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Nuclear weapon

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Nuclear_weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions (thermonuclear bomb), producing a nuclear explosion. Both bomb types release large quantities of energy from relatively small amounts of matter.

The first test of a fission ("atomic") bomb released an amount of energy approximately equal to 20,000 tons of TNT (84 TJ). The first thermonuclear ("hydrogen") bomb test released energy approximately equal to 10 million tons of TNT (42 PJ). Nuclear bombs have had yields between 10 tons TNT (the W54) and 50 megatons for the Tsar Bomba (see TNT equivalent). A thermonuclear weapon weighing as little as 600 pounds (270 kg) can release energy equal to more than 1.2 megatonnes of TNT (5.0 PJ).

A nuclear device no larger than a conventional bomb can devastate an entire city by blast, fire, and radiation. Since they are weapons of mass destruction, the proliferation of nuclear weapons is a focus of international relations policy. Nuclear weapons have been deployed twice in war, by the United States against the Japanese cities of Hiroshima and Nagasaki in 1945 during World War II.

Testing and deployment

Nuclear weapons have only twice been used in warfare, both times by the United States against Japan at the end of World War II. On August 6, 1945, the United States Army Air Forces (USAAF) detonated a uranium gun-type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima; three days later, on August 9, the USAAF detonated a plutonium implosion-type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki. These bombings caused injuries that resulted in the deaths of approximately 200,000 civilians and military personnel. The ethics of these bombings and their role in Japan's surrender are to this day, still subjects of debate.

Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated over 2,000 times for testing and demonstration. Only a few nations possess such weapons or are suspected of seeking them. The only countries known to have detonated nuclear weapons—and acknowledge possessing them—are (chronologically by date of first test) the United States, the Soviet Union (succeeded as a nuclear power by Russia), the United Kingdom, France, China, India, Pakistan, and North Korea. Israel is believed to possess nuclear weapons, though, in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Italy, Turkey, Belgium, the Netherlands, and Belarus are nuclear weapons sharing states. South Africa is the only country to have independently developed and then renounced and dismantled its nuclear weapons.

The Treaty on the Non-Proliferation of Nuclear Weapons aims to reduce the spread of nuclear weapons, but there are different views of its effectiveness.

Types

There are two basic types of nuclear weapons: those that derive the majority of their energy from nuclear fission reactions alone, and those that use fission reactions to begin nuclear fusion reactions that produce a large amount of the total energy output.

Fission weapons

All existing nuclear weapons derive some of their explosive energy from nuclear fission reactions. Weapons whose explosive output is exclusively from fission reactions are commonly referred to as atomic bombs or atom bombs (abbreviated as A-bombs). This has long been noted as something of a misnomer, as their energy comes from the nucleus of the atom, just as it does with fusion weapons.

In fission weapons, a mass of fissile material (enriched uranium or plutonium) is forced into supercriticality—allowing an exponential growth of nuclear chain reactions—either by shooting one piece of sub-critical material into another (the "gun" method) or by compression of a sub-critical sphere or cylinder of fissile material using chemically fueled explosive lenses. The latter approach, the "implosion" method, is more sophisticated and more efficient (smaller, less massive, and requiring less of the expensive fissile fuel) than the former.

A major challenge in all nuclear weapon designs is to ensure that a significant fraction of the fuel is consumed before the weapon destroys itself. The amount of energy released by fission bombs can range from the equivalent of just under a ton to upwards of 500,000 tons (500 kilotons) of TNT (4.2 to 2.1×10⁶ GJ).

All fission reactions generate fission products, the remains of the split atomic nuclei. Many fission products are either highly radioactive (but short-lived) or moderately radioactive (but long-lived), and as such, they are a serious form of radioactive contamination. Fission products are the principal radioactive component of nuclear fallout. Another source of radioactivity is the burst of free neutrons produced by the weapon. When they collide with other nuclei in the surrounding material, the neutrons transmute those nuclei into other isotopes, altering their stability and making them radioactive.

The most commonly used fissile materials for nuclear weapons applications have been uranium-235 and plutonium-239. Less commonly used has been uranium-233. Neptunium-237 and some isotopes of americium may be usable for nuclear explosives as well, but it is not clear that this has ever been implemented, and their plausible use in nuclear weapons is a matter of dispute.

Fusion weapons

The other basic type of nuclear weapon produces a large proportion of its energy in nuclear fusion reactions. Such fusion weapons are generally referred to as thermonuclear weapons or more colloquially as hydrogen bombs (abbreviated as H-bombs), as they rely on fusion reactions between isotopes of hydrogen (deuterium and tritium). All such weapons derive a significant portion of their energy from fission reactions used to "trigger" fusion reactions, and fusion reactions can themselves trigger additional fission reactions.

Only six countries—the United States, Russia, the United Kingdom, China, France, and India—have conducted thermonuclear weapon tests. Whether India has detonated a "true" multi-staged thermonuclear weapon is controversial. North Korea claims to have tested a fusion weapon as of January 2016, though this claim is disputed. Thermonuclear weapons are considered much more difficult to successfully design and execute than primitive fission weapons. Almost all of the nuclear weapons deployed today use the thermonuclear design because it is more efficient.

Thermonuclear bombs work by using the energy of a fission bomb to compress and heat fusion fuel. In the Teller-Ulam design, which accounts for all multi-megaton yield hydrogen bombs, this is accomplished by placing a fission bomb and fusion fuel (tritium, deuterium, or lithium deuteride) in proximity within a special, radiation-reflecting container. When the fission bomb is detonated, gamma rays and X-rays emitted first compress the fusion fuel, then heat it to thermonuclear temperatures. The ensuing fusion reaction creates enormous numbers of high-speed neutrons, which can then induce fission in materials not normally prone to it, such as depleted uranium. Each of these components is known as a "stage", with the fission bomb as the "primary" and the fusion capsule as the "secondary". In large, megaton-range hydrogen bombs, about half of the yield comes from the final fissioning of depleted uranium.

Virtually all thermonuclear weapons deployed today use the "two-stage" design described to the right, but it is possible to add additional fusion stages—each stage igniting a larger amount of fusion fuel in the next stage. This technique can be used to construct thermonuclear weapons of arbitrarily large yield. This is in contrast to fission bombs, which are limited in their explosive power due to criticality danger (premature nuclear chain reaction caused by too-large amounts of pre-assembled fissile fuel). The largest nuclear weapon ever detonated, the Tsar Bomba of the USSR, which released an energy equivalent of over 50 megatons of TNT (210 PJ), was a three-stage weapon. Most thermonuclear weapons are considerably smaller than this, due to practical constraints from missile warhead space and weight requirements. In the early 1950s the Livermore Laboratory in the United States had plans for the testing of two massive bombs, Gnomon and Sundial, 1 gigaton of TNT and 10 gigatons of TNT respectively.

Fusion reactions do not create fission products, and thus contribute far less to the creation of nuclear fallout than fission reactions, but because all thermonuclear weapons contain at least one fission stage, and many high-yield thermonuclear devices have a final fission stage, thermonuclear weapons can generate at least as much nuclear fallout as fission-only weapons. Furthermore, high yield thermonuclear explosions (most dangerously ground bursts) have the force to lift radioactive debris upwards past the tropopause into the stratosphere, where the calm non-turbulent winds permit the debris to travel great distances from the burst, eventually settling and unpredictably contaminating areas far removed from the target of the explosion.

Other types

There are other types of nuclear weapons as well. For example, a boosted fission weapon is a fission bomb that increases its explosive yield through a small number of fusion reactions, but it is not a fusion bomb. In the boosted bomb, the neutrons produced by the fusion reactions serve primarily to increase the efficiency of the fission bomb. There are two types of boosted fission bomb: internally boosted, in which a deuterium-tritium mixture is injected into the bomb core, and externally boosted, in which concentric shells of lithium-deuteride and depleted uranium are layered on the outside of the fission bomb core. The external method of boosting enabled the USSR to field the first partially thermonuclear weapons, but it is now obsolete because it demands a spherical bomb geometry, which was adequate during the 1950s arms race when bomber aircraft were the only available delivery vehicles.

The detonation of any nuclear weapon is accompanied by a blast of neutron radiation. Surrounding a nuclear weapon with suitable materials (such as cobalt or gold) creates a weapon known as a salted bomb. This device can produce exceptionally large quantities of long-lived radioactive contamination. It has been conjectured that such a device could serve as a "doomsday weapon" because such a large quantity of radioactivities with half-lives of decades, lifted into the stratosphere where winds would distribute it around the globe, would make all life on the planet extinct.

In connection with the Strategic Defense Initiative, research into the nuclear pumped laser was conducted under the DOD program Project Excalibur but this did not result in a working weapon. The concept involves the tapping of the energy of an exploding nuclear bomb to power a single-shot laser that is directed at a distant target.

