function can be partially restored if the degraded devices are annealed, that is, if they are heated to a high temperature for a period of time (minutes to hours depending upon the initial neutron fluence and the material affected). Nuclear detonations within even the outer fringes of the Earth’s atmosphere produce an electromagnetic pulse (EMP) due to the interactions of gamma rays with the upper regions of the atmosphere and of the resuting Compton (knock-on) electrons with the Earth’s magnetic field. The EMP can be transmitted over large distances both on the Earth and in near-Earth space with minimal attenuation. This is the most serious threat to the LEO base and COTVs at lower altitudes from nuclear detonations not aimed directly at these elements of SPS. Energy in the EMP can couple to any antenna, cable, or other conducting structure. The effects are transient but can lead to catastrophic damage due to burnout of electronic devices. The EMP is described by the electric field strength (in volts/meter) as a function of time. (In the frequency domain, EMP signals from high altitude bursts characteristically span 100 KHz to 100 MHz.) Another indirect nuclear radiation effect is the result of a severe increase in the number of energetic particles, particularly electrons, trapped in the Earth’s magnetic field. These can cause significant ionization and atomic displacement in materials. The effect is due to the trapping of beta particles from decay of fission products from weapon debris in the Earth’s magnetic field. These negatively charged particles spread around the Earth within minutes after a detonation, with a rapid initial decay followed by a gradual decay over periods of hundreds of days.
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