In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak interaction participates in nuclear fission, and the theory describing it in terms of both its behaviour and effects is sometimes called quantum flavourdynamics (QFD). However, the term QFD is rarely used, because the weak force is better understood in terms of electroweak theory (EWT). The effective range of the weak force is limited to subatomic distances, and is less than the diameter of a proton.
Introduction
Weak interaction
Weak interaction | Introduction
In the Nuclear Fission Nuclear fission In an atom. Between subatomic particles
where do the nuclear and weak interaction forces occur
user
In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak interaction participates in nuclear fission, and the theory describing it in terms of both its behaviour and effects is sometimes called quantum flavourdynamics (QFD). However, the term QFD is rarely used, because the weak force is better understood in terms of electroweak theory (EWT). The effective range of the weak force is limited to subatomic distances, and is less than the diameter of a proton.
Weak interaction | Introduction
Artificially, the energy associated with the nuclear force is partially released in nuclear power and nuclear weapons, both in uranium or plutonium-based fission weapons and in fusion weapons like the hydrogen bomb.
Strong interaction | Introduction
In this capacity it is called the nuclear force (or "residual strong force"). So the residuum from the strong interaction within protons and neutrons also binds nuclei together. As such, the residual strong interaction obeys a distance-dependent behavior between nucleons that is quite different from that when it is acting to bind quarks within nucleons. Additionally, distinctions exist in the binding energies of the nuclear force of nuclear fusion vs nuclear fission. Nuclear fusion accounts for most energy production in the Sun and other stars. Nuclear fission allows for decay of radioactive elements and isotopes, although it is often mediated by the weak interaction.
Strong interaction | Introduction
Scaled up by just one and a half orders of magnitude, at distances of around 3×10 m, the weak interaction becomes 10,000 times weaker. The weak interaction affects all the fermions of the Standard Model, as well as the Higgs boson; neutrinos interact only through gravity and the weak interaction. The weak interaction does not produce bound states nor does it involve binding energy – something that gravity does on an astronomical scale, that the electromagnetic force does at the atomic level, and that the strong nuclear force does inside nuclei.
Weak interaction | Properties
The "weak interaction" or "weak nuclear force" is responsible for some nuclear phenomena such as beta decay. Electromagnetism and the weak force are now understood to be two aspects of a unified electroweak interaction — this discovery was the first step toward the unified theory known as the Standard Model. In the theory of the electroweak interaction, the carriers of the weak force are the massive gauge bosons called the W and Z bosons. The weak interaction is the only known interaction that does not conserve parity; it is left-right asymmetric. The weak interaction even violates CP symmetry but does conserve CPT.
Fundamental interaction | Weak interaction
In nuclear physics and particle physics, the strong interaction is the mechanism responsible for the strong nuclear force, and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation. At the range of 10 m (1 femtometer), the strong force is approximately 137 times as strong as electromagnetism, a million times as strong as the weak interaction, and 10 times as strong as gravitation. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as the proton and neutron. In addition, the strong force binds these neutrons and protons to create atomic nuclei.
Strong interaction | Introduction
The Standard Model of particle physics provides a uniform framework for understanding the electromagnetic, weak, and strong interactions. An interaction occurs when two particles (typically but not necessarily half-integer spin fermions) exchange integer-spin, force-carrying bosons. The fermions involved in such exchanges can be either elementary (e.g. electrons or quarks) or composite (e.g. protons or neutrons), although at the deepest levels, all weak interactions ultimately are between elementary particles. In the weak interaction, fermions can exchange three types of force carriers, namely W, W, and Z bosons. The masses of these bosons are far greater than the mass of a proton or neutron, which is consistent with the short range of the weak force.
Weak interaction | Background
Unlike the strong force itself, the residual strong force, "does" diminish in strength, and it in fact diminishes rapidly with distance. The decrease is approximately as a negative exponential power of distance, though there is no simple expression known for this; see Yukawa potential. The rapid decrease with distance of the attractive residual force and the less-rapid decrease of the repulsive electromagnetic force acting between protons within a nucleus, causes the instability of larger atomic nuclei, such as all those with atomic numbers larger than 82 (the element lead).
Strong interaction | Residual strong force
The nuclear force is a residual effect of the more fundamental strong force, or strong interaction. The strong interaction is the attractive force that binds the elementary particles called quarks together to form the nucleons (protons and neutrons) themselves. This more powerful force, one of the fundamental forces of nature, is mediated by particles called gluons. Gluons hold quarks together through color charge which is analogous to electric charge, but far stronger. Quarks, gluons, and their dynamics are mostly confined within nucleons, but residual influences extend slightly beyond nucleon boundaries to give rise to the nuclear force. The nuclear forces arising between nucleons are analogous to the forces in chemistry between neutral atoms or molecules called London forces.
Nuclear force | The nuclear force as a residual of the strong force
There are two "nuclear forces", which today are usually described as interactions that take place in quantum theories of particle physics. The strong nuclear force is the force responsible for the structural integrity of atomic nuclei while the weak nuclear force is responsible for the decay of certain nucleons into leptons and other types of hadrons. The strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics (QCD). The strong force is the fundamental force mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves. The (aptly named) strong interaction is the "strongest" of the four fundamental forces.
Force | Strong nuclear
It manifests as a force between the "colorless" hadrons, and is sometimes known as the strong nuclear force or simply nuclear force. The nuclear force acts between hadrons, known as mesons and baryons. This "residual strong force", acting indirectly, transmits gluons that form part of the virtual "π" and "ρ" mesons, which, in turn, transmit the force between nucleons that holds the nucleus (beyond protium) together. The residual strong force is thus a minor residuum of the strong force that binds quarks together into protons and neutrons. This same force is much weaker "between" neutrons and protons, because it is mostly neutralized "within" them, in the same way that electromagnetic forces between neutral atoms (van der Waals forces) are much weaker than the electromagnetic forces that hold electrons in association with the nucleus, forming the atoms.