During the Starfish Prime high-altitude nuclear test in 1962, an unexpected effect was produced which is called a nuclear electromagnetic pulse. This is an intense flash of electromagnetic energy produced by a rain of high-energy electrons which in turn are produced by a nuclear bomb's gamma rays. This flash of energy can permanently destroy or disrupt electronic equipment if insufficiently shielded. It has been proposed to use this effect to disable an enemy's military and civilian infrastructure as an adjunct to other nuclear or conventional military operations. By itself it could as well be useful to terrorists for crippling a nation's economic electronics-based infrastructure. Because the effect is most effectively produced by high altitude nuclear detonations (by military weapons delivered by air, though ground bursts also produce EMP effects over a localized area), it can produce damage to electronics over a wide, even continental, geographical area.

Research has been done into the possibility of pure fusion bombs: nuclear weapons that consist of fusion reactions without requiring a fission bomb to initiate them. Such a device might provide a simpler path to thermonuclear weapons than one that required the development of fission weapons first, and pure fusion weapons would create significantly less nuclear fallout than other thermonuclear weapons because they would not disperse fission products. In 1998, the United States Department of Energy divulged that the United States had, "...made a substantial investment" in the past to develop pure fusion weapons, but that, "The U.S. does not have and is not developing a pure fusion weapon", and that, "No credible design for a pure fusion weapon resulted from the DOE investment".

Nuclear isomers provide a possible pathway to fissionless fusion bombs. These are naturally occurring isotopes (^178m2Hf being a prominent example) which exist in an elevated energy state. Mechanisms to release this energy as bursts of gamma radiation (as in the hafnium controversy) have been proposed as possible triggers for conventional thermonuclear reactions.

Antimatter, which consists of particles resembling ordinary matter particles in most of their properties but having opposite electric charge, has been considered as a trigger mechanism for nuclear weapons. A major obstacle is the difficulty of producing antimatter in large enough quantities, and there is no evidence that it is feasible beyond the military domain. However, the U.S. Air Force funded studies of the physics of antimatter in the Cold War, and began considering its possible use in weapons, not just as a trigger, but as the explosive itself. A fourth generation nuclear weapon design is related to, and relies upon, the same principle as antimatter-catalyzed nuclear pulse propulsion.

Most variation in nuclear weapon design is for the purpose of achieving different yields for different situations, and in manipulating design elements to attempt to minimize weapon size, radiation hardness or requirements for special materials, especially fissile fuel or tritium.

Tactical nuclear weapons

Soviet OTR-21 Tochka missile. Capable of firing a 100-kiloton nuclear warhead a distance of 185 km

Some nuclear weapons are designed for special purposes; most of these are for non-strategic (decisively war-winning) purposes and are referred to as tactical nuclear weapons.

The neutron bomb purportedly conceived by Sam Cohen is a thermonuclear weapon that yields a relatively small explosion but a relatively large amount of neutron radiation. Such a weapon could, according to tacticians, be used to cause massive biological casualties while leaving inanimate infrastructure mostly intact and creating minimal fallout. Because high energy neutrons are capable of penetrating dense matter, such as tank armor, neutron warheads were procured in the 1980s (though not deployed in Europe) for use as tactical payloads for US Army artillery shells (200 mm W79 and 155 mm W82) and short range missile forces. Soviet authorities announced similar intentions for neutron warhead deployment in Europe; indeed, they claimed to have originally invented the neutron bomb, but their deployment on USSR tactical nuclear forces is unverifiable.

A type of nuclear explosive most suitable for use by ground special forces was the Special Atomic Demolition Munition, or SADM, sometimes popularly known as a suitcase nuke. This is a nuclear bomb that is man-portable, or at least truck-portable, and though of a relatively small yield (one or two kilotons) is sufficient to destroy important tactical targets such as bridges, dams, tunnels, important military or commercial installations, etc. either behind enemy lines or pre-emptively on friendly territory soon to be overtaken by invading enemy forces. These weapons require plutonium fuel and are particularly "dirty". They also demand especially stringent security precautions in their storage and deployment.

Small "tactical" nuclear weapons were deployed for use as antiaircraft weapons. Examples include the USAF AIR-2 Genie, the AIM-26 Falcon and US Army Nike Hercules. Missile interceptors such as the Sprint and the Spartan also used small nuclear warheads (optimized to produce neutron or X-ray flux) but were for use against enemy strategic warheads.

Other small, or tactical, nuclear weapons were deployed by naval forces for use primarily as antisubmarine weapons. These included nuclear depth bombs or nuclear armed torpedoes. Nuclear mines for use on land or at sea are also possibilities.

Weapons delivery

The system used to deliver a nuclear weapon to its target is an important factor affecting both nuclear weapon design and nuclear strategy. The design, development, and maintenance of delivery systems are among the most expensive parts of a nuclear weapons program; they account, for example, for 57% of the financial resources spent by the United States on nuclear weapons projects since 1940.

The simplest method for delivering a nuclear weapon is a gravity bomb dropped from aircraft; this was the method used by the United States against Japan. This method places few restrictions on the size of the weapon. It does, however, limit attack range, response time to an impending attack, and the number of weapons that a country can field at the same time. With miniaturization, nuclear bombs can be delivered by both strategic bombers and tactical fighter-bombers. This method is the primary means of nuclear weapons delivery; the majority of U.S. nuclear warheads, for example, are free-fall gravity bombs, namely the B61, which is being improved upon to this day.

Preferable from a strategic point of view is a nuclear weapon mounted on a missile, which can use a ballistic trajectory to deliver the warhead over the horizon. Although even short-range missiles allow for a faster and less vulnerable attack, the development of long-range intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs) has given some nations the ability to plausibly deliver missiles anywhere on the globe with a high likelihood of success.

More advanced systems, such as multiple independently targetable reentry vehicles (MIRVs), can launch multiple warheads at different targets from one missile, reducing the chance of a successful missile defense. Today, missiles are most common among systems designed for delivery of nuclear weapons. Making a warhead small enough to fit onto a missile, though, can be difficult.

Tactical weapons have involved the most variety of delivery types, including not only gravity bombs and missiles but also artillery shells, land mines, and nuclear depth charges and torpedoes for anti-submarine warfare. An atomic mortar has been tested by the United States. Small, two-man portable tactical weapons (somewhat misleadingly referred to as suitcase bombs), such as the Special Atomic Demolition Munition, have been developed, although the difficulty of combining sufficient yield with portability limits their military utility.

Nuclear strategy

Nuclear warfare strategy is a set of policies that deal with preventing or fighting a nuclear war. The policy of trying to prevent an attack by a nuclear weapon from another country by threatening nuclear retaliation is known as the strategy of nuclear deterrence. The goal in deterrence is to always maintain a second strike capability (the ability of a country to respond to a nuclear attack with one of its own) and potentially to strive for first strike status (the ability to destroy an enemy's nuclear forces before they could retaliate). During the Cold War, policy and military theorists considered the sorts of policies that might prevent a nuclear attack, and they developed game theory models that could lead to stable deterrence conditions.

Different forms of nuclear weapons delivery (see above) allow for different types of nuclear strategies. The goals of any strategy are generally to make it difficult for an enemy to launch a pre-emptive strike against the weapon system and difficult to defend against the delivery of the weapon during a potential conflict. This can mean keeping weapon locations hidden, such as deploying them on submarines or land mobile transporter erector launchers whose locations are difficult to track, or it can mean protecting weapons by burying them in hardened missile silo bunkers. Other components of nuclear strategies included using missile defenses to destroy the missiles before they land or implementing civil defense measures using early-warning systems to evacuate citizens to safe areas before an attack.

Weapons designed to threaten large populations or to deter attacks are known as strategic weapons. Nuclear weapons for use on a battlefield in military situations are called tactical weapons.

Critics of nuclear war strategy often suggest that a nuclear war between two nations would result in mutual annihilation. From this point of view, the significance of nuclear weapons is to deter war because any nuclear war would escalate out of mutual distrust and fear, resulting in mutually assured destruction. This threat of national, if not global, destruction has been a strong motivation for anti-nuclear weapons activism.

Critics from the peace movement and within the military establishment have questioned the usefulness of such weapons in the current military climate. According to an advisory opinion issued by the International Court of Justice in 1996, the use of (or threat of use of) such weapons would generally be contrary to the rules of international law applicable in armed conflict, but the court did not reach an opinion as to whether or not the threat or use would be lawful in specific extreme circumstances such as if the survival of the state were at stake.

