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Neutron bomb

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Energy distribution of weapon
Standard Enhanced
Blast 50% 40%[1] or as low as 30%[2]
Thermal energy 35% 25%[3] or as low as 20%[2]
Instant radiation 5% 30[4]–45%
Residual radiation 10% 5%[5]

A neutron bomb or officially known as one type of Enhanced Radiation Weapon is a low yield 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.[6] The weapon's radiation case, usually made from relatively thick uranium, lead or steel in a standard bomb, are 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[7] 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, closer to (14 MeV) than those released during a fission reaction (1–2 MeV).[8] Technically speaking, all low yield nuclear weapons are radiation weapons, that is including the non-enhanced variant, from 0 up to about 10 kilotons in yield, all have prompt neutron radiation,[9] as their most far reaching lethal component, after which point the lethal blast and thermal effects radius begins to out-range the lethal ionizing radiation radius.[10][11][12] 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

Conception of the neutron bomb is generally credited to Samuel T. Cohen of the Lawrence Livermore National Laboratory, who developed the concept in 1958.[13] Testing was authorized and carried out in 1963 at an underground Nevada test facility.[14] 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.[15] On November 17, 1978, the USSR detonated its first neutron bomb.[16] President Ronald Reagan restarted production in 1981.[15] The Soviet Union began a propaganda campaign against the US's neutron bomb in 1981 following Reagan's announcement, a campaign which gained immense popularity, with the new threat from the ”American capitalist machine”. 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.[16]

Three types of enhanced radiation weapons (ERW) were built by the United States.[17] 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.[18][19] The last W70 Mod 3 warhead was dismantled in 1996,[20] and the last W79 Mod 0 was dismantled by 2003, when the dismantling of all W79 variants was completed.[21]

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[22] and an "actual" neutron bomb in 1980.[23] The 1999 Cox Report indicates that China is able to produce neutron bombs,[24] although no country is currently known to deploy them, 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 neutron bombs in actuality if not in name.

By 1984, according to Mordechai Vanunu, Israel was mass-producing neutron bombs.[25] A number of analysts believe that the Vela incident was an Israeli neutron bomb experiment.[26]

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.[27][28] 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.[29][30] 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."[31]

Use of neutron bomb

Neutron bombs are purposely designed with explosive yields lower than other nuclear weapons. Since neutrons are absorbed by air,[9] neutron radiation effects drop off very rapidly with distance in air, there is a sharper distinction, as opposed to thermal effects, between areas of high lethality and areas with minimal radiation doses.[6] All high yield (more than ~10 kiloton) "neutron bombs", such as the extreme example of 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 airburst and so are no longer classified as neutron bombs. As it is the intense pulse of high-energy neutrons that are generated by a neutron bomb that are intended as the principal killing mechanism, not the fallout, heat or blast.

For example, the inventor of the neutron bomb, Samuel Cohen, criticized the description of the W70 as a "neutron bomb" as it could be configured to produce a yield of 100 kiloton:

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."[32]

The Soviet/Warsaw pact invasion plan, "Seven Days to the River Rhine" to seize West Germany. Under such a scenario, neutron bombs, according to their inventor, would hopefully blunt the Warsaw pact tank, and more thinly armored BMP-1 thrusts, without causing as much damage to the people and infrastructure of Germany as alternative tactical nuclear weapons would. They would likely be used if the mass conventional weapon NATO REFORGER response to the invasion had yet to find time to be organized or found ineffective in battle.

Although neutron bombs are commonly believed to "leave the infrastructure intact", with current designs that have explosive yields in the low kiloton range,[33] the detonation of which, in a built up area, would still cause considerable, although not total, destruction through blast and heat effects out to a considerable radius.[34]

Neutron bombs could be used as strategic anti-ballistic missile weapons,[35] 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.[36] 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,[37] such as the contemporary M47 Dragon and BGM-71 TOW missiles.