Strong interaction | Residual strong force
The nuclear force (or nucleon–nucleon interaction or residual strong force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nuclear force almost identically. Since protons have charge +1 "e", they experience an electric force that tends to push them apart, but at short range the attractive nuclear force is strong enough to overcome the electromagnetic force. The nuclear force binds nucleons into atomic nuclei. The nuclear force is powerfully attractive between nucleons at distances of about 1 femtometre (fm, or 1.0 × 10 metres), but it rapidly decreases to insignificance at distances beyond about 2.5 fm.
Nuclear force | Introduction
Nevertheless, they are strong enough to bind neutrons and protons over short distances, and overcome the electrical repulsion between protons in the nucleus. Sometimes, the nuclear force is called the residual strong force, in contrast to the strong interactions which arise from QCD. This phrasing arose during the 1970s when QCD was being established. Before that time, the "strong nuclear force" referred to the inter-nucleon potential. After the verification of the quark model, "strong interaction" has come to mean QCD.
Nuclear force | The nuclear force as a residual of the strong force
In fact, the force is termed "weak" because its field strength over a given distance is typically several orders of magnitude less than that of the strong nuclear force or electromagnetic force. Quarks, which make up composite particles like neutrons and protons, come in six "flavours" – up, down, strange, charm, top and bottom – which give those composite particles their properties. The weak interaction is unique in that it allows quarks to swap their flavour for another. The swapping of those properties is mediated by the force carrier bosons. For example, during beta minus decay, a down quark within a neutron is changed into an up quark, thus converting the neutron to a proton and resulting in the emission of an electron and an electron antineutrino.
Weak interaction | Background
The weak force is due to the exchange of the heavy W and Z bosons. Its most familiar effect is beta decay (of neutrons in atomic nuclei) and the associated radioactivity. The word "weak" derives from the fact that the field strength is some 10 times less than that of the strong force. Still, it is stronger than gravity over short distances. A consistent electroweak theory has also been developed, which shows that electromagnetic forces and the weak force are indistinguishable at a temperatures in excess of approximately 10 kelvins. Such temperatures have been probed in modern particle accelerators and show the conditions of the universe in the early moments of the Big Bang.
Force | Weak nuclear
It is one of the four known force-related fundamental interactions of nature, alongside the strong interaction, electromagnetism, and gravitation.
Weak interaction | Introduction
The "strong interaction", or "strong nuclear force", is the most complicated interaction, mainly because of the way it varies with distance. At distances greater than 10 femtometers, the strong force is practically unobservable. Moreover, it holds only inside the atomic nucleus. After the nucleus was discovered in 1908, it was clear that a new force, today known as the nuclear force, was needed to overcome the electrostatic repulsion, a manifestation of electromagnetism, of the positively charged protons. Otherwise, the nucleus could not exist. Moreover, the force had to be strong enough to squeeze the protons into a volume whose diameter is about 10 m, much smaller than that of the entire atom.
Fundamental interaction | Strong interaction
The weak force tries to make the number of neutrons and protons into the most energetically stable configuration. For nuclei containing less than 40 particles, these numbers are usually about equal. Protons and neutrons are closely related and are collectively known as nucleons. As the number of particles increases toward a maximum of about 209, the number of neutrons to maintain stability begins to outstrip the number of protons, until the ratio of neutrons to protons is about three to two. The protons of hydrogen combine to helium only if they have enough velocity to overcome each other's mutual repulsion sufficiently to get within range of the strong nuclear attraction.
Nuclear binding energy | Nuclear fusion
But if the particles' spins are anti-aligned the nuclear force is too weak to bind them, even if they are of different types.
Nuclear force | Field strength
The weak force is best known for its role in nuclear decay. It has very short range but (apart from gravity) is the only force to interact with neutrinos. The weak force is communicated via exchange particles like other subatomic forces. Perhaps the most well known of the exchange particles for the weak force is the W particle which is involved in beta decay. W particles have electric charge – there are both positive and negative W particles – however the Z boson is also an exchange particle for the weak force but does "not" have any electrical charge. Exchange of a Z boson transfers momentum, spin, and energy, but leaves the interacting particles’ quantum numbers unaffected – charge, flavor, baryon number, lepton number, etc.
Neutral current | In simple terms
The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force, unless it is too large. Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nucleus, or beta decay, in which they eject an electron (or positron). After one of these decays the resultant nucleus may be left in an excited state, and in this case it decays to its ground state by emitting high-energy photons (gamma decay). The study of the strong and weak nuclear forces (the latter explained by Enrico Fermi via Fermi's interaction in 1934) led physicists to collide nuclei and electrons at ever higher energies.
Nuclear physics | Yukawa's meson postulated to bind nuclei
An important characteristic of the weak interaction is its violation of parity symmetry. This characteristic was established through the measurement of the spins and parities of many hyperons. Steinberger and his collaborators contributed several such measurements using large (75 cm) liquid-hydrogen bubble chambers and separated hadron beams at Brookhaven. One example is the measurement of the invariant mass distribution of electron-positron pairs produced in the decay of Sigma-zero hyperons to Lambda-zero hyperons.
Jack Steinberger | Investigations of strange particles
The strong force only acts "directly" upon elementary particles. However, a residual of the force is observed between hadrons (the best known example being the force that acts between nucleons in atomic nuclei) as the nuclear force. Here the strong force acts indirectly, transmitted as gluons, which form part of the virtual pi and rho mesons, which classically transmit the nuclear force (see this topic for more). The failure of many searches for free quarks has shown that the elementary particles affected are not directly observable. This phenomenon is called color confinement.
