|Energy type||Standard %||Enhanced %|
|Blast||50||40 to minimum 30|
|Thermal energy||35||25 to minimum 20|
|Prompt radiation||5||45 to minimum 30|
A neutron bomb, officially termed one type of Enhanced Radiation Weapon (ERW), is a low yield thermonuclear weapon in which a burst of neutrons generated by a nuclear fusion reaction is intentionally allowed to escape the weapon, rather than being absorbed by its other components. The neutron bomb was to be used as a tactical nuclear weapon intended for use against armored forces. Originally conceived by the U.S. military, their design goals were to stop massed Soviet armored divisions from overrunning allied nations without destroying the infrastructure of the allied nation.
The weapon's radiation case, usually made from relatively thick uranium, lead or steel in a standard bomb, is, instead, made of as thin a material as possible, to facilitate the greatest escape of fusion-produced neutrons. The usual nuclear weapon yield—expressed as kilotons of 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 pure fission bomb with an identical explosive yield, a neutron bomb would emit about ten times the amount of neutron radiation. In a fission bomb, at sea level, the total radiation pulse energy which is composed of both gamma rays and neutrons is approximately 5% of the entire energy released; in the neutron bomb it would be closer to 40%. Furthermore, the neutrons emitted by a neutron bomb have a much higher average energy level (close to 14 MeV) than those released during a fission reaction (1–2 MeV). Technically speaking, all low yield nuclear weapons are radiation weapons, including non-enhanced variants. Up to about 10 kilotons in yield, all nuclear weapons have prompt neutron radiation as their furthest-reaching lethal component, after which point the lethal blast and thermal effects radius begins to out-range the lethal ionizing radiation radius. Enhanced radiation weapons also fall into this same yield range and simply enhance the intensity and range of the neutron dose for a given yield.
History and deployment to present
Conception of the neutron bomb is generally credited to Samuel T. Cohen of the Lawrence Livermore National Laboratory, who developed the concept in 1958. Initial development was carried out as part of projects DOVE and STARLING, and an early device was tested underground in early 1962. Designs of a "weaponized" version were carried out in 1963.
Development of two production designs for the Army's MGM-52 Lance short-range missile began in July 1964, the W63 at Livermore and the W64 at Los Alamos. Both entered Phase 3 testing in July 1964, and the W64 was cancelled in favor of the W63 in September 1964. The W63 was in turn cancelled in November 1965 in favor of the W70 (Mod 0), a conventional design. By this time, the same concepts were being used to develop warheads for the Sprint missile, an anti-ballistic missile (ABM), with Livermore designing the W65 and Los Alamos the W66. Both entered Phase 3 testing in October 1965, but the W65 was cancelled in favor of the W66 in November 1968. Testing of the W66 was carried out in the late 1960s, and entered production in June 1974, the first neutron bomb to do so. Approximately 120 were built, with about 70 of these being on active duty during 1975 and 1976 as part of the Safeguard Program. When that program was shut down they were placed in storage, and eventually decommissioned in the early 1980s.
Development of ER warheads for Lance continued, but in the early 1970s attention had turned to using modified versions of the W70, the W70 Mod 3. Development was subsequently postponed by President Jimmy Carter in 1978 following protests against his administration's plans to deploy neutron warheads to ground forces in Europe. On November 17, 1978, in a test the USSR detonated its first similar-type bomb. President Ronald Reagan restarted production in 1981. The Soviet Union began a propaganda campaign against the US's neutron bomb in 1981 following Reagan's announcement. In 1983 Reagan then announced the Strategic Defense Initiative, which surpassed neutron bomb production in ambition and vision and with that the neutron bomb quickly faded from the center of the public's attention.
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 MGM-52 Lance missile, and the W79 Mod 0 was developed for nuclear 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.
According to the Cox Report, as of 1999 the United States had never deployed a neutron weapon. The nature of this statement is not clear; it reads "The stolen information also includes classified design information for an enhanced radiation weapon (commonly known as the "neutron bomb"), which neither the United States, nor any other nation, has ever deployed." However, the fact that neutron bombs had been produced by the US was well known at this time and part of the public record. Sam Cohen suggests the report is playing with the definitions; the US bombs were never deployed to Europe, they remained stockpiled in the US.