Another deterrence position is that nuclear proliferation can be desirable. In this case, it is argued that, unlike conventional weapons, nuclear weapons deter all-out war between states, and they succeeded in doing this during the Cold War between the U.S. and the Soviet Union. In the late 1950s and early 1960s, Gen. Pierre Marie Gallois of France, an adviser to Charles de Gaulle, argued in books like The Balance of Terror: Strategy for the Nuclear Age (1961) that mere possession of a nuclear arsenal was enough to ensure deterrence, and thus concluded that the spread of nuclear weapons could increase international stability. Some prominent neo-realist scholars, such as Kenneth Waltz and John Mearsheimer, have argued, along the lines of Gallois, that some forms of nuclear proliferation would decrease the likelihood of total war, especially in troubled regions of the world where there exists a single nuclear-weapon state. Aside from the public opinion that opposes proliferation in any form, there are two schools of thought on the matter: those, like Mearsheimer, who favored selective proliferation, and Waltz, who was somewhat more non-interventionist. Interest in proliferation and the stability-instability paradox that it generates continues to this day, with ongoing debate about indigenous Japanese and South Korean nuclear deterrent against North Korea.

The threat of potentially suicidal terrorists possessing nuclear weapons (a form of nuclear terrorism) complicates the decision process. The prospect of mutually assured destruction might not deter an enemy who expects to die in the confrontation. Further, if the initial act is from a stateless terrorist instead of a sovereign nation, there might not be a nation or specific target to retaliate against. It has been argued, especially after the September 11, 2001, attacks, that this complication calls for a new nuclear strategy, one that is distinct from that which gave relative stability during the Cold War. Since 1996, the United States has had a policy of allowing the targeting of its nuclear weapons at terrorists armed with weapons of mass destruction.

Robert Gallucci argues that although traditional deterrence is not an effective approach toward terrorist groups bent on causing a nuclear catastrophe, Gallucci believes that "the United States should instead consider a policy of expanded deterrence, which focuses not solely on the would-be nuclear terrorists but on those states that may deliberately transfer or inadvertently leak nuclear weapons and materials to them. By threatening retaliation against those states, the United States may be able to deter that which it cannot physically prevent.".

Graham Allison makes a similar case, arguing that the key to expanded deterrence is coming up with ways of tracing nuclear material to the country that forged the fissile material. "After a nuclear bomb detonates, nuclear forensics cops would collect debris samples and send them to a laboratory for radiological analysis. By identifying unique attributes of the fissile material, including its impurities and contaminants, one could trace the path back to its origin." The process is analogous to identifying a criminal by fingerprints. "The goal would be twofold: first, to deter leaders of nuclear states from selling weapons to terrorists by holding them accountable for any use of their weapons; second, to give leaders every incentive to tightly secure their nuclear weapons and materials."

According to the Pentagon's June 2019 "Doctrine for Joint Nuclear Operations" of the Joint Chiefs of Staffs website Publication, "Integration of nuclear weapons employment with conventional and special operations forces is essential to the success of any mission or operation."

Governance, control, and law

Because they are weapons of mass destruction, the proliferation and possible use of nuclear weapons are important issues in international relations and diplomacy. In most countries, the use of nuclear force can only be authorized by the head of government or head of state. Despite controls and regulations governing nuclear weapons, there is an inherent danger of "accidents, mistakes, false alarms, blackmail, theft, and sabotage".

In the late 1940s, lack of mutual trust prevented the United States and the Soviet Union from making progress on arms control agreements. The Russell–Einstein Manifesto was issued in London on July 9, 1955, by Bertrand Russell in the midst of the Cold War. It highlighted the dangers posed by nuclear weapons and called for world leaders to seek peaceful resolutions to international conflict. The signatories included eleven pre-eminent intellectuals and scientists, including Albert Einstein, who signed it just days before his death on April 18, 1955. A few days after the release, philanthropist Cyrus S. Eaton offered to sponsor a conference—called for in the manifesto—in Pugwash, Nova Scotia, Eaton's birthplace. This conference was to be the first of the Pugwash Conferences on Science and World Affairs, held in July 1957.

By the 1960s, steps were taken to limit both the proliferation of nuclear weapons to other countries and the environmental effects of nuclear testing. The Partial Nuclear Test Ban Treaty (1963) restricted all nuclear testing to underground nuclear testing, to prevent contamination from nuclear fallout, whereas the Treaty on the Non-Proliferation of Nuclear Weapons (1968) attempted to place restrictions on the types of activities signatories could participate in, with the goal of allowing the transference of non-military nuclear technology to member countries without fear of proliferation.

In 1957, the International Atomic Energy Agency (IAEA) was established under the mandate of the United Nations to encourage development of peaceful applications of nuclear technology, provide international safeguards against its misuse, and facilitate the application of safety measures in its use. In 1996, many nations signed the Comprehensive Nuclear-Test-Ban Treaty, which prohibits all testing of nuclear weapons. A testing ban imposes a significant hindrance to nuclear arms development by any complying country. The Treaty requires the ratification by 44 specific states before it can go into force; as of 2012, the ratification of eight of these states is still required.

Additional treaties and agreements have governed nuclear weapons stockpiles between the countries with the two largest stockpiles, the United States and the Soviet Union, and later between the United States and Russia. These include treaties such as SALT II (never ratified), START I (expired), INF, START II (never in effect), SORT, and New START, as well as non-binding agreements such as SALT I and the Presidential Nuclear Initiatives of 1991. Even when they did not enter into force, these agreements helped limit and later reduce the numbers and types of nuclear weapons between the United States and the Soviet Union/Russia.

Nuclear weapons have also been opposed by agreements between countries. Many nations have been declared Nuclear-Weapon-Free Zones, areas where nuclear weapons production and deployment are prohibited, through the use of treaties. The Treaty of Tlatelolco (1967) prohibited any production or deployment of nuclear weapons in Latin America and the Caribbean, and the Treaty of Pelindaba (1964) prohibits nuclear weapons in many African countries. As recently as 2006 a Central Asian Nuclear Weapon Free Zone was established among the former Soviet republics of Central Asia prohibiting nuclear weapons.

In 1996, the International Court of Justice, the highest court of the United Nations, issued an Advisory Opinion concerned with the "Legality of the Threat or Use of Nuclear Weapons". The court ruled that the use or threat of use of nuclear weapons would violate various articles of international law, including the Geneva Conventions, the Hague Conventions, the UN Charter, and the Universal Declaration of Human Rights. Given the unique, destructive characteristics of nuclear weapons, the International Committee of the Red Cross calls on States to ensure that these weapons are never used, irrespective of whether they consider them lawful or not.

Additionally, there have been other, specific actions meant to discourage countries from developing nuclear arms. In the wake of the tests by India and Pakistan in 1998, economic sanctions were (temporarily) levied against both countries, though neither were signatories with the Nuclear Non-Proliferation Treaty. One of the stated casus belli for the initiation of the 2003 Iraq War was an accusation by the United States that Iraq was actively pursuing nuclear arms (though this was soon discovered not to be the case as the program had been discontinued). In 1981, Israel had bombed a nuclear reactor being constructed in Osirak, Iraq, in what it called an attempt to halt Iraq's previous nuclear arms ambitions; in 2007, Israel bombed another reactor being constructed in Syria.

In 2013, Mark Diesendorf said that governments of France, India, North Korea, Pakistan, UK, and South Africa have used nuclear power or research reactors to assist nuclear weapons development or to contribute to their supplies of nuclear explosives from military reactors.

In 2017, 122 countries mainly in the Global South voted in favor of adopting the Treaty on the Prohibition of Nuclear Weapons, which eventually entered into force in 2021.

The Doomsday Clock measures the likelihood of a human-made global catastrophe and is published annually by the Bulletin of the Atomic Scientists. The two years with the highest likelihood had previously been 1953, when the Clock was set to two minutes until midnight after the U.S. and the Soviet Union began testing hydrogen bombs, and 2018, following the failure of world leaders to address tensions relating to nuclear weapons and climate change issues. In 2023, following the escalation of nuclear threats during the Russian invasion of Ukraine, the doomsday clock was set to 90 seconds, the highest likelihood of global catastrophe since the existence of the Doomsday Clock.

Disarmament

Nuclear disarmament refers to both the act of reducing or eliminating nuclear weapons and to the end state of a nuclear-free world, in which nuclear weapons are eliminated.

Beginning with the 1963 Partial Test Ban Treaty and continuing through the 1996 Comprehensive Nuclear-Test-Ban Treaty, there have been many treaties to limit or reduce nuclear weapons testing and stockpiles. The 1968 Nuclear Non-Proliferation Treaty has as one of its explicit conditions that all signatories must "pursue negotiations in good faith" towards the long-term goal of "complete disarmament". The nuclear-weapon states have largely treated that aspect of the agreement as "decorative" and without force.

Only one country—South Africa—has ever fully renounced nuclear weapons they had independently developed. The former Soviet republics of Belarus, Kazakhstan, and Ukraine returned Soviet nuclear arms stationed in their countries to Russia after the collapse of the USSR.