Force | Strong nuclear
The and bosons are carrier particles that mediate the weak nuclear force, much as the photon is the carrier particle for the electromagnetic force.
W and Z bosons | Weak nuclear force
Before the 1970s, physicists were uncertain as to how the atomic nucleus was bound together. It was known that the nucleus was composed of protons and neutrons and that protons possessed positive electric charge, while neutrons were electrically neutral. By the understanding of physics at that time, positive charges would repel one another and the positively charged protons should cause the nucleus to fly apart. However, this was never observed. New physics was needed to explain this phenomenon. A stronger attractive force was postulated to explain how the atomic nucleus was bound despite the protons' mutual electromagnetic repulsion. This hypothesized force was called the "strong force", which was believed to be a fundamental force that acted on the protons and neutrons that make up the nucleus.
Strong interaction | History
The fact that the maximum binding energy is found in medium-sized nuclei is a consequence of the trade-off in the effects of two opposing forces that have different range characteristics. The attractive nuclear force (strong nuclear force), which binds protons and neutrons equally to each other, has a limited range due to a rapid exponential decrease in this force with distance. However, the repelling electromagnetic force, which acts between protons to force nuclei apart, falls off with distance much more slowly (as the inverse square of distance). For nuclei larger than about four nucleons in diameter, the additional repelling force of additional protons more than offsets any binding energy that results between further added nucleons as a result of additional strong force interactions.
Nuclear binding energy | Binding energy and nuclide masses
Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is a minor residuum of the strong interaction which binds quarks together to form protons and neutrons. This force is much weaker "between" neutrons and protons because it is mostly neutralized within them, in the same way that electromagnetic forces "between" neutral atoms (such as van der Waals forces that act between two inert gas atoms) are much weaker than the electromagnetic forces that hold the parts of the atoms together internally (for example, the forces that hold the electrons in an inert gas atom bound to its nucleus).
Atomic nucleus | Forces
Such forces between atoms are much weaker than the attractive electrical forces that hold the atoms themselves together (i.e., that bind electrons to the nucleus), and their range between atoms is shorter, because they arise from small separation of charges inside the neutral atom. Similarly, even though nucleons are made of quarks in combinations which cancel most gluon forces (they are "color neutral"), some combinations of quarks and gluons nevertheless leak away from nucleons, in the form of short-range nuclear force fields that extend from one nucleon to another nearby nucleon. These nuclear forces are very weak compared to direct gluon forces ("color forces" or strong forces) inside nucleons, and the nuclear forces extend only over a few nuclear diameters, falling exponentially with distance.
Nuclear force | The nuclear force as a residual of the strong force
A large part of the practical difficulties met in mean field theories is the definition (or calculation) of the potential of the mean field itself. One can very roughly distinguish between two approaches: In the case of the Hartree–Fock approaches, the trouble is not to find the mathematical function which describes best the nuclear potential, but that which describes best the nucleon–nucleon interaction. Indeed, in contrast with atomic physics where the interaction is known (it is the Coulomb interaction), the nucleon–nucleon interaction within the nucleus is not known analytically. There are two main reasons for this fact. First, the strong interaction acts essentially among the quarks forming the nucleons.
Nuclear structure | Nuclear potential and effective interaction
Most of the mass of a common proton or neutron is the result of the strong force field energy; the individual quarks provide only about 1% of the mass of a proton. The strong interaction is observable at two ranges and mediated by two force carriers. On a larger scale (about 1 to 3 fm), it is the force (carried by mesons) that binds protons and neutrons (nucleons) together to form the nucleus of an atom. On the smaller scale (less than about 0.8 fm, the radius of a nucleon), it is the force (carried by gluons) that holds quarks together to form protons, neutrons, and other hadron particles.
Strong interaction | Introduction
The latter is basically a mathematical function with several arbitrary parameters, which are adjusted to agree with experimental data.
Nuclear structure | Nuclear potential and effective interaction
Nuclear fusion occurs when the nuclei of two atoms approach closely enough for the nuclear force to pull them together into a single larger nucleus. The strong force is opposed by the electrostatic force created by the positive charge of the nuclei's protons, pushing the nuclei apart. The amount of energy that is needed to overcome this repulsion is known as the Coulomb barrier. The amount of energy released by the fusion reaction when it occurs may be greater or less than the Coulomb barrier. Generally, lighter nuclei with a smaller number of protons and greater number of neutrons will have the greatest ratio of energy released to energy required, and the majority of fusion power research focusses on the use of deuterium and tritium, two isotopes of hydrogen.
Beta (plasma physics) | Fusion basics
The electrically charged weak interaction is unique in a number of respects: Due to their large mass (approximately 90 GeV/c) these carrier particles, called the W and Z bosons, are short-lived with a lifetime of under 10 seconds. The weak interaction has a coupling constant (an indicator of interaction strength) of between 10 and 10, compared to the strong interaction's coupling constant of 1 and the electromagnetic coupling constant of about 10; consequently the weak interaction is ‘weak’ in terms of strength. The weak interaction has a very short effective range (around 10 to 10 m). At distances around 10 meters, the weak interaction has a strength of a similar magnitude to the electromagnetic force, but this starts to decrease exponentially with increasing distance.
Weak interaction | Properties
There are two types of weak interaction (called "vertices"). The first type is called the "charged-current interaction" because it is mediated by particles that carry an electric charge (the or bosons). It is responsible for the beta decay phenomenon. The second type is called the "neutral-current interaction" because it is mediated by a neutral particle, the boson. It is responsible for the (rare) deflection of neutrinos. The two types of interaction follow different selection rules.
