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A neutron bomb is a type of nuclear weapon specifically designed not to produce a massive explosion and significant infrastructure damage, but rather to completely irradiate an area with neutron radiation, killing all living things while leaving the infrastructure relatively intact. Though a neutron bomb does cause infrastructure damage and some long term radiation (it is still a nuclear weapon after all), they are specifically designed so that ~48 hours after the device is detonated, friendly troops can move into an area and utilize the infrastructure without fear of radiation. Thus, neutron bombs are generally seen more as a tactical nuclear weapon than a strategic one. Due to this focus on tactical use, neutron bombs comprise some of the smallest nuclear weapons ever built (though the American bazooka-launched Davy Crockett warhead is not, as is sometimes claimed, a neutron bomb device -- it employs a standard fission warhead).
Neutron bombs, also called enhanced radiation weapons (ER), are small nuclear weapons in which the burst of neutrons generated by the fusion reaction is intentionally not absorbed inside the weapon, but allowed to escape. The X-ray mirrors and shell of the weapon are made of chromium or nickel so that the neutrons are permitted to escape. This intense burst of high-energy neutrons is the principle destructive mechanism. Neutrons are more penetrating than other types of radiation so many shielding materials that work well against gamma rays do not work nearly as well. The term "enhanced radiation" refers only to the burst of ionizing radiation released at the moment of detonation, not to any enhancement of residual radiation in fallout (contrast this with salted bombs).
Neutron Bomb Tactics
Neutron bombs could be used as strategic anti-missile weapons, and as tactical weapons intended for use against armored forces. As an anti-missile weapon ER weapons were developed to protect U.S. ICBM silos from incoming Soviet warheads by damaging the electronic components of the incoming warhead with the intense neutron flux. Tactical neutron bombs are primarily intended to kill soldiers who are protected by armor. Armored vehicles are extremely resistant to blast and heat produced by nuclear weapons, so the effective range of a nuclear weapon against tanks is determined by the lethal range of the radiation, although this is also reduced by the armor. By emitting large amounts of lethal radiation of the most penetrating kind, ER warheads maximize the lethal range of a given yield of nuclear warhead against armored targets.
One problem with using radiation as a tactical anti-personnel weapon is that to bring about rapid incapacitation of the target, a radiation dose that is many times the lethal level must be administered. A radiation dose of 600 rads is normally considered lethal (it will kill at least half of those who are exposed to it), but no effect is noticeable for several hours. Neutron bombs were intended to deliver a dose of 8000 rads to produce immediate and permanent incapacitation. A 1 kt ER warhead can do this to a T-72 tank crew at a range of 690 m, compared to 360 m for a pure fission bomb. For a "mere" 600 rad dose the distances are 1100 m and 700 m respectively, and for unprotected soldiers 600 rad exposures occur at 1350 m and 900 m. The lethal range for tactical neutron bombs exceeds the lethal range for blast and heat even for unprotected troops.
The neutron flux can induce significant amounts of short lived secondary radioactivity in the environment in the high flux region near the burst point. The alloy steels used in armor can develop radioactivity that is dangerous for 24-48 hours. If a tank exposed to a 1 kt neutron bomb at 690 m (the effective range for immediate crew incapacitation) is immediately occupied by a new crew, they will receive a lethal dose of radiation within 24 hours.
Some authorities say that due to the rapid attenuation of neutron energy by the atmosphere (it drops by a factor of 10 every 500 m in addition to the effects of spreading) ER weapons are only effective at short ranges, and thus are practical only in relatively low yields. These ER warheads are said to be designed to minimize the amount of fission energy and blast effect produced relative to the neutron yield. The principal reason is said to be to allow their use close to friendly forces.
These same authorities say that the common perception of the neutron bomb as a "landlord bomb" that would kill people but leave buildings undamaged is greatly overstated. At the conventional effective combat range (690 m) the blast from a 1 kt neutron bomb will destroy or damage to the point of unusability almost any civilian building. Thus the use of neutron bombs to stop an enemy attack, which requires exploding large numbers of them to blanket the enemy forces, would also destroy all buildings in the area.
Another view of the neutron bomb and its tactics exists. The inventor of the neutron bomb, Samuel Cohen, wrote a book in which he stated that the effective range of a pure neutron bomb exceeded 10 Km of altitude. Samuel Cohen stated explicitly that "enhanced radiation" weapons deployed in Germany during the cold war were political compromises designed to have substantial blast, with radiation effects deliberately reduced to eliminate any possibility of surviving structures. He also quoted radiation releases of 100KRads at the ground from pure neutron weapons exploded at 10Km.
The neutron absorption spectra of air is disputed by some authorities, and may depend in part on absorption by hydrogen from water vapor. It therefore might vary exponentially with humidity, making high-altitude neutron bombs immensely more deadly in desert climates than in humid ones. This effect also varies with altitude.
According to Samuel Cohen, one possible tactic of using such "true" neutron bombs is therefore to launch them as defensive weapons against armored attacks. Civilians enter radiation shelters, and the bomb is exploded 10Km over the armored attack. Portable armor is said to be unable to shield tank and aircraft crews. In such an event, a city's trees and grass would have been killed by radiation, but buildings would remain undamaged for the emerging civilians.
Such neutron bombs would be very potent anti-ship weapons. A major support of Cohen's research was the U.S. Navy.