Proponents of nuclear disarmament say that it would lessen the probability of nuclear war, especially accidentally. Critics of nuclear disarmament say that it would undermine the present nuclear peace and deterrence and would lead to increased global instability. Various American elder statesmen, who were in office during the Cold War period, have been advocating the elimination of nuclear weapons. These officials include Henry Kissinger, George Shultz, Sam Nunn, and William Perry. In January 2010, Lawrence M. Krauss stated that "no issue carries more importance to the long-term health and security of humanity than the effort to reduce, and perhaps one day, rid the world of nuclear weapons".

In January 1986, Soviet leader Mikhail Gorbachev publicly proposed a three-stage program for abolishing the world's nuclear weapons by the end of the 20th century. In the years after the end of the Cold War, there have been numerous campaigns to urge the abolition of nuclear weapons, such as that organized by the Global Zero movement, and the goal of a "world without nuclear weapons" was advocated by United States President Barack Obama in an April 2009 speech in Prague. A CNN poll from April 2010 indicated that the American public was nearly evenly split on the issue.

Some analysts have argued that nuclear weapons have made the world relatively safer, with peace through deterrence and through the stability–instability paradox, including in south Asia. Kenneth Waltz has argued that nuclear weapons have helped keep an uneasy peace, and further nuclear weapon proliferation might even help avoid the large scale conventional wars that were so common before their invention at the end of World War II. But former Secretary Henry Kissinger says there is a new danger, which cannot be addressed by deterrence: "The classical notion of deterrence was that there was some consequences before which aggressors and evildoers would recoil. In a world of suicide bombers, that calculation doesn't operate in any comparable way". George Shultz has said, "If you think of the people who are doing suicide attacks, and people like that get a nuclear weapon, they are almost by definition not deterrable".

As of early 2019, more than 90% of world's 13,865 nuclear weapons were owned by Russia and the United States.

United Nations

The UN Office for Disarmament Affairs (UNODA) is a department of the United Nations Secretariat established in January 1998 as part of the United Nations Secretary-General Kofi Annan's plan to reform the UN as presented in his report to the General Assembly in July 1997.

Its goal is to promote nuclear disarmament and non-proliferation and the strengthening of the disarmament regimes in respect to other weapons of mass destruction, chemical and biological weapons. It also promotes disarmament efforts in the area of conventional weapons, especially land mines and small arms, which are often the weapons of choice in contemporary conflicts.

Controversy

Ethics

Even before the first nuclear weapons had been developed, scientists involved with the Manhattan Project were divided over the use of the weapon. The role of the two atomic bombings of the country in Japan's surrender and the U.S.'s ethical justification for them has been the subject of scholarly and popular debate for decades. The question of whether nations should have nuclear weapons, or test them, has been continually and nearly universally controversial.

Notable nuclear weapons accidents

August 21, 1945: While conducting experiments on a plutonium-gallium core at Los Alamos National Laboratory, physicist Harry Daghlian received a lethal dose of radiation when an error caused it to enter prompt criticality. He died 25 days later, on September 15, 1945, from radiation poisoning.
May 21, 1946: While conducting further experiments on the same core at Los Alamos National Laboratory, physicist Louis Slotin accidentally caused the core to become briefly supercritical. He received a lethal dose of gamma and neutron radiation, and died nine days later on May 30, 1946. After the death of Daghlian and Slotin, the mass became known as the "demon core". It was ultimately used to construct a bomb for use on the Nevada Test Range.
February 13, 1950: a Convair B-36B crashed in northern British Columbia after jettisoning a Mark IV atomic bomb. This was the first such nuclear weapon loss in history. The accident was designated a "Broken Arrow"—an accident involving a nuclear weapon, but which does not present a risk of war. Experts believe that up to 50 nuclear weapons were lost during the Cold War.
May 22, 1957: a 42,000-pound (19,000 kg) Mark-17 hydrogen bomb accidentally fell from a bomber near Albuquerque, New Mexico. The detonation of the device's conventional explosives destroyed it on impact and formed a crater 25 feet (7.6 m) in diameter on land owned by the University of New Mexico. According to a researcher at the Natural Resources Defense Council, it was one of the most powerful bombs made to date.
June 7, 1960: the 1960 Fort Dix IM-99 accident destroyed a Boeing CIM-10 Bomarc nuclear missile and shelter and contaminated the BOMARC Missile Accident Site in New Jersey.
January 24, 1961: the 1961 Goldsboro B-52 crash occurred near Goldsboro, North Carolina. A Boeing B-52 Stratofortress carrying two Mark 39 nuclear bombs broke up in mid-air, dropping its nuclear payload in the process.
1965 Philippine Sea A-4 crash, where a Skyhawk attack aircraft with a nuclear weapon fell into the sea. The pilot, the aircraft, and the B43 nuclear bomb were never recovered. It was not until 1989 that the Pentagon revealed the loss of the one-megaton bomb.
January 17, 1966: the 1966 Palomares B-52 crash occurred when a B-52G bomber of the USAF collided with a KC-135 tanker during mid-air refuelling off the coast of Spain. The KC-135 was completely destroyed when its fuel load ignited, killing all four crew members. The B-52G broke apart, killing three of the seven crew members aboard. Of the four Mk28 type hydrogen bombs the B-52G carried, three were found on land near Almería, Spain. The non-nuclear explosives in two of the weapons detonated upon impact with the ground, resulting in the contamination of a 2-square-kilometer (490-acre) (0.78 square mile) area by radioactive plutonium. The fourth, which fell into the Mediterranean Sea, was recovered intact after a 21⁄2-month-long search.
January 21, 1968: the 1968 Thule Air Base B-52 crash involved a United States Air Force (USAF) B-52 bomber. The aircraft was carrying four hydrogen bombs when a cabin fire forced the crew to abandon the aircraft. Six crew members ejected safely, but one who did not have an ejection seat was killed while trying to bail out. The bomber crashed onto sea ice in Greenland, causing the nuclear payload to rupture and disperse, which resulted in widespread radioactive contamination. One of the bombs remains lost.
September 18–19, 1980: the Damascus Accident occurred in Damascus, Arkansas, where a Titan Missile equipped with a nuclear warhead exploded. The accident was caused by a maintenance man who dropped a socket from a socket wrench down an 80-foot (24 m) shaft, puncturing a fuel tank on the rocket. Leaking fuel resulted in a hypergolic fuel explosion, jettisoning the W-53 warhead beyond the launch site.

Nuclear testing and fallout

Over 500 atmospheric nuclear weapons tests were conducted at various sites around the world from 1945 to 1980. Radioactive fallout from nuclear weapons testing was first drawn to public attention in 1954 when the Castle Bravo hydrogen bomb test at the Pacific Proving Grounds contaminated the crew and catch of the Japanese fishing boat Lucky Dragon. One of the fishermen died in Japan seven months later, and the fear of contaminated tuna led to a temporary boycotting of the popular staple in Japan. The incident caused widespread concern around the world, especially regarding the effects of nuclear fallout and atmospheric nuclear testing, and "provided a decisive impetus for the emergence of the anti-nuclear weapons movement in many countries".

As public awareness and concern mounted over the possible health hazards associated with exposure to the nuclear fallout, various studies were done to assess the extent of the hazard. A Centers for Disease Control and Prevention/ National Cancer Institute study claims that fallout from atmospheric nuclear tests would lead to perhaps 11,000 excess deaths among people alive during atmospheric testing in the United States from all forms of cancer, including leukemia, from 1951 to well into the 21st century. As of March 2009, the U.S. is the only nation that compensates nuclear test victims. Since the Radiation Exposure Compensation Act of 1990, more than $1.38 billion in compensation has been approved. The money is going to people who took part in the tests, notably at the Nevada Test Site, and to others exposed to the radiation.

In addition, leakage of byproducts of nuclear weapon production into groundwater has been an ongoing issue, particularly at the Hanford site.

Effects of nuclear explosions

Effects of nuclear explosions on human health

Some scientists estimate that a nuclear war with 100 Hiroshima-size nuclear explosions on cities could cost the lives of tens of millions of people from long-term climatic effects alone. The climatology hypothesis is that if each city firestorms, a great deal of soot could be thrown up into the atmosphere which could blanket the earth, cutting out sunlight for years on end, causing the disruption of food chains, in what is termed a nuclear winter.

People near the Hiroshima explosion and who managed to survive the explosion subsequently suffered a variety of horrible medical effects. Some of these effects are still present to this day:

Initial stage—the first 1–9 weeks, in which are the greatest number of deaths, with 90% due to thermal injury or blast effects and 10% due to super-lethal radiation exposure.
Intermediate stage—from 10 to 12 weeks. The deaths in this period are from ionizing radiation in the median lethal range – LD50
Late period—lasting from 13 to 20 weeks. This period has some improvement in survivors' condition.
Delayed period—from 20+ weeks. Characterized by numerous complications, mostly related to healing of thermal and mechanical injuries, and if the individual was exposed to a few hundred to a thousand millisieverts of radiation, it is coupled with infertility, sub-fertility and blood disorders. Furthermore, ionizing radiation above a dose of around 50–100 millisievert exposure has been shown to statistically begin increasing one's chance of dying of cancer sometime in their lifetime over the normal unexposed rate of ~25%, in the long term, a heightened rate of cancer, proportional to the dose received, would begin to be observed after ~5+ years, with lesser problems such as eye cataracts and other more minor effects in other organs and tissue also being observed over the long term.