Weak interaction | Interaction types
In the latter context, it is often known as the color force. The strong force inherently has such a high strength that hadrons bound by the strong force can produce new massive particles. Thus, if hadrons are struck by high-energy particles, they give rise to new hadrons instead of emitting freely moving radiation (gluons). This property of the strong force is called color confinement, and it prevents the free "emission" of the strong force: instead, in practice, jets of massive particles are produced. In the context of atomic nuclei, the same strong interaction force (that binds quarks within a nucleon) also binds protons and neutrons together to form a nucleus.
Strong interaction | Introduction
This process of "unifying" forces continues today, and electromagnetism and the weak nuclear force are now considered to be two aspects of the electroweak interaction. Physics hopes to find an ultimate reason (theory of everything) for why nature is as it is (see section "Current research" below for more information).
Physics | Scope and aims
The other three are discrete quantum fields, and their interactions are mediated by elementary particles described by the Standard Model of particle physics. Within the Standard Model, the strong interaction is carried by a particle called the gluon, and is responsible for quarks binding together to form hadrons, such as protons and neutrons. As a residual effect, it creates the nuclear force that binds the latter particles to form atomic nuclei. The weak interaction is carried by particles called W and Z bosons, and also acts on the nucleus of atoms, mediating radioactive decay. The electromagnetic force, carried by the photon, creates electric and magnetic fields, which are responsible for the attraction between orbital electrons and atomic nuclei which holds atoms together, as well as chemical bonding and electromagnetic waves, including visible light, and forms the basis for electrical technology.
Fundamental interaction | Introduction
Between two electrons, it begins roughly 10 times weaker than the weak interaction, and grows exponentially weaker at non-zero distances.
Fundamental interaction | Higgs interaction
High-energy particle physics observations made during the 1970s and 1980s confirmed that the weak and electromagnetic forces are expressions of a more fundamental electroweak interaction.
Force | Development of the concept
The neutrons and protons that constitute nuclei, as well as other particles that approach close enough to them, are governed by several interactions. The strong nuclear force, not observed at the familiar macroscopic scale, is the most powerful force over subatomic distances. The electrostatic force is almost always significant, and, in the case of beta decay, the weak nuclear force is also involved. The combined effects of these forces produces a number of different phenomena in which energy may be released by rearrangement of particles in the nucleus, or else the change of one type of particle into others. These rearrangements and transformations may be hindered energetically, so that they do not occur immediately.
Radioactive decay | Theoretical basis of decay phenomena
Therefore, another force, called the "nuclear force" (or "residual strong force") holds the nucleons of nuclei together. This force is a residuum of the strong interaction, which binds quarks into nucleons at an even smaller level of distance.
Nuclear binding energy | The nuclear force
Under the strong link/weak link system, "weak links" are constructed between critical nuclear weapon components (the "hard links"). In the event of an accident the weak links are designed to fail first in a manner that precludes energy transfer between them. Then, if a hard link fails in a manner that transfers or release energy, energy can't be transferred into other weapon systems, potentially starting a nuclear detonation. Hard links are usually critical weapon components that have been hardened to survive extreme environments, while weak links can be both components deliberately inserted into the system to act as a weak link and critical nuclear components that can fail predictably.
Nuclear weapon design | Strong link/weak link
At distances larger than 0.7 fm the force becomes attractive between spin-aligned nucleons, becoming maximal at a center–center distance of about 0.9 fm. Beyond this distance the force drops exponentially, until beyond about 2.0 fm separation, the force is negligible. Nucleons have a radius of about 0.8 fm. At short distances (less than 1.7 fm or so), the attractive nuclear force is stronger than the repulsive Coulomb force between protons; it thus overcomes the repulsion of protons within the nucleus. However, the Coulomb force between protons has a much greater range as it varies as the inverse square of the charge separation, and Coulomb repulsion thus becomes the only significant force between protons when their separation exceeds about 2 to 2.5 fm.
Nuclear force | Field strength
Nuclear matrix elements of relevant operators as extracted from data and from a shell-model and theoretical approximations both for the two-neutrino and neutrinoless modes of decay are used to explain the weak interaction and nuclear structure aspects of nuclear double beta decay.
Theoretical astronomy | Weak interaction and nuclear double beta decay
The weak and electromagnetic interactions are unified in electroweak theory, which has been theoretically unified with the strong force in the standard model, but experimental results are yet to prove this.
Interaction | Physics
In nuclear transitions governed by strong and electromagnetic interactions (which are invariant under parity), the physical laws would be the same if the interaction was reflected in a mirror. Hence the sum of a vector and a pseudovector is not meaningful. However, the weak force, which governs beta decay and the corresponding nuclear transitions, "does" depend on the chirality of the interaction, and in this case pseudovectors and vectors "are" added. The Gamow–Teller transition is a pseudovector transition, that is, the selection rules for beta decay caused by such a transition involve no parity change of the nuclear state. The spin of the parent nucleus can either remain unchanged or change by ±1.
Beta decay transition | Gamow–Teller transition
While the nuclear force is usually associated with nucleons, more generally this force is felt between hadrons, or particles composed of quarks. At small separations between nucleons (less than ~ 0.7 fm between their centers, depending upon spin alignment) the force becomes repulsive, which keeps the nucleons at a certain average separation. For identical nucleons (such as two neutrons or two protons) this repulsion arises from the Pauli exclusion force. A Pauli repulsion also occurs between quarks of the same flavour from different nucleons (a proton and a neutron).
Nuclear force | Description
In 1933, Enrico Fermi proposed the first theory of the weak interaction, known as Fermi's interaction. He suggested that beta decay could be explained by a four-fermion interaction, involving a contact force with no range. However, it is better described as a non-contact force field having a finite range, albeit very short. In 1968, Sheldon Glashow, Abdus Salam and Steven Weinberg unified the electromagnetic force and the weak interaction by showing them to be two aspects of a single force, now termed the electroweak force. The existence of the W and Z bosons was not directly confirmed until 1983.