In addition to the two superpowers, France and China are known to have tested neutron or enhanced radiation bombs. France conducted an early test of the technology in 1967 and tested an "actual" neutron bomb in 1980. China conducted a successful test of neutron bomb principles in 1984 and a successful test of a neutron bomb in 1988. However, neither of those countries chose to deploy the neutron bomb. Chinese nuclear scientists stated before the 1988 test that China had no need for the neutron bomb, but it was developed to serve as a "technology reserve", in case the need arose in the future.
Although no country is currently known to deploy them in an offensive manner, all thermonuclear dial-a-yield warheads that have about 10 kiloton and lower as one dial option, with a considerable fraction of that yield derived from fusion reactions, can be considered able to be neutron bombs in use, if not in name. The only country definitely known to deploy dedicated (that is, not dial-a-yield) neutron warheads for any length of time is Russia, which inherited the USSR's neutron warhead equipped ABM-3 Gazelle missile program. This anti-ballistic missile (ABM) system contains at least 68 neutron warheads with a 10 kiloton yield each and it has been in service since 1995, with inert missile testing approximately every other year since then (2014). The system is designed to destroy incoming endoatmospheric level nuclear warheads aimed at Moscow and other targets and is the lower-tier/last umbrella of the A-135 anti-ballistic missile system (NATO reporting name: ABM-3).
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.[need quotation to verify] Science fiction author and commentator Isaac Asimov also stated that "Such a neutron bomb or N bomb seems desirable to those who worry about property and hold life cheap."
Neutron bombs are purposely designed with explosive yields lower than other nuclear weapons. Since neutrons are scattered and absorbed by air, neutron radiation effects drop off rapidly with distance in air. As such, there is a sharper distinction, relative to thermal effects, between areas of high lethality and areas with minimal radiation doses. All high yield (more than ~10 kiloton) neutron bombs, such as the extreme example of a device that derived 97% of its energy from fusion, the 50 megaton Tsar Bomba, are not able to radiate sufficient neutrons beyond their lethal blast range when detonated as a surface burst or low altitude air burst and so are no longer classified as neutron bombs, thus limiting the yield of neutron bombs to a maximum of about 10 kilotons. The intense pulse of high-energy neutrons generated by a neutron bomb is the principal killing mechanism, not the fallout, heat or blast.
The inventor of the neutron bomb, Sam Cohen, criticized the description of the W70 as a neutron bomb since it could be configured to yield 100 kilotons:
the W-70 ... is not even remotely a "neutron bomb." Instead of being the type of weapon that, in the popular mind, "kills people and spares buildings" it is one that both kills and physically destroys on a massive scale. The W-70 is not a discriminate weapon, like the neutron bomb—which, incidentally, should be considered a weapon that "kills enemy personnel while sparing the physical fabric of the attacked populace, and even the populace too."
Although neutron bombs are commonly believed to "leave the infrastructure intact", with current designs that have explosive yields in the low kiloton range, detonation in a built up area would still cause considerable, although not total, destruction through blast and heat effects out to a considerable radius.
As the Warsaw Pact tank strength was over twice that of NATO, and Soviet Deep Battle doctrine was likely to be to use this numerical advantage to rapidly sweep across continental Europe if the Cold War ever turned hot, any weapon that could break up their intended mass tank formation deployments and force them to deploy their tanks in a thinner, more easily dividable manner, would aid ground forces in the task of hunting down solitary tanks and firing anti-tank missiles upon them, such as the contemporary M47 Dragon and BGM-71 TOW missiles, which NATO had hundreds of thousands of.
Rather than making extensive preparations for battlefield nuclear combat in Central Europe, "The Soviet military leadership believed that conventional superiority provided the Warsaw Pact with the means to approximate the effects of nuclear weapons and achieve victory in Europe without resort to those weapons."
Neutron bombs, or more precisely, enhanced [neutron] radiation weapons were also to find use as strategic anti-ballistic missile weapons, and in this role they are believed to remain in active service within Russia's Gazelle (missile).