Fallout exposure—depending on if further afield individuals shelter in place or evacuate perpendicular to the direction of the wind, and therefore avoid contact with the fallout plume, and stay there for the days and weeks after the nuclear explosion, their exposure to fallout, and therefore their total dose, will vary. With those who do shelter in place, and or evacuate, experiencing a total dose that would be negligible in comparison to someone who just went about their life as normal.

Staying indoors until after the most hazardous fallout isotope, I-131 decays away to 0.1% of its initial quantity after ten half-lifes—which is represented by 80 days in I-131s case, would make the difference between likely contracting Thyroid cancer or escaping completely from this substance depending on the actions of the individual.

Effects of nuclear war

Nuclear war could yield unprecedented human death tolls and habitat destruction. Detonating large numbers of nuclear weapons would have an immediate, short term and long-term effects on the climate, potentially causing cold weather known as a "nuclear winter". In 1982, Brian Martin estimated that a US–Soviet nuclear exchange might kill 400–450 million directly, mostly in the United States, Europe and Russia, and maybe several hundred million more through follow-up consequences in those same areas. Many scholars have posited that a global thermonuclear war with Cold War-era stockpiles, or even with the current smaller stockpiles, may lead to the extinction of the human race. The International Physicians for the Prevention of Nuclear War believe that nuclear war could indirectly contribute to human extinction via secondary effects, including environmental consequences, societal breakdown, and economic collapse. It has been estimated that a relatively small-scale nuclear exchange between India and Pakistan involving 100 Hiroshima yield (15 kilotons) weapons, could cause a nuclear winter and kill more than a billion people.

According to a peer-reviewed study published in the journal Nature Food in August 2022, a full-scale nuclear war between the U.S. and Russia would directly kill 360 million people and more than 5 billion people would die from starvation. More than 2 billion people could die from a smaller-scale nuclear war between India and Pakistan.

Public opposition

Peace movements emerged in Japan and in 1954 they converged to form a unified "Japan Council against Atomic and Hydrogen Bombs." Japanese opposition to nuclear weapons tests in the Pacific Ocean was widespread, and "an estimated 35 million signatures were collected on petitions calling for bans on nuclear weapons".

In the United Kingdom, the first Aldermaston March organised by the Campaign for Nuclear Disarmament(CND) took place at Easter 1958, when, according to the CND, several thousand people marched for four days from Trafalgar Square, London, to the Atomic Weapons Research Establishment close to Aldermaston in Berkshire, England, to demonstrate their opposition to nuclear weapons. The Aldermaston marches continued into the late 1960s when tens of thousands of people took part in the four-day marches.

In 1959, a letter in the Bulletin of the Atomic Scientists was the start of a successful campaign to stop the Atomic Energy Commission dumping radioactive waste in the sea 19 kilometres from Boston. In 1962, Linus Pauling won the Nobel Peace Prize for his work to stop the atmospheric testing of nuclear weapons, and the "Ban the Bomb" movement spread.

In 1963, many countries ratified the Partial Test Ban Treaty prohibiting atmospheric nuclear testing. Radioactive fallout became less of an issue and the anti-nuclear weapons movement went into decline for some years. A resurgence of interest occurred amid European and American fears of nuclear war in the 1980s.

Costs and technology spin-offs

According to an audit by the Brookings Institution, between 1940 and 1996, the U.S. spent $10.9 trillion in present-day terms on nuclear weapons programs. 57% of which was spent on building nuclear weapons delivery systems. 6.3% of the total$, 681 billion in present-day terms, was spent on environmental remediation and nuclear waste management, for example cleaning up the Hanford site, and 7% of the total$, 763 billion was spent on making nuclear weapons themselves.

Non-weapons uses

Peaceful nuclear explosions are nuclear explosions conducted for non-military purposes, such as activities related to economic development including the creation of canals. During the 1960s and 1970s, both the United States and the Soviet Union conducted a number of PNEs. Six of the explosions by the Soviet Union are considered to have been of an applied nature, not just tests.

The United States and the Soviet Union later halted their programs. Definitions and limits are covered in the Peaceful Nuclear Explosions Treaty of 1976. The stalled Comprehensive Nuclear-Test-Ban Treaty of 1996 would prohibit all nuclear explosions, regardless of whether they are for peaceful purposes or not.

History of development

In the first decades of the 20th century, physics was revolutionized with developments in the understanding of the nature of atoms including the discoveries in atomic theory by John Dalton. Around the turn of the 20th century, it was discovered by Hans Geiger and Ernest Marsden and then Ernest Rutherford, that atoms had a highly dense, very small, charged central core called an atomic nucleus. In 1898, Pierre and Marie Curie discovered that pitchblende, an ore of uranium, contained a substance—which they named radium—that emitted large amounts of radiation. Ernest Rutherford and Frederick Soddy identified that atoms were breaking down and turning into different elements. Hopes were raised among scientists and laymen that the elements around us could contain tremendous amounts of unseen energy, waiting to be harnessed.

In Paris in 1934, Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles; in Italy Enrico Fermi reported similar results when bombarding uranium with neutrons.

In December 1938, Otto Hahn and Fritz Strassmann reported that they had detected the element barium after bombarding uranium with neutrons. Lise Meitner and Otto Robert Frisch correctly interpreted these results as being due to the splitting of the uranium atom. Frisch confirmed this experimentally on January 13, 1939. They gave the process the name "fission" because of its similarity to the splitting of a cell into two new cells. Even before it was published, news of Meitner's and Frisch's interpretation crossed the Atlantic. In their second publication on nuclear fission in February of 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction.

After learning about the German fission in 1939, Leo Szilard concluded that uranium would be the element which can realize his 1933 idea about nuclear chain reaction.

Uranium appears in nature primarily in two isotopes: uranium-238 and uranium-235. When the nucleus of uranium-235 absorbs a neutron, it undergoes nuclear fission, releasing energy and, on average, 2.5 neutrons. Because uranium-235 releases more neutrons than it absorbs, it can support a chain reaction and so is described as fissile. Uranium-238, on the other hand, is not fissile as it does not normally undergo fission when it absorbs a neutron.

By the start of the war in September 1939, many scientists likely to be persecuted by the Nazis had already escaped. Physicists on both sides were well aware of the possibility of utilizing nuclear fission as a weapon, but no one was quite sure how it could be engineered. In August 1939, concerned that Germany might have its own project to develop fission-based weapons, Albert Einstein signed a letter to U.S. President Franklin D. Roosevelt warning him of the threat.

Roosevelt responded by setting up the Uranium Committee under Lyman James Briggs but, with little initial funding ($6,000), progress was slow. It was not until the U.S. entered the war in December 1941 that Washington decided to commit the necessary resources to a top-secret high priority bomb project.

Organized research first began in Britain and Canada as part of the Tube Alloys project: the world's first nuclear weapons project. The Maud Committee was set up following the work of Frisch and Rudolf Peierls who calculated uranium-235's critical mass and found it to be much smaller than previously thought which meant that a deliverable bomb should be possible. In the February 1940 Frisch–Peierls memorandum they stated that: "The energy liberated in the explosion of such a super-bomb...will, for an instant, produce a temperature comparable to that of the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city."

Edgar Sengier, a director of Shinkolobwe Mine in the Congo which produced by far the highest quality uranium ore in the world, had become aware of uranium's possible use in a bomb. In late 1940, fearing that it might be seized by the Germans, he shipped the mine's entire stockpile of ore to a warehouse in New York.

Antiproton

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Antiproton

Antiproton
The quark content of the antiproton.
Classification	Antibaryon
Composition	2 up antiquarks, 1 down antiquark
Statistics	Fermionic
Family	Hadron
Interactions	Strong, weak, electromagnetic, gravity
Symbol	p
Antiparticle	Proton
Theorised	Paul Dirac (1933)
Discovered	Emilio Segrè & Owen Chamberlain (1955)
Mass	1.67262192369(51)×10⁻²⁷ kg 938.27208816(29) MeV/c²
Electric charge	−1 e
Magnetic moment	−2.7928473441(42) μ_N
Spin	1⁄2
Isospin	−1⁄2

The antiproton,
p
, (pronounced p-bar) is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy.

The existence of the antiproton with electric charge of −1 e, opposite to the electric charge of +1 e of the proton, was predicted by Paul Dirac in his 1933 Nobel Prize lecture. Dirac received the Nobel Prize for his 1928 publication of his Dirac equation that predicted the existence of positive and negative solutions to Einstein's energy equation ( $E = m c^{2}$ ) and the existence of the positron, the antimatter analog of the electron, with opposite charge and spin.