Weak interaction | History
Nuclear interaction length is the mean distance travelled by a hadronic particle before undergoing an inelastic nuclear interaction.
Nuclear interaction length | Introduction
The nuclear force has a spin-dependent component. The force is stronger for particles with their spins aligned than for those with their spins anti-aligned. If two particles are the same, such as two neutrons or two protons, the force is not enough to bind the particles, since the spin vectors of two particles of the same type must point in opposite directions when the particles are near each other and are (save for spin) in the same quantum state. This requirement for fermions stems from the Pauli exclusion principle. For fermion particles of different types, such as a proton and neutron, particles may be close to each other and have aligned spins without violating the Pauli exclusion principle, and the nuclear force may bind them (in this case, into a deuteron), since the nuclear force is much stronger for spin-aligned particles.
Nuclear force | Field strength
All of the known forces of the universe are classified into four fundamental interactions. The strong and the weak forces are nuclear forces that act only at very short distances, and are responsible for the interactions between subatomic particles, including nucleons and compound nuclei. The electromagnetic force acts between electric charges, and the gravitational force acts between masses. All other forces in nature derive from these four fundamental interactions. For example, friction is a manifestation of the electromagnetic force acting between atoms of two surfaces, and the Pauli exclusion principle, which does not permit atoms to pass through each other.
Force | Fundamental forces
This is because in the original quantum numbers, when formula_4 is parallel to formula_5, the interaction energy is positive; and in this case "j" = "ℓ" + "s" = "ℓ" + . When formula_4 is anti-parallel to formula_5 (i.e. aligned oppositely), the interaction energy is negative, and in this case . Furthermore, the strength of the interaction is roughly proportional to "ℓ". For example, consider the states at level 4:
Nuclear shell model | Including a spin-orbit interaction
In chemistry, a C–H···O interaction is occasionally described as a special type of weak hydrogen bond. These interactions frequently occur in the structures of important biomolecules like amino acids, proteins, sugars, DNA and RNA.
C–H···O interaction | Introduction
The variation in specific binding energy with atomic number is due to the interplay of the two fundamental forces acting on the component nucleons (protons and neutrons) that make up the nucleus. Nuclei are bound by an attractive nuclear force between nucleons, which overcomes the electrostatic repulsion between protons. However, the nuclear force acts only over relatively short ranges (a few nucleon diameters), since it follows an exponentially decaying Yukawa potential which makes it insignificant at longer distances. The electrostatic repulsion is of longer range, since it decays by an inverse-square rule, so that nuclei larger than about 12 nucleons in diameter reach a point that the total electrostatic repulsion overcomes the nuclear force and causes them to be spontaneously unstable.
Nuclear fission | Origin of the active energy and the curve of binding energy
The nucleon–nucleon interaction in vacuum is a mere "consequence" of the quark–quark interaction. While the latter is well understood in the framework of the Standard Model at high energies, it is much more complicated in low energies due to color confinement and asymptotic freedom. Thus there is yet no fundamental theory allowing one to deduce the nucleon–nucleon interaction from the quark–quark interaction. Furthermore, even if this problem were solved, there would remain a large difference between the ideal (and conceptually simpler) case of two nucleons interacting in vacuum, and that of these nucleons interacting in the nuclear matter. To go further, it was necessary to invent the concept of effective interaction.
Nuclear structure | Nuclear potential and effective interaction
In nuclear fusion, two low-mass nuclei come into very close contact with each other so that the strong force fuses them. It requires a large amount of energy for the strong or nuclear forces to overcome the electrical repulsion between the nuclei in order to fuse them; therefore nuclear fusion can only take place at very high temperatures or high pressures. When nuclei fuse, a very large amount of energy is released and the combined nucleus assumes a lower energy level. The binding energy per nucleon increases with mass number up to nickel-62. Stars like the Sun are powered by the fusion of four protons into a helium nucleus, two positrons, and two neutrinos.
Nuclear physics | Nuclear fusion
For this reason, work on unification, for much of the twentieth century, focused on understanding the three "quantum" forces: electromagnetism and the weak and strong forces. The first two were combined in 1967–68 by Sheldon Glashow, Steven Weinberg, and Abdus Salam into the "electroweak" force. Electroweak unification is a broken symmetry: the electromagnetic and weak forces appear distinct at low energies because the particles carrying the weak force, the W and Z bosons, have non-zero masses of and , whereas the photon, which carries the electromagnetic force, is massless. At higher energies Ws and Zs can be created easily and the unified nature of the force becomes apparent.
Theory of everything | Late 20th century and the nuclear interactions
In neutral current interactions, a quark or a lepton (e.g., an electron or a muon) emits or absorbs a neutral Z boson. For example: Like the bosons, the boson also decays rapidly, for example: Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, and / or weak isospin, the neutral-current interaction can cause any two fermions in the standard model to deflect: Either particles and anti-particles of any electric charge, and both left- and right-chirality, although the strength of the interaction differs.
Weak interaction | Neutral-current interaction
Fusion takes place when atoms come into close proximity and the nuclear strong force pulls their nuclei together. Counteracting this process is the fact that the nuclei are all positively charged, and thus repel each other due to the electrostatic force. In order for fusion to occur, the nuclei must have enough energy to overcome this coulomb barrier. The barrier is lowered for atoms with less positive charge, those with the fewest protons, and the strong force is increased with additional nucleons, the total number of protons and neutrons. This means that a combination of deuterium and tritium has the lowest coulomb barrier, at about 100 keV (see requirements for fusion).