Upon detonation, a 1 kiloton neutron bomb near the ground, in an airburst 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 all non-reinforced concrete structures, at the conventional effective combat range against modern main battle tanks and armored personnel carriers (<690–900 m) the blast from a 1 kt neutron bomb will destroy or damage to the point of non-usability almost all un-reinforced civilian buildings. Thus the use of neutron bombs to stop an enemy armored attack by rapidly incapacitating the crew with a dose of 8000+ rads of radiation, which would require exploding large numbers of them to blanket the enemy forces, would also destroy all normal civilian buildings in the same immediate area ~600 meters, and via neutron activation it would make many building materials in the city radioactive, such as zinc coated steel/galvanized steel (see area denial use below). Although at this ~600 meter distance the 4-5 psi blast overpressure would cause very few direct casualties because liquid-filled objects such as a human body are resistant to gross overpressure, the powerful winds produced by this overpressure can throw bodies into objects or throw objects, including window glass at high velocity, both with potentially lethal results, rendering casualties highly dependent on surroundings, including on if the building they are in collapses. The pulse of neutron radiation would cause immediate and permanent incapacitation to unprotected outdoor humans in the open out to 900 meters, with death occurring in one or two days. The median lethal dose (LD50) of 600 rads would extend to about 1350–1400 meters for those unprotected and outdoors, where approximately half of those exposed would die of radiation sickness after several weeks.
However a human residing within, or simply shielded by, at least one of the aforementioned concrete buildings with walls and ceilings 30 cm (12 in) thick, or alternatively of damp soil 24 inches thick, would receive a neutron radiation exposure reduced by a factor of 10. Even near ground zero, basement sheltering or buildings with similar radiation shielding characteristics, would drastically reduce the radiation dose.
Furthermore, the neutron absorption spectrum of air is disputed by some authorities and depends in part on absorption by hydrogen from water vapor. Thus, it might vary exponentially with humidity, making neutron bombs far more deadly in desert climates than in humid ones.
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 fully protected from radiation effects. Thus, 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. However this assertion was regarded as dubious in a reply in 1986 by a member of the Royal Military College of Science as neutron radiation from a 1 kiloton neutron bomb would incapacitate the crew of a tank with a protection factor of 35 out to a range of 280 meters, but the incapacitating blast range, depending on the exact weight of the tank, is much less, from 70 to 130 meters. However although the author did note that effective neutron absorbers and neutron poisons such as boron carbide can be incorporated into conventional armor and strap on neutron moderating hydrogenous material (hydrogen atom containing substances), such as explosive reactive armor, can both increase the protection factor, the author holds that in practice combined with neutron scattering, the actual average total tank area protection factor is rarely higher than 15.5 to 35. According to the Federation of American Scientists, the neutron protection factor of a "tank" can be as low as 2, without qualifying whether the statement implies a light tank, medium tank, or main battle tank.
A composite high density concrete, or alternatively, a laminated Graded Z shield, 24 units thick of which 16 units are iron and 8 units are polyethylene containing boron (BPE), and additional mass behind it to attenuate neutron capture gamma rays is more effective than just 24 units of pure iron or BPE alone, due to the advantages of both iron and BPE in combination. Iron is effective in slowing down/scattering high-energy neutrons in the 14-MeV energy range and attenuating gamma rays, while the hydrogen in polyethylene is effective in slowing down these now slower fast neutrons in the few MeV range, and boron 10 has a high absorption cross section for thermal neutrons and a low production yield of gamma rays when it absorbs a neutron. The Soviet T72 tank, in response to the neutron bomb threat, is cited as having fitted a boronated, polyethylene liner, which has had its neutron shielding properties simulated.
However, some tank armor material contains depleted uranium (DU), common in the US's M1A1 Abrams tank, which "incorporates steel-encased depleted uranium armour", a substance that will fast fission when it captures a fast, fusion-generated neutron, and thus on fissioning will produce fission neutrons and fission products embedded within the armor, products which emit among other things, penetrating gamma rays. Although the neutrons emitted by the neutron bomb may not penetrate to the tank crew in lethal quantities, the fast fission of DU within the armor could still ensure a lethal environment for the crew and maintenance personnel by fission neutron and gamma ray exposure, largely depending on the exact thickness and elemental composition of the armor—information usually hard to attain. Despite this, Ducrete—which has an elemental composition similar to, but not identical to the ceramic 2nd generation heavy metal Chobham armor of the Abrams tank—is an effective radiation shield, to both fission neutrons and gamma rays due to it being a graded Z material. Uranium being about twice as dense as lead is thus nearly twice as effective at shielding gamma ray radiation per unit thickness.