The antiproton was first experimentally confirmed in 1955 at the Bevatron particle accelerator by University of California, Berkeley, physicists Emilio Segrè and Owen Chamberlain, for which they were awarded the 1959 Nobel Prize in Physics.

In terms of valence quarks, an antiproton consists of two up antiquarks and one down antiquark (
u

u

d
). The properties of the antiproton that have been measured all match the corresponding properties of the proton, with the exception that the antiproton has electric charge and magnetic moment that are the opposites of those in the proton, which is to be expected from the antimatter equivalent of a proton. The questions of how matter is different from antimatter, and the relevance of antimatter in explaining how our universe survived the Big Bang, remain open problems—open, in part, due to the relative scarcity of antimatter in today's universe.

Occurrence in nature

Antiprotons have been detected in cosmic rays beginning in 1979, first by balloon-borne experiments and more recently by satellite-based detectors. The standard picture for their presence in cosmic rays is that they are produced in collisions of cosmic ray protons with atomic nuclei in the interstellar medium, via the reaction, where A represents a nucleus:

p
+ A →
p
+
p
+
p
+ A

The secondary antiprotons (
p
) then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium, and antiprotons can also be lost by "leaking out" of the galaxy.

The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions. These experimental measurements set upper limits on the number of antiprotons that could be produced in exotic ways, such as from annihilation of supersymmetric dark matter particles in the galaxy or from the Hawking radiation caused by the evaporation of primordial black holes. This also provides a lower limit on the antiproton lifetime of about 1–10 million years. Since the galactic storage time of antiprotons is about 10 million years, an intrinsic decay lifetime would modify the galactic residence time and distort the spectrum of cosmic ray antiprotons. This is significantly more stringent than the best laboratory measurements of the antiproton lifetime:

LEAR collaboration at CERN: 0.08 years
Antihydrogen Penning trap of Gabrielse et al.: 0.28 years
BASE experiment at CERN: 10.2 years
APEX collaboration at Fermilab: 50000 years for
p
→
μ⁻
+ anything
APEX collaboration at Fermilab: 300000 years for
p
→
e⁻
+
γ

The magnitude of properties of the antiproton are predicted by CPT symmetry to be exactly related to those of the proton. In particular, CPT symmetry predicts the mass and lifetime of the antiproton to be the same as those of the proton, and the electric charge and magnetic moment of the antiproton to be opposite in sign and equal in magnitude to those of the proton. CPT symmetry is a basic consequence of quantum field theory and no violations of it have ever been detected.

List of recent cosmic ray detection experiments

BESS: balloon-borne experiment, flown in 1993, 1995, 1997, 2000, 2002, 2004 (Polar-I) and 2007 (Polar-II).
CAPRICE: balloon-borne experiment, flown in 1994 and 1998.
HEAT: balloon-borne experiment, flown in 2000.
AMS: space-based experiment, prototype flown on the Space Shuttle in 1998, intended for the International Space Station, launched May 2011.
PAMELA: satellite experiment to detect cosmic rays and antimatter from space, launched June 2006. Recent report discovered 28 antiprotons in the South Atlantic Anomaly.

Modern experiments and applications

Production

Antiprotons were routinely produced at Fermilab for collider physics operations in the Tevatron, where they were collided with protons. The use of antiprotons allows for a higher average energy of collisions between quarks and antiquarks than would be possible in proton–proton collisions. This is because the valence quarks in the proton, and the valence antiquarks in the antiproton, tend to carry the largest fraction of the proton or antiproton's momentum.

Formation of antiprotons requires energy equivalent to a temperature of 10 trillion K (10¹³ K), and this does not tend to happen naturally. However, at CERN, protons are accelerated in the Proton Synchrotron to an energy of 26 G eV and then smashed into an iridium rod. The protons bounce off the iridium nuclei with enough energy for matter to be created. A range of particles and antiparticles are formed, and the antiprotons are separated off using magnets in vacuum.

Measurements

In July 2011, the ASACUSA experiment at CERN determined the mass of the antiproton to be 1836.1526736(23) times that of the electron. This is the same as the mass of a proton, within the level of certainty of the experiment.

In October 2017, scientists working on the BASE experiment at CERN reported a measurement of the antiproton magnetic moment to a precision of 1.5 parts per billion. It is consistent with the most precise measurement of the proton magnetic moment (also made by BASE in 2014), which supports the hypothesis of CPT symmetry. This measurement represents the first time that a property of antimatter is known more precisely than the equivalent property in matter.

In January 2022, by comparing the charge-to-mass ratios between antiproton and negatively charged hydrogen ion, the BASE experiment has determined the antiproton's charge-to-mass ratio is identical to the proton's, down to 16 parts per trillion.

Possible applications

Antiprotons have been shown within laboratory experiments to have the potential to treat certain cancers, in a similar method currently used for ion (proton) therapy. The primary difference between antiproton therapy and proton therapy is that following ion energy deposition the antiproton annihilates, depositing additional energy in the cancerous region.

Isotopic labeling

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Isotopic_labeling

Isotopic labeling (or isotopic labelling) is a technique used to track the passage of an isotope (an atom with a detectable variation in neutron count) through chemical reaction, metabolic pathway, or a biological cell. The reactant is 'labeled' by replacing one or more specific atoms with their isotopes. The reactant is then allowed to undergo the reaction. The position of the isotopes in the products is measured to determine the sequence the isotopic atom followed in the reaction or the cell's metabolic pathway. The nuclides used in isotopic labeling may be stable nuclides or radionuclides. In the latter case, the labeling is called radiolabeling.

In isotopic labeling, there are multiple ways to detect the presence of labeling isotopes; through their mass, vibrational mode, or radioactive decay. Mass spectrometry detects the difference in an isotope's mass, while infrared spectroscopy detects the difference in the isotope's vibrational modes. Nuclear magnetic resonance detects atoms with different gyromagnetic ratios. The radioactive decay can be detected through an ionization chamber or autoradiographs of gels.

An example of the use of isotopic labeling is the study of phenol (C₆H₅OH) in water by replacing common hydrogen (protium) with deuterium (deuterium labeling). Upon adding phenol to deuterated water (water containing D₂O in addition to the usual H₂O), the substitution of deuterium for the hydrogen is observed in phenol's hydroxyl group (resulting in C₆H₅OD), indicating that phenol readily undergoes hydrogen-exchange reactions with water. Only the hydroxyl group is affected, indicating that the other 5 hydrogen atoms do not participate in the exchange reactions.

Isotopic tracer

A carbon-13 label was used to determine the mechanism in the 1,2- to 1,3-didehydrobenzene conversion of the phenyl substituted aryne precursor 1 to acenaphthylene.

An isotopic tracer, (also "isotopic marker" or "isotopic label"), is used in chemistry and biochemistry to help understand chemical reactions and interactions. In this technique, one or more of the atoms of the molecule of interest is substituted for an atom of the same chemical element, but of a different isotope (like a radioactive isotope used in radioactive tracing). Because the labeled atom has the same number of protons, it will behave in almost exactly the same way as its unlabeled counterpart and, with few exceptions, will not interfere with the reaction under investigation. The difference in the number of neutrons, however, means that it can be detected separately from the other atoms of the same element.

Nuclear magnetic resonance (NMR) and mass spectrometry (MS) are used to investigate the mechanisms of chemical reactions. NMR and MS detects isotopic differences, which allows information about the position of the labeled atoms in the products' structure to be determined. With information on the positioning of the isotopic atoms in the products, the reaction pathway the initial metabolites utilize to convert into the products can be determined. Radioactive isotopes can be tested using the autoradiographs of gels in gel electrophoresis. The radiation emitted by compounds containing the radioactive isotopes darkens a piece of photographic film, recording the position of the labeled compounds relative to one another in the gel.

Isotope tracers are commonly used in the form of isotope ratios. By studying the ratio between two isotopes of the same element, we avoid effects involving the overall abundance of the element, which usually swamp the much smaller variations in isotopic abundances. Isotopic tracers are some of the most important tools in geology because they can be used to understand complex mixing processes in earth systems. Further discussion of the application of isotopic tracers in geology is covered under the heading of isotope geochemistry.

Isotopic tracers are usually subdivided into two categories: stable isotope tracers and radiogenic isotope tracers. Stable isotope tracers involve only non-radiogenic isotopes and usually are mass-dependent. In theory, any element with two stable isotopes can be used as an isotopic tracer. However, the most commonly used stable isotope tracers involve relatively light isotopes, which readily undergo fractionation in natural systems. See also isotopic signature. A radiogenic isotope tracer involves an isotope produced by radioactive decay, which is usually in a ratio with a non-radiogenic isotope (whose abundance in the earth does not vary due to radioactive decay).