Migma | Conventional fusion
At distances less than 0.7 fm, the nuclear force becomes repulsive. This repulsive component is responsible for the physical size of nuclei, since the nucleons can come no closer than the force allows. By comparison, the size of an atom, measured in angstroms (Å, or 1.0 × 10 m), is five orders of magnitude larger. The nuclear force is not simple, however, since it depends on the nucleon spins, has a tensor component, and may depend on the relative momentum of the nucleons. The nuclear force plays an essential role in storing energy that is used in nuclear power and nuclear weapons.
Nuclear force | Introduction
The fields from the external magnets push the nuclei back towards the vessel ends. Magnetic field strength is an increasing function of distance from the center. This implies that as the plasma pressure causes the plasma to expand, the magnetic field becomes stronger at the plasma edge, increasing containment.
Lockheed Martin Compact Fusion Reactor | Design
In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different aspects of the same force. Above the unification energy, on the order of 246 GeV, they would merge into a single force. Thus, if the universe is hot enough (approximately 10 K, a temperature not exceeded since shortly after the Big Bang), then the electromagnetic force and weak force merge into a combined electroweak force.
Electroweak interaction | Introduction
Typically the strongest neutron flux occurs in the middle of the reactor core, becoming lower toward the edges. The higher the neutron flux the greater the chance of a nuclear reaction occurring as there are more neutrons going through an area per unit time.
Neutron flux | Artificial neutron flux
This technique is based on the study of magnetic properties of atomic nuclei, thus determining physical and chemical properties of the correspondent atoms or the molecules. Nuclear magnetic resonance is advantageous for characterizing weak PPIs.
Protein–protein interaction | Structure
A Strong link/weak link and Exclusion zone nuclear detonation mechanism is a type of safety mechanism employed in the arming and firing mechanisms of modern nuclear weapons. The safety mechanism starts by enclosing the electronics and mechanical components used to arm and fire the nuclear weapon with a mechanical and electrical isolation barrier, forming the "Exclusion zone". This is insulated from mechanical, thermal, and electrical disruptions (such as static electricity, lightning, fire). Between the exclusion zone and the actual detonators, a normally disconnected link mechanism is used, such as a switch which has a built in motor to activate it.
Strong link/weak link | Introduction
Direct evidence of a 3-body force in helium-3 is known: . The existence of stable [PNP] cluster calls into question models of the atomic nucleus that restrict nucleon interactions within shells to 2-body phenomenon. The three-nucleon-interaction is fundamentally possible because gluons, the mediators of the strong interaction, can couple to themselves. In particle physics, the interactions between the three quarks that compose hadrons can be described in a diquark model which might be equivalent to the hypothesis of a three-body force. There is growing evidence in the field of nuclear physics that three-body forces exist among the nucleons inside atomic nuclei for many different isotopes (three-nucleon force).
Three-body force | Introduction
The interaction between nucleons, which is a consequence of strong interactions and binds the nucleons within the nucleus, exhibits the peculiar behaviour of having a finite range: it vanishes when the distance between two nucleons becomes too large; it is attractive at medium range, and repulsive at very small range. This last property correlates with the Pauli exclusion principle according to which two fermions (nucleons are fermions) cannot be in the same quantum state. This results in a very large mean free path predicted for a nucleon within the nucleus. The main idea of the Independent Particle approach is that a nucleon moves inside a certain potential well (which keeps it bound to the nucleus) independently from the other nucleons.
Nuclear structure | The independent-particle model (IPM)
Attractive nuclear forces in this region, as atomic mass increases, are nearly balanced by repellent electromagnetic forces between protons, as the atomic number increases. Finally, in elements heavier than xenon, there is a decrease in binding energy per nucleon as atomic number increases. In this region of nuclear size, electromagnetic repulsive forces are beginning to overcome the strong nuclear force attraction.
Nuclear binding energy | Nuclear binding energy curve
Charged current interaction is the most easily detected of the interactions of the weak force. The weak force is best known for mediating nuclear decay. It has very short range, but is the only force (apart from gravity) to interact with neutrinos. The weak force is communicated via the W and Z exchange particles. Of these, the W-boson has either a positive or negative electric charge, and mediates neutrino absorption and emission by or with an electrically charged particle. During these processes, the W-boson induces electron or positron emission or absorption, or changing the flavour of a quark as well as its electrical charge.
Charged current | In simple terms
It is thought that this interaction occurs through hydrophobic interactions.
FADD | Structure
In the initial collision which begins the reaction, the particles must approach closely enough so that the short range strong force can affect them. As most common nuclear particles are positively charged, this means they must overcome considerable electrostatic repulsion before the reaction can begin. Even if the target nucleus is part of a neutral atom, the other particle must penetrate well beyond the electron cloud and closely approach the nucleus, which is positively charged. Thus, such particles must be first accelerated to high energy, for example by: Also, since the force of repulsion is proportional to the product of the two charges, reactions between heavy nuclei are rarer, and require higher initiating energy, than those between a heavy and light nucleus; while reactions between two light nuclei are the most common ones.
Nuclear reaction | Neutrons vs. ions
From there modern electroweak theory was developed, which described the weak interaction in terms of massive gauge bosons which was required for describing high energy particle cross-sections.
Beta decay transition | The weak interaction and beta decay
In physics, there are four observed fundamental forces or interactions that form the basis of all known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories. The characteristics of this fifth force depend on the hypothesis being advanced. Many postulate a force roughly the strength of gravity ("i.e.", it is much weaker than electromagnetism or the nuclear forces) with a range of anywhere from less than a millimeter to cosmological scales. Another proposal is a new weak force mediated by W′ and Z′ bosons.
Fifth force | Introduction
This research became the science of particle physics, the crown jewel of which is the standard model of particle physics, which describes the strong, weak, and electromagnetic forces.