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 all non-reinforced concrete structures.[38] 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.[39] The pulse of neutron radiation would cause immediate and permanent incapacitation to unprotected humans in the open out to 900 meters,[7] with death occurring in one or two days. The lethal dose would extend out past 1400 meters for those in the open, where approximately half of those exposed would die of radiation sickness after several weeks. However with humans residing within the aforementioned concrete buildings with walls thicker than 12 inches, or 24 inches of damp earth, the neutron radiation exposure would be reduced by a factor of 10.[40][41]

Questionable effectiveness in modern anti-tank role

See also: Centurion Tank § Nuclear tests
The Neutron cross section/ absorption probability in barns of the two natural Boron isotopes found in nature (top curve is for 10B and bottom curve for 11B. As neutron energy increases to 14 MeV, the absorption effectiveness, in general, decreases. Therefore for boron containing armor to be effective, fast neutrons must first be slowed by another element by neutron scattering.

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.[42] However this assertion was regarded as dubious in a reply in 1986 [3] 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 the actual average total tank area protection factor is rarely higher than 15.5 to 35.[43]

A composite high density concrete,[44][45] 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.[46][47][48][49] The Soviet T72 tank, in response to the neutron bomb threat, is cited as having fitted a boronated,[50] polyethylene liner.[41][51]

Maintenance

The bombs require considerable maintenance for their capabilities, requiring some tritium for fusion boosting[citation needed] and tritium in the secondary stage (yielding more neutrons), in amounts on the order of a few tens of grams[52] (10–30 grams[53] 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[54] must be supplied.) Moreover, tritium decays into helium-3, which absorbs neutrons[55] and will thus further reduce the bomb's neutron yield.

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.[56] Ionization greater than 5,000 rads in silicon chips delivered over seconds to minutes will degrade the function of semiconductors for long periods.[57] 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 airburst, where 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), soft x-rays are the chief nuclear effects threat to the survival of incoming missiles and warheads rather than neutrons.[58] A factor exploited by the other warhead of the Safeguard Program, the enhanced (X-ray) radiation W71.

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 warheads 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/"harden" nuclear warheads from the effects of externally generated neutrons.[59][60] Radiation hardening of the warheads 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 TREE(Transient Radiation effects on Electronics) mechanism.[61][62]

Use as an area denial weapon

In November 2012, a former British Labour defence minister (Lord Gilbert), suggested that enhanced radiation reduced blast (ERRB) warheads could be detonated in the mountain region of the Afghanistan/Pakistan border to prevent infiltration.[63] He proposed to warn the inhabitants to evacuate, then irradiate the area, making it unusable and impassible.[64] 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 the majority of 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 with that of neutron bombs producing 1/2, or less, of the quantity of fission products when compared to the same yield pure fission bomb. Radiological warfare with neutron bombs that rely on fission primaries would therefore still produce fission fallout, albeit a comparatively "cleaner" and shorter lasting version of it in the area if airbursts were utilized, 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 from a military point of view, as when activated the Zinc-65 that is created is a gamma emitter, with a half life of 244 days.[65]

See also

References

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  38. ^ Calculated from http://nuclearweaponarchive.org/Nwfaq/Nfaq5.html assuming 0.5 kt combined blast and thermal
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  54. ^ 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.
  55. ^ 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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  59. ^ "Section 12.0 Useful Tables Nuclear Weapons Frequently Asked Questions". 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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  62. ^ "Transient Radiation Effects on Electronics (TREE) Handbook Formerly Design Handbook for TREE, Chapters 1-6".
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  64. ^ "Lord Gilbert obituary, by Andrew Roth, 3 June 2013. "Nobody lives up in the mountains on the border between Afghanistan and Pakistan except for a few goats and a handful of people herding them," he observed. "If you told them that some ... warheads were going to be dropped there and that it would be a very unpleasant place to go, they would not go there."".
  65. ^ "1.6 Cobalt Bombs and other Salted Bombs, Nuclear Weapons Archive, Carey Sublette".

Further reading

External links

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