Stable isotope labeling

Isotopic tracing through reactions in the pentose phosphate pathway. The blue circles indicate a labeled carbon atom, while white circles are an unlabeled carbon atom.

Stable isotope labeling involves the use of non-radioactive isotopes that can act as a tracers used to model several chemical and biochemical systems. The chosen isotope can act as a label on that compound that can be identified through nuclear magnetic resonance (NMR) and mass spectrometry (MS). Some of the most common stable isotopes are ²H, ¹³C, and ¹⁵N, which can further be produced into NMR solvents, amino acids, nucleic acids, lipids, common metabolites and cell growth media. The compounds produced using stable isotopes are either specified by the percentage of labeled isotopes (i.e. 30% uniformly labeled ¹³C glucose contains a mixture that is 30% labeled with ¹³ carbon isotope and 70% naturally labeled carbon) or by the specifically labeled carbon positions on the compound (i.e. 1-¹³C glucose which is labeled at the first carbon position of glucose).

A network of reactions adopted from the glycolysis pathway and the pentose phosphate pathway is shown in which the labeled carbon isotope rearranges to different carbon positions throughout the network of reactions. The network starts with fructose 6-phosphate (F6P), which has 6 carbon atoms with a label ¹³C at carbon position 1 and 2. 1,2-¹³C F6P becomes two glyceraldehyde 3-phosphate (G3P), one 2,3-¹³C T3P and one unlabeled T3P. The 2,3-¹³C T3P can now be reacted with sedoheptulose 7-phosphate (S7P) to form an unlabeled erythrose 4-phosphate(E4P) and a 5,6-¹³C F6P. The unlabeled T3P will react with the S7P to synthesize unlabeled products. The figure demonstrates the use of stable isotope labeling to discover the carbon atom rearrangement through reactions using position specific labeled compounds.

Metabolic flux analysis using stable isotope labeling

Determining the percent of isotope labeling throughout a reaction. If a 50% labeled and 50% unlabeled metabolite is split in the manner shown, the expected percent of each outcome can be found. The blue circles indicate a labeled atom, while a white circle indicates an unlabeled atom.

Metabolic flux analysis (MFA) using stable isotope labeling is an important tool for explaining the flux of certain elements through the metabolic pathways and reactions within a cell. An isotopic label is fed to the cell, then the cell is allowed to grow utilizing the labeled feed. For stationary metabolic flux analysis the cell must reach a steady state (the isotopes entering and leaving the cell remain constant with time) or a quasi-steady state (steady state is reached for a given period of time). The isotope pattern of the output metabolite is determined. The output isotope pattern provides valuable information, which can be used to find the magnitude of flux, rate of conversion from reactants to products, through each reaction.

The figure demonstrates the ability to use different labels to determine the flux through a certain reaction. Assume the original metabolite, a three carbon compound, has the ability to either split into a two carbon metabolite and one carbon metabolite in one reaction then recombine or remain a three carbon metabolite. If the reaction is provided with two isotopes of the metabolite in equal proportion, one completely labeled (blue circles), commonly known as uniformly labeled, and one completely unlabeled (white circles). The pathway down the left side of the diagram does not display any change in the metabolites, while the right side shows the split and recombination. As shown, if the metabolite only takes the pathway down the left side, it remains in a 50–50 ratio of uniformly labeled to unlabeled metabolite. If the metabolite only takes the right side new labeling patterns can occur, all in equal proportion. Other proportions can occur depending on how much of the original metabolite follows the left side of the pathway versus the right side of the pathway. Here the proportions are shown for a situation in which half of the metabolites take the left side and half the right, but other proportions can occur. These patterns of labeled atoms and unlabeled atoms in one compound represent isotopomers. By measuring the isotopomer distribution of the differently labeled metabolites, the flux through each reaction can be determined.

MFA combines the data harvested from isotope labeling with the stoichiometry of each reaction, constraints, and an optimization procedure resolve a flux map. The irreversible reactions provide the thermodynamic constraints needed to find the fluxes. A matrix is constructed that contains the stoichiometry of the reactions. The intracellular fluxes are estimated by using an iterative method in which simulated fluxes are plugged into the stoichiometric model. The simulated fluxes are displayed in a flux map, which shows the rate of reactants being converted to products for each reaction. In most flux maps, the thicker the arrow, the larger the flux value of the reaction.

Isotope labeling measuring techniques

Any technique in measuring the difference between isotopomers can be used. The two primary methods, nuclear magnetic resonance (NMR) and mass spectrometry (MS), have been developed for measuring mass isotopomers in stable isotope labeling.

Proton NMR was the first technique used for ¹³C-labeling experiments. Using this method, each single protonated carbon position inside a particular metabolite pool can be observed separately from the other positions. This allows the percentage of isotopomers labeled at that specific position to be known. The limit to proton NMR is that if there are n carbon atoms in a metabolite, there can only be at most n different positional enrichment values, which is only a small fraction of the total isotopomer information. Although the use of proton NMR labeling is limiting, pure proton NMR experiments are much easier to evaluate than experiments with more isotopomer information.

In addition to Proton NMR, using ¹³C NMR techniques will allow a more detailed view of the distribution of the isotopomers. A labeled carbon atom will produce different hyperfine splitting signals depending on the labeling state of its direct neighbors in the molecule. A singlet peak emerges if the neighboring carbon atoms are not labeled. A doublet peak emerges if only one neighboring carbon atom is labeled. The size of the doublet split depends on the functional group of the neighboring carbon atom. If two neighboring carbon atoms are labeled, a doublet of doublets may degenerate into a triplet if the doublet splittings are equal.

The drawbacks to using NMR techniques for metabolic flux analysis purposes is that it is different from other NMR applications because it is a rather specialized discipline. An NMR spectrometer may not be directly available for all research teams. The optimization of NMR measurement parameters and proper analysis of peak structures requires a skilled NMR specialist. Certain metabolites also may require specialized measurement procedures to obtain additional isotopomer data. In addition, specially adapted software tools are needed to determine the precise quantity of peak areas as well as identifying the decomposition of entangled singlet, doublet, and triplet peaks.

As opposed to nuclear magnetic resonance, mass spectrometry (MS) is another method that is more applicable and sensitive to metabolic flux analysis experiments. MS instruments are available in different variants. Different from two-dimensional nuclear magnetic resonance (2D-NMR), the MS instruments work directly with hydrolysate.

In gas chromatography-mass spectrometry (GC-MS), the MS is coupled to a gas chromatograph to separate the compounds of the hydrolysate. The compounds eluting from the GC column are then ionized and simultaneously fragmented. The benefit in using GC-MS is that not only are the mass isotopomers of the molecular ion measured but also the mass isotopomer spectrum of several fragments, which significantly increases the measured information.

In liquid chromatography-mass spectrometry (LC-MS), the GC is replaced with a liquid chromatograph. The main difference is that chemical derivatization is not necessary. Applications of LC-MS to MFA, however, are rare.

In each case, MS instruments divide a particular isotopomer distribution by its molecular weight. All isotopomers of a particular metabolite that contain the same number of labeled carbon atoms are collected in one peak signal. Because every isotopomer contributes to exactly one peak in the MS spectrum, the percentage value can then be calculated for each peak, yielding the mass isotopomer fraction. For a metabolite with n carbon atoms, n+1 measurements are produced. After normalization, exactly n informative mass isotopomer quantities remain.

The drawback to using MS techniques is that for gas chromatography, the sample must be prepared by chemical derivatization in order to obtain molecules with charge. There are numerous compounds used to derivatize samples. N,N-Dimethylformamide dimethyl acetal (DMFDMA) and N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide (MTBSTFA) are two examples of compounds that have been used to derivatize amino acids.

In addition, strong isotope effects observed affect the retention time of differently labeled isotopomers in the GC column. Overloading of the GC column also must be prevented.

Lastly, the natural abundance of other atoms than carbon also leads to a disturbance in the mass isotopomer spectrum. For example, each oxygen atom in the molecule might also be present as a ¹⁷O isotope and as a ¹⁸O isotope. A more significant impact of the natural abundance of isotopes is the effect of silicon with a natural abundance of the isotopes ²⁹Si and ³⁰Si. Si is used in derivatizing agents for MS techniques.

Radioisotopic labeling

Radioisotopic labeling is a technique for tracking the passage of a sample of substance through a system. The substance is "labeled" by including radionuclides in its chemical composition. When these decay, their presence can be determined by detecting the radiation emitted by them. Radioisotopic labeling is a special case of isotopic labeling.

For these purposes, a particularly useful type of radioactive decay is positron emission. When a positron collides with an electron, it releases two high-energy photons traveling in diametrically opposite directions. If the positron is produced within a solid object, it is likely to do this before traveling more than a millimeter. If both of these photons can be detected, the location of the decay event can be determined very precisely.

Strictly speaking, radioisotopic labeling includes only cases where radioactivity is artificially introduced by experimenters, but some natural phenomena allow similar analysis to be performed. In particular, radiometric dating uses a closely related principle.