Nuclear physics | Yukawa's meson postulated to bind nuclei
Weak neutral current interactions are one of the ways in which subatomic particles can interact by means of the weak force. These interactions are mediated by the boson. The discovery of weak neutral currents was a significant step toward the unification of electromagnetism and the weak force into the electroweak force, and led to the discovery of the W and Z bosons.
Neutral current | Introduction
The requirements of this objective, however, assume that the reactor was built to stringent safety factors.
Nuclear MASINT | Structural weakening
Consider a system that starts in the quantum state formula_1 and an ancilla that starts in the state formula_2, the combined initial state is formula_3. These two systems interact via the Hamiltonian formula_4, which generates the time evolutions formula_5 (in units where formula_6), where formula_7 is the "interaction strength", which has units of inverse time. Assume a fixed interaction time formula_8 and that formula_9 is small, such that formula_10. A series expansion of formula_11 in formula_12 gives Because it was only necessary to expand the unitary to a low order in perturbation theory, we call this is a weak interaction.
Weak measurement | Weak interaction and ancilla coupled measurement
A hydrogen nucleus is a single proton, so simultaneous nuclear interactions, which would occur within a heavier nucleus, don't need to be considered for the detection experiment. Within a cubic metre of water placed right outside a nuclear reactor, only relatively few such interactions can be recorded, but the setup is now used for measuring the reactor's plutonium production rate.
Neutrino | Nuclear reactions
These nuclear binding energies and forces are on the order of a million times greater than the electron binding energies of light atoms like hydrogen.
Nuclear binding energy | Introduction
The strong force grows rapidly once the nuclei are close enough, and the fusing nucleons can essentially "fall" into each other and the result is fusion and net energy produced. The fusion of lighter nuclei, which creates a heavier nucleus and often a free neutron or proton, generally releases more energy than it takes to force the nuclei together; this is an exothermic process that can produce self-sustaining reactions.
Nuclear fusion | Process
However, at low energies, this gauge symmetry is spontaneously broken down to the U(1) symmetry of electromagnetism, since one of the Higgs fields acquires a vacuum expectation value. This symmetry-breaking would be expected to produce three massless bosons, but instead they become integrated by the other three fields and acquire mass through the Higgs mechanism. These three boson integrations produce the , and bosons of the weak interaction. The fourth gauge boson is the photon of electromagnetism, and remains massless. This theory has made a number of predictions, including a prediction of the masses of the Z and W-bosons before their discovery and detection in 1983.
Weak interaction | Electroweak theory
In view of the non-perturbative nature of strong interaction, and also in view of the fact that the exact potential form of this interaction between groups of nucleons is relatively badly known, the use of such an approach in the case of atomic nuclei requires drastic approximations. The main simplification consists in replacing in the equations all field terms (which are operators in the mathematical sense) by their mean value (which are functions). In this way, one gets a system of coupled integro-differential equations, which can be solved numerically, if not analytically.
Nuclear structure | The relativistic mean field approaches
This spin-orbit interaction is nearly negligible unless the electron passes very close to the nucleus (small formula_2, large formula_3). This force bends the trajectory of the electron, preventing any collision with the nucleus.
Michał Gryziński | Free-fall atomic model
While the strong and electroweak forces peacefully coexist in the Standard Model of particle physics, they remain distinct. So far, the quest for a theory of everything is thus unsuccessful on two points: neither a unification of the strong and electroweak forces – which Laplace would have called 'contact forces' – nor a unification of these forces with gravitation has been achieved.
Theory of everything | Late 20th century and the nuclear interactions
This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus splits and leaves behind different elements. This is a form of nuclear decay.
Atom | Introduction
In any given interaction, weak isospin is conserved: The sum of the weak isospin numbers of the particles entering the interaction equals the sum of the weak isospin numbers of the particles exiting that interaction. For example, a (left-handed) , with a weak isospin of +1 normally decays into a (with = ) and a (as a right-handed antiparticle, ). For the development of the electroweak theory, another property, weak hypercharge, was invented, defined as: where is the weak hypercharge of a particle with electrical charge (in elementary charge units) and weak isospin . Weak hypercharge is the generator of the U(1) component of the electroweak gauge group; whereas some particles have a weak isospin of zero, all known spin particles have a non-zero weak hypercharge.
Weak interaction | Weak isospin and weak hypercharge
Fusion reactions occur when two or more atomic nuclei come close enough for long enough that the nuclear force pulling them together exceeds the electrostatic force pushing them apart, fusing them into heavier nuclei. For nuclei lighter than iron-56, the reaction is exothermic, releasing energy. For nuclei heavier than iron-56, the reaction is endothermic, requiring an external source of energy. Hence, nuclei smaller than iron-56 are more likely to fuse while those heavier than iron-56 are more likely to break apart. The strong force acts only over short distances, while the repulsive electrostatic force acts over longer distances. In order to undergo fusion, the fuel atoms need to be given enough energy to approach each other close enough for the strong force to become active.
Fusion power | Mechanism
A substantial energy barrier of electrostatic forces must be overcome before fusion can occur. At large distances, two naked nuclei repel one another because of the repulsive electrostatic force between their positively charged protons. If two nuclei can be brought close enough together, however, the electrostatic repulsion can be overcome by the quantum effect in which nuclei can tunnel through coulomb forces. When a nucleon such as a proton or neutron is added to a nucleus, the nuclear force attracts it to all the other nucleons of the nucleus (if the atom is small enough), but primarily to its immediate neighbours due to the short range of the force.
