A neutron bomb is a fission-fusion thermonuclear weapon (hydrogen bomb) in which the burst of neutrons generated by a fusion reaction is intentionally allowed to escape the weapon, rather than being absorbed by its other components. The weapon's X-ray mirrors and radiation case, made of uranium or lead in a standard bomb, are instead made of chromium or nickel so that the neutrons can escape. The bombs also require some tritium for fusion boosting (yielding more neutrons), in amounts on the order of a few tens of grams (10–30 grams estimated). Because tritium has a relatively short half-life of 12.32 years (after that time, half the tritium has decayed), it is necessary to replenish it periodically in order to keep the bomb effective. (For instance: to maintain a constant level of 24 grams of tritium in a warhead, about 1 gram per bomb per year must be supplied.) Moreover, tritium decays into helium-3, which absorbs neutrons and will thus further reduce the bomb's neutron yield.
The "usual" nuclear weapon yield—expressed as kt TNT equivalent—is not a measure of a neutron weapon's destructive power. It refers only to the energy released (mostly heat and blast), and does not express the lethal effect of neutron radiation on living organisms. Compared to a fission bomb with the identical explosive yield, a neutron bomb would emit about ten times the amount of neutron radiation. In a fission bomb, the radiation pulse energy is approximately 5% of the entire energy released; in the neutron bomb it would be closer to 50%. Furthermore, the neutrons emitted by the latter type are of much higher energy (14 MeV) than those released during a fission reaction (1–2 MeV).
Conception of the neutron bomb is generally credited to Samuel T. Cohen of the Lawrence Livermore National Laboratory, who developed the concept in 1958. Testing was authorized and carried out in 1963 at an underground Nevada test facility. Development was subsequently postponed by President Jimmy Carter in 1978 following protests against his administration's plans to deploy neutron warheads in Europe. President Ronald Reagan restarted production in 1981.
Three types of enhanced radiation weapons (ERW) were built by the United States. The W66 warhead, for the anti-ICBM Sprint missile system, was deployed in 1975 and retired the next year, along with the missile system. The W70 Mod 3 warhead was developed for the short-range, tactical Lance missile, and the W79 Mod 0 was developed for artillery shells. The latter two types were retired by President George H. W. Bush in 1992, following the end of the Cold War. The last W70 Mod 3 warhead was dismantled in 1996, and the last W79 Mod 0 was dismantled by 2003, when the dismantling of all W79 variants was completed.
Besides the United States and Soviet Union, France and China are understood to have tested neutron or enhanced radiation bombs in the past, with France apparently leading the field with an early test of the technology in 1967 and an "actual" neutron bomb in 1980. The 1999 Cox Report indicates that China is able to produce neutron bombs, although no country is currently known to deploy them.
Considerable controversy arose in the U.S. and Western Europe, following a June 1977 Washington Post exposé describing U.S. government plans to purchase the bomb. The article focused on the fact that it was the first weapon specifically intended to kill humans with radiation. Lawrence Livermore National Laboratory director Harold Brown and Soviet General Secretary Leonid Brezhnev both described the neutron bomb as a "capitalist bomb", because it was designed to destroy people while preserving property. Science fiction author Isaac Asimov also stated that "Such a neutron bomb or N bomb seems desirable to those who worry about property and hold life cheap." 
Use of neutron bomb
Neutron bombs are purposely designed with explosive yields lower than other nuclear weapons. Since neutrons are absorbed by air, even a high-yield neutron bomb is not able to radiate neutrons beyond its blast range and so would theoretically have no destructive advantage over a normal hydrogen bomb. However, the intense pulse of high-energy neutrons that is generated is intended as the principal killing mechanism, not the fallout, heat or blast. Although neutron bombs are commonly believed to "leave the infrastructure intact", current designs have explosive yields in the kiloton range, the detonation of which would cause considerable destruction through blast and heat effects.
Neutron bombs could be used as strategic anti-ballistic missile weapons or as tactical weapons intended for use against armored forces. The neutron bomb was originally conceived by the U.S. military as a weapon that could stop massed Soviet armored divisions from overrunning allied nations without destroying the infrastructure of the allied nation.
Effects of a neutron bomb detonation
Upon detonation, a 1 kiloton neutron bomb would produce a large blast wave, and a powerful pulse of both thermal radiation and ionizing radiation, mostly in the form of fast (14.1 MeV) neutrons. The thermal pulse would cause third degree burns to unprotected skin out to approximately 500 meters. The blast would create at least 4.6 PSI out to a radius of 600 meters, which would severely damage non-reinforced structures. At this distance the blast would cause very few direct casualties as the human body is resistant to sheer overpressure, however, the powerful winds produced by this overpressure are capable of throwing human bodies into objects or throwing objects at high velocity, both with lethal results, rendering casualties highly dependent on surroundings. The pulse of neutron radiation would cause immediate and permanent incapacitation to unprotected humans out to 900 meters, with death occurring in one or two days. The lethal dose would extend out past 1400 meters, where approximately half of those exposed would die of radiation sickness after several weeks.
Questionable effectiveness in modern anti-tank role
The questionable effectiveness of ER weapons against modern tanks is cited as one of the main reasons that these weapons are no longer fielded or stockpiled. With the increase in average tank armor thickness since the first ER weapons were fielded, tank armor protection approaches the level where tank crews are now almost completely protected from radiation effects. Therefore for an ER weapon to incapacitate a modern tank crew through irradiation, the weapon must now be detonated at such a close proximity to the tank that the nuclear explosion's blast would now be equally effective at incapacitating it and its crew.
Use against ballistic missiles
As an anti-ballistic missile weapon, an ER warhead was developed for the Sprint missile system as part of the Safeguard Program to protect United States cities and missile silos from incoming Soviet warheads by damaging their electronic components with the intense neutron flux.
In November 2012, a former British Labour defence minister Lord Gilbert, made the suggestion that enhanced radiation reduced blast (ERRB) warheads could be detonated in the mountain region of the Afghanistan/Pakistan border to protect against infiltration.
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- After 12.32 years, half the 24g has decayed and thus about 12g is missing: to replenish these 12g during the 12 years they decayed, adding about 1g per year is needed.
- When absorbing neutrons, helium-3 produces back some tritium, but it comes too late in the reaction for fusion boosting and doesn't compensate for the decayed tritium missing at the start of the reaction.
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