Use against ballistic missiles
As an anti-ballistic missile weapon, the first fielded ER warhead, the W66, 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. Ionization greater than 5,000 rads in silicon chips delivered over seconds to minutes will degrade the function of semiconductors for long periods. Due to the rarefied atmosphere encountered high above the earth at the most likely intercept point of an incoming warhead by a neutron bomb/warhead, whether it be the retired Sprint missile's W66 neutron warhead or the still in service Russian counterpart, the ABM-3 Gazelle, at the Terminal phase point (10–30 km) of the incoming warheads flight, the neutrons generated by a mid- to high-altitude nuclear explosion (HANE) have an even greater range than that encountered after a low altitude air burst, as in the high altitude case, there is a lower density of air molecules that produces, by comparison, an appreciable reduction in the air shielding effect/half-value thickness.
However, although this neutron transparency advantage attained only increases at increased altitudes, neutron effects lose importance in the exoatmospheric environment, being overtaken by the range of another effect of a nuclear detonation, at approximately the same altitude as the end of the incoming missile's boost phase (~150 km), ablation producing soft X-rays are the chief nuclear effects threat to the survival of incoming missiles and warheads rather than neutrons. A factor exploited by the other warhead of the Safeguard Program, the enhanced (X-ray) radiation W71 and its USSR/Russian counterpart, the warhead on the A-135 Gorgon missile.
Another method by which neutron radiation can be used to destroy incoming nuclear warheads is by serving as an intense neutron generator and to thus initiate fission in the incoming warhead's fissionable components by fast fission, potentially causing the incoming warhead to prematurely detonate in a fizzle if within sufficient proximity, but in most likely interception ranges, requiring only that enough fissionable material in the warhead fissions to interfere with the functioning of the incoming warhead when it is later fuzed to explode (see related physics: Subcritical reactor).
Lithium-6 hydride (Li6H) is cited as being used as a countermeasure to reduce the vulnerability and "harden" nuclear warheads from the effects of externally generated neutrons. Radiation hardening of the warhead's electronic components as a countermeasure to high altitude neutron warheads somewhat reduces the range that a neutron warhead could successfully cause an unrecoverable glitch by the transient radiation effects on electronics (TREE) effects.
Use as an area denial weapon
In November 2012, during the planning stages of Operation Hammer of God, British Labour Peer Lord Gilbert suggested that multiple enhanced radiation reduced blast (ERRB) warheads could be detonated in the mountain region of the Afghanistan/Pakistan border to prevent infiltration. He proposed to warn the inhabitants to evacuate, then irradiate the area, making it unusable and impassable. Used in this manner, the neutron bomb(s), regardless of burst height, would release neutron activated casing materials used in the bomb, and depending on burst height, create radioactive soil activation products.
In much the same fashion as the area denial effect resulting from fission product (the substances that make up most fallout) contamination in an area following a conventional surface burst nuclear explosion, as considered in the Korean War by Douglas MacArthur, it would thus be a form of radiological warfare - with the difference that neutron bombs produce half, or less, of the quantity of fission products relative to the same-yield pure fission bomb. Radiological warfare with neutron bombs that rely on fission primaries would thus still produce fission fallout, albeit a comparatively cleaner and shorter lasting version of it in the area if air bursts were used, as little to no fission products would be deposited on the direct immediate area, instead becoming diluted global fallout.
However the most effective use of a neutron bomb with respect to area denial would be to encase it in a thick shell of material that could be neutron activated, and use a surface burst. In this manner the neutron bomb would be turned into a salted bomb; a case of zinc-64, produced as a byproduct of depleted zinc oxide enrichment, would for example probably be the most attractive fror military use, as when activated, the zinc-65 so formed is a gamma emitter, with a half life of 244 days.
Neutron bombs-warheads require considerable maintenance for their abilities, requiring some tritium for fusion boosting and tritium in the secondary stage (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 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.
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Due to moderating ability and light weight, used to harden weapons against outside neutron fluxes (especially in combination with Li-6)...The very high cross section of this reaction for thermalized neutrons, combined with the light weight of the Li-6 atom, make it useful in the form of lithium hydride for hardening of nuclear weapons against external neutron fluxes.
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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 reaction start.
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