Applications

Applications in human mineral nutrition research

The use of stable isotope tracers to study mineral nutrition and metabolism in humans was first reported in the 1960s. While radioisotopes had been used in human nutrition research for several decades prior, stable isotopes presented a safer option, especially in subjects for which there is elevated concern about radiation exposure, e.g. pregnant and lactating women and children. Other advantages offered by stable isotopes include the ability to study elements having no suitable radioisotopes and to study long-term tracer behavior. Thus the use of stable isotopes became commonplace with the increasing availability of isotopically enriched materials and inorganic mass spectrometers. The use of stable isotopes instead of radioisotopes does have several drawbacks: larger quantities of tracer are required, having the potential of perturbing the naturally existing mineral; analytical sample preparation is more complex and mass spectrometry instrumentation more costly; the presence of tracer in whole bodies or particular tissues cannot be measured externally. Nonetheless, the advantages have prevailed making stable isotopes the standard in human studies.

Most of the minerals that are essential for human health and of particular interest to nutrition researchers have stable isotopes, some well-suited as biological tracers because of their low natural abundance. Iron, zinc, calcium, copper, magnesium, selenium and molybdenum are among the essential minerals having stable isotopes to which isotope tracer methods have been applied. Iron, zinc and calcium in particular have been extensively studied.

Aspects of mineral nutrition/metabolism that are studied include absorption (from the gastrointestinal tract into the body), distribution, storage, excretion and the kinetics of these processes. Isotope tracers are administered to subjects orally (with or without food, or with a mineral supplement) and/or intravenously. Isotope enrichment is then measured in blood plasma, erythrocytes, urine and/or feces. Enrichment has also been measured in breast milk and intestinal contents. Tracer experiment design sometimes differs between minerals due to differences in their metabolism. For example, iron absorption is usually determined from incorporation of tracer in erythrocytes whereas zinc or calcium absorption is measured from tracer appearance in plasma, urine or feces. The administration of multiple isotope tracers in a single study is common, permitting the use of more reliable measurement methods and simultaneous investigations of multiple aspects of metabolism.

The measurement of mineral absorption from the diet, often conceived of as bioavailability, is the most common application of isotope tracer methods to nutrition research. Among the purposes of such studies are the investigations of how absorption is influenced by type of food (e.g. plant vs animal source, breast milk vs formula), other components of the diet (e.g. phytate), disease and metabolic disorders (e.g. environmental enteric dysfunction), the reproductive cycle, quantity of mineral in diet, chronic mineral deficiency, subject age and homeostatic mechanisms. When results from such studies are available for a mineral, they may serve as a basis for estimations of the human physiological and dietary requirements of the mineral.

When tracer is administered with food for the purpose of observing mineral absorption and metabolism, it may be in the form of an intrinsic or extrinsic label. An intrinsic label is isotope that has been introduced into the food during its production, thus enriching the natural mineral content of the food, whereas extrinsic labeling refers to the addition of tracer isotope to the food during the study. Because it is a very time-consuming and expensive approach, intrinsic labeling is not routinely used. Studies comparing measurements of absorption using intrinsic and extrinsic labeling of various foods have generally demonstrated good agreement between the two labeling methods, supporting the hypothesis that extrinsic and natural minerals are handled similarly in the human gastrointestinal tract.

Enrichment is quantified from the measurement of isotope ratios, the ratio of the tracer isotope to a reference isotope, by mass spectrometry. Multiple definitions and calculations of enrichment have been adopted by different researchers. Calculations of enrichment become more complex when multiple tracers are used simultaneously. Because enriched isotope preparations are never isotopically pure, i.e. they contain all the element's isotopes in unnatural abundances, calculations of enrichment of multiple isotope tracers must account for the perturbation of each isotope ratio by the presence of the other tracers.

Due to the prevalence of mineral deficiencies and their critical impact on human health and well-being in resource-poor countries, the International Atomic Energy Agency has recently published detailed and comprehensive descriptions of stable isotope methods to facilitate the dissemination of this knowledge to researchers beyond western academic centers.

Applications in proteomics

In proteomics, the study of the full set of proteins expressed by a genome, identifying diseases biomarkers can involve the usage of stable isotope labeling by amino acids in cell culture (SILAC), that provides isotopic labeled forms of amino acid used to estimate protein levels. In protein recombinant, manipulated proteins are produced in large quantities and isotope labeling is a tool to test for relevant proteins. The method used to be about selectively enrich nuclei with ¹³C or ¹⁵N or deplete ¹H from them. The recombinant would be expressed in E.coli with media containing ¹⁵N-ammonium chloride as a source of nitrogen. The resulting ¹⁵N labeled proteins are then purified by immobilized metal affinity and their percentage estimated. In order to increase the yield of labeled proteins and cut down the cost of isotope labeled media, an alternative procedure primarily increases the cell mass using unlabeled media before introducing it in a minimal amount of labeled media. Another application of isotope labeling would be in measuring DNA synthesis, that is cell proliferation in vitro. Uses H₃-thymidine labeling to compare pattern of synthesis (or sequence) in cells.

Applications for ecosystem process analysis

Isotopic tracers are used to examine processes in natural systems, especially terrestrial and aquatic environments. In soil science ¹⁵N tracers are used extensively to study nitrogen cycling, whereas ¹³C and ¹⁴C, stable and radioisotopes of carbon respectively, are used for studying turnover of organic compounds and fixation of CO₂ by autotrophs. For example, Marsh et al. (2005) used dual labeled (¹⁵N- and ¹⁴C) urea to demonstrate utilization of the compound by ammonia oxidizers as both an energy source (ammonia oxidation) and carbon source (chemoautotrophic carbon fixation). Deuterated water is also used for tracing the fate and ages of water in a tree or in an ecosystem.

Applications for oceanography

Tracers are also used extensively in oceanography to study a wide array of processes. The isotopes used are typically naturally occurring with well-established sources and rates of formation and decay. However, anthropogenic isotopes may also be used with great success. The researchers measure the isotopic ratios at different locations and times to infer information about the physical processes of the ocean.

Particle transport

The ocean is an extensive network of particle transport. Thorium isotopes can help researchers decipher the vertical and horizontal movement of matter. ²³⁴Th has a constant, well-defined production rate in the ocean and a half-life of 24 days. This naturally occurring isotope has been shown to vary linearly with depth. Therefore, any changes in this linear pattern can be attributed to the transport of ²³⁴Th on particles. For example, low isotopic ratios in surface water with very high values a few meters down would indicate a vertical flux in the downward direction. Furthermore, the thorium isotope may be traced within a specific depth to decipher the lateral transport of particles.

Circulation

Circulation within local systems, such as bays, estuaries, and groundwater, may be examined with radium isotopes. ²²³Ra has a half-life of 11 days and can occur naturally at specific locations in rivers and groundwater sources. The isotopic ratio of radium will then decrease as the water from the source river enters a bay or estuary. By measuring the amount of ²²³Ra at a number of different locations, a circulation pattern can be deciphered. This same exact process can also be used to study the movement and discharge of groundwater.

Various isotopes of lead can be used to study circulation on a global scale. Different oceans (i.e. the Atlantic, Pacific, Indian, etc.) have different isotopic signatures. This results from differences in isotopic ratios of sediments and rocks within the different oceans. Because the different isotopes of lead have half-lives of 50–200 years, there is not enough time for the isotopic ratios to be homogenized throughout the whole ocean. Therefore, precise analysis of Pb isotopic ratios can be used to study the circulation of the different oceans.

Tectonic processes and climate change

Isotopes with extremely long half-lives and their decay products can be used to study multi-million year processes, such as tectonics and extreme climate change. For example, in rubidium–strontium dating, the isotopic ratio of strontium (⁸⁷Sr/⁸⁶Sr) can be analyzed within ice cores to examine changes over the earth's lifetime. Differences in this ratio within the ice core would indicate significant alterations in the earth's geochemistry.

Isotopes related to nuclear weapons

The aforementioned processes can be measured using naturally occurring isotopes. Nevertheless, anthropogenic isotopes are also extremely useful for oceanographic measurements. Nuclear weapons tests released a plethora of uncommon isotopes into the world's oceans. ³H, ¹²⁹I, and ¹³⁷Cs can be found dissolved in seawater, while ²⁴¹Am and ²³⁸Pu are attached to particles. The isotopes dissolved in water are particularly useful in studying global circulation. For example, differences in lateral isotopic ratios within an ocean can indicate strong water fronts or gyres. Conversely, the isotopes attached to particles can be used to study mass transport within water columns. For instance, high levels of Am or Pu can indicate downwelling when observed at great depths, or upwelling when observed at the surface.

Methods for isotopic labeling

Chemical synthesis
Enzyme-mediated exchange
Recombinant protein expression in isotopic labeled media.

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Tuesday, January 30, 2024

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