Editing Neutron bomb (section)

===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 silo]]s from incoming Soviet warheads.

A problem faced by Sprint and similar ABMs was that the blast effects of their warheads change greatly as they climb and the atmosphere thins out. At higher altitudes, starting around {{convert|60,000|feet}} and above, the blast effects begin to drop off rapidly as the air density becomes very low. This can be countered by using a larger warhead, but then it becomes too powerful when used at lower altitudes. An ideal system would use a mechanism that was less sensitive to changes in air density.

Neutron-based attacks offer one solution to this problem. The burst of neutrons released by an ER weapon can induce fission in the fissile materials of primary in the target warhead. The energy released by these reactions may be enough to melt the warhead, but even at lower fission rates, the "burning up" of some of the fuel in the primary can cause it to fail to explode properly, or "fizzle".<ref name=w47>{{cite book |url=https://books.google.com/books?id=6m43DAAAQBAJ&pg=PA323 |title=British Nuclear Weapons and the Test Ban 1954–1973 |first=John |last=Walker |publisher=Routledge |date=2016 |pages=23, 323–325|isbn=978-1-317-17170-6}}</ref> Thus, a small ER warhead can be effective across a wide altitude band, using blast effects at lower altitudes and the increasingly long-ranged neutrons as the engagement rises.

The use of neutron-based attacks was discussed as early as the 1950s, with the US [[United States Atomic Energy Commission|Atomic Energy Commission]] mentioning weapons with a "clean, enhanced neutron output" for use as "antimissile defensive warheads."<ref name=bomarc>{{cite magazine |url=http://www.rcaf-arc.forces.gc.ca/en/cf-aerospace-warfare-centre/elibrary/journal/2014-vol3-iss4-08-secrets-of-the-bomarc-part-2.page#note47 |title=Secrets of the BOMARC: Re-examining Canada's Misunderstood Missile |date=Fall 2014 |first=Sean |last=Maloney |access-date=2018-10-05 |archive-date=2018-08-10 |archive-url=https://web.archive.org/web/20180810021236/http://www.rcaf-arc.forces.gc.ca/en/cf-aerospace-warfare-centre/elibrary/journal/2014-vol3-iss4-08-secrets-of-the-bomarc-part-2.page#note47 |url-status=dead }}</ref> Studying, improving and defending against such attacks was a major area of research during the 1950s and '60s. A particular example of this is the US [[UGM-27 Polaris|Polaris A-3]] missile, which delivered three warheads travelling on roughly the same trajectory, and thus with a short distance between them. A single ABM could conceivably destroy all three through neutron flux. Developing warheads that were less sensitive to these attacks was also a major area of research in the US and UK during the 1960s.<ref name=w47 />

Some sources claim that the neutron flux attack was also the main design goal of the various nuclear-tipped anti-aircraft weapons like the [[AIM-26 Falcon]] and [[CIM-10 Bomarc]]. One [[F-102]] pilot noted:

{{blockquote|GAR-11/AIM-26 was primarily a weapon-killer. The bomber(s, if any) was collateral damage. The weapon was proximity-fused to ensure detonation close enough so an intense flood of neutrons would result in an instantaneous nuclear reaction (NOT full-scale) in the enemy weapon's pit; rendering it incapable of functioning as designed ... [O]ur first "neutron bombs" were the GAR-11 and MB-1 Genie.<ref name=bomarc />}}

It has also been suggested that neutron flux's effects on the warhead electronics are another attack vector for ER warheads in the ABM role. [[Ionization]] greater than 50 [[Gray (unit)|Gray]] in [[silicon chips]] delivered over seconds to minutes will degrade the function of [[semiconductors]] for long periods.<ref>{{cite web |url=https://www.fas.org/nuke/intro/nuke/radiation.htm |title=FAS Nuclear Weapon Radiation Effects |url-status=live |archive-url=https://web.archive.org/web/20130721195639/http://www.fas.org/nuke/intro/nuke/radiation.htm |archive-date=2013-07-21}}</ref> However, while such attacks might be useful against guidance systems, which used relatively advanced electronics, in the ABM role, these components have long ago separated from the warheads by the time they come within range of the interceptors. The electronics in the warheads themselves tend to be very simple, and hardening them was one of the many issues studied in the 1960s.<ref name=w47 />

[[Lithium hydride|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.<ref>{{cite web |url=http://nuclearweaponarchive.org/Nwfaq/Nfaq12.html |title=Section 12.0 Useful Tables Nuclear Weapons Frequently Asked Questions |quote=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. |url-status=live |archive-url=http://archive.wikiwix.com/cache/20110224033350/http://nuclearweaponarchive.org/Nwfaq/Nfaq12.html |archive-date=2011-02-24}}</ref><ref>{{cite web |url=https://www.osti.gov/opennet/forms.jsp?formurl=document/rdd-1/drwcrtf4.html |title=''Restricted Data Declassification Policy, 1946 to the Present RDD-1'' |quote=The fact that Li6H is used in unspecified weapons for hardening |url-status=live |archive-url=https://web.archive.org/web/20131020234426/https://www.osti.gov/opennet/forms.jsp?formurl=document%2Frdd-1%2Fdrwcrtf4.html |archive-date=2013-10-20}}</ref> [[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.<ref>{{cite web |url=http://www.acq.osd.mil/ncbdp/nm/nm_book_5_11/appendix_F.htm |title=The Nuclear Matters Handbook, F.13 |url-status=dead |archive-url=https://web.archive.org/web/20130302091606/http://www.acq.osd.mil/ncbdp/nm/nm_book_5_11/appendix_F.htm |archive-date=2013-03-02}}</ref><ref>{{cite web |url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA302734 |title=Transient Radiation Effects on Electronics (TREE) Handbook Formerly Design Handbook for TREE, Chapters 1-6 |url-status=dead |archive-url=https://web.archive.org/web/20140506094200/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA302734 |archive-date=2014-05-06 |access-date=2014-05-06}}</ref>

At very high altitudes, at the edge of the atmosphere and above it, another effect comes into play. At lower altitudes, the [[X-ray]]s generated by the bomb are absorbed by the air and have [[mean free path]]s on the order of meters. But as the air thins out, the X-rays can travel further, eventually outpacing the area of effect of the neutrons. In [[exoatmospheric]] explosions, this can be on the order of {{convert|10|km}} in radius. In this sort of attack, it is the X-rays promptly delivering energy on the warhead surface that is the active mechanism; the rapid ablation (or "blowoff") of the surface creates shock waves that can break up the warhead.<ref>{{cite web |url=http://www.acq.osd.mil/ncbdp/nm/nm_book_5_11/appendix_G.htm |title=Nuclear Matters Handbook |quote=Nuclear weapon-generated X-rays are the chief threat to the survival of strategic missiles in-flight above the atmosphere and to satellites ... The Neutron and gamma ray effects dominate at lower altitudes where the air absorbs most of the X-rays. |url-status=dead |archive-url=https://web.archive.org/web/20140506095559/http://www.acq.osd.mil/ncbdp/nm/nm_book_5_11/appendix_G.htm |archive-date=2014-05-06}}</ref>