Nuclear fusion | Requirements
In 1935 Hideki Yukawa proposed the first significant theory of the strong force to explain how the nucleus holds together. In the Yukawa interaction a virtual particle, later called a meson, mediated a force between all nucleons, including protons and neutrons. This force explained why nuclei did not disintegrate under the influence of proton repulsion, and it also gave an explanation of why the attractive strong force had a more limited range than the electromagnetic repulsion between protons. Later, the discovery of the pi meson showed it to have the properties of Yukawa's particle. With Yukawa's papers, the modern model of the atom was complete.
Nuclear physics | Yukawa's meson postulated to bind nuclei
By inverse beta decay, the predicted neutrino, more correctly an electron antineutrino (formula_1), should interact with a proton () to produce a neutron () and positron (formula_2), The chance of this reaction occurring was small. The probability for any given reaction to occur is in proportion to its cross section. Cowan and Reines predicted a cross section for the reaction to be about . The usual unit for a cross section in nuclear physics is a barn, which is and 20 orders of magnitudes larger. Despite the low probability of the neutrino interaction, the signatures of the interaction are unique, making detection of the rare interactions possible.
Cowan–Reines neutrino experiment | Potential for experiment
It has been pointed out that it is an ill-defined problem because the standard formulation assumes that the nucleus has a magnetic dipolar moment, which is not always the case.
Fermi contact interaction | Introduction
In a simple picture of an electron-nucleus two-spin system, the solid effect occurs when a transition involving an electron-nucleus mutual flip (called Zero Quantum or Double Quantum) is excited by a microwave irradiation, in the presence of relaxation. This kind of transition is in general weakly allowed, meaning that the transition moment for the above microwave excitation results from a second-order effect of the electron-nuclear interactions and thus requires stronger microwave power to be significant, and its intensity is decreased by an increase of the external magnetic field B. As a result, the DNP enhancement from the solid effect scales as B when all the relaxation parameters are kept constant.
Dynamic nuclear polarization | Static sample case
Nuclear spins exhibit a magnetic dipole moment, which generates a magnetic field that interacts with the dipole moments of other nuclei (dipolar coupling). The magnitude of the interaction is dependent on the spin species, the internuclear distance "r", and the orientation, with respect to the external magnetic field "B", of the vector connecting the two nuclear spins (see figure). The maximum dipolar coupling is given by the dipolar coupling constant "d", where γ and γ are the gyromagnetic ratios of the nuclei. In a strong magnetic field, the dipolar coupling depends on the angle "θ" between the internuclear vector and the external magnetic field "B" (see the figure) according to "D" becomes zero for "formula_3" = 54.7°.
Solid-state nuclear magnetic resonance | Dipolar coupling
The nuclear force holding an atomic nucleus together is very strong, in general much stronger than the repulsive electromagnetic forces between the protons. However, the nuclear force is also short-range, dropping quickly in strength beyond about 1 femtometre, while the electromagnetic force has an unlimited range. The strength of the attractive nuclear force keeping a nucleus together is thus proportional to the number of nucleons, but the total disruptive electromagnetic force trying to break the nucleus apart is roughly proportional to the square of its atomic number. A nucleus with 210 or more nucleons is so large that the strong nuclear force holding it together can just barely counterbalance the electromagnetic repulsion between the protons it contains.
Alpha decay | Mechanism
The ultimate goal of nuclear physics would be to describe all nuclear interactions from the basic interactions between nucleons. This is called the "microscopic" or "ab initio" approach of nuclear physics. There are two major obstacles to overcome: This is an active area of research with ongoing advances in computational techniques leading to better first-principles calculations of the nuclear shell structure. Two- and three-nucleon potentials have been implemented for nuclides up to "A" = 12.
Nuclear force | From nucleons to nuclei
Absence of a glossary of technical terms so as to let the readers know about how the nuclear fuel cycle is achieved and how such terms as UF6, UF4, UCF, and P1 and P2 centrifuges are different. Absence of suitable and relevant photos as well as a summary part at the beginning or the end of every chapter is another weakness. The contents of the book are apparently a combination of dictated and written parts as a result of which certain topics have been frequently, and unnecessarily, repeated throughout the text.
National Security and Nuclear Diplomacy | Negative points
A successful way of describing nuclear interactions is to construct one potential for the whole nucleus instead of considering all its nucleon components. This is called the "macroscopic" approach. For example, scattering of neutrons from nuclei can be described by considering a plane wave in the potential of the nucleus, which comprises a real part and an imaginary part. This model is often called the optical model since it resembles the case of light scattered by an opaque glass sphere. Nuclear potentials can be "local" or "global": local potentials are limited to a narrow energy range and/or a narrow nuclear mass range, while global potentials, which have more parameters and are usually less accurate, are functions of the energy and the nuclear mass and can therefore be used in a wider range of applications.
Nuclear force | Nuclear potentials
An example of a weak link would be an electrical connector that contains electrical wires made from a low melting point alloy. During a fire, those wires would melt breaking any electrical connection.
Nuclear weapon design | Strong link/weak link
If GUT is correct, after the Big Bang and during the electroweak epoch of the universe, the electroweak force separated from the strong force. Accordingly, a grand unification epoch is hypothesized to have existed prior to this.
Strong interaction | Unification
Born first in the 1970s with the works of John Dirk Walecka on quantum hadrodynamics, the relativistic models of the nucleus were sharpened up towards the end of the 1980s by P. Ring and coworkers. The starting point of these approaches is the relativistic quantum field theory. In this context, the nucleon interactions occur via the exchange of virtual particles called mesons. The idea is, in a first step, to build a Lagrangian containing these interaction terms. Second, by an application of the least action principle, one gets a set of equations of motion. The real particles (here the nucleons) obey the Dirac equation, whilst the virtual ones (here the mesons) obey the Klein–Gordon equations.
Nuclear structure | The relativistic mean field approaches