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In nuclear physics, a nuclear reaction is a process in which two nuclei or nuclear particles collide to produce products different from the initial particles. Nuclear physics is the field of Physics that studies the building blocks and interactions of Atomic nuclei. The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom A subatomic particle is an elementary or composite Particle smaller than an Atom. In principle a reaction can involve more than two particles colliding, but because the probability of three or more nuclei to meet at the same time at the same place is much less than for two nuclei, such an event is exceptionally rare. While the transformation is spontaneous in the case of radioactive decay, it is initiated by a particle in the case of a nuclear reaction. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. If the particles collide and separate without changing, the process is called an elastic collision rather than a reaction. A collision is an isolated event in which two or more bodies (colliding bodies exert relatively strong forces on each other for a relatively short time

In the symbolic figure shown to the right, 63Li and deuterium react to form the highly excited intermediate nucleus 84Be which then decays immediately into two alpha particles. Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Protons are symbolically represented by red spheres, and neutrons by blue spheres. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron.

Contents

The reaction equation

A nuclear reaction can be written in terms of a formula just like a chemical reaction. Nuclear decays can be written in a similar way, but with only one nucleus on the left side.

Every particle partaking in the reaction is written with its chemical symbol, with the mass number at the upper left and the atomic number at the lower left. The mass number ( A) also called atomic mass number or nucleon number, is the total number of Protons and Neutrons (together known as See also List of elements by atomic number In Chemistry and Physics, the atomic number (also known as the proton The neutron is written "n"; the proton can be written "1H" or "p".

The equation is correct only if the sums of the mass numbers on both sides are identical (as required by the conservation law for baryon number), and if the sums of the atomic numbers on both sides are identical (as required by the conservation law for electric charge). In Physics, a conservation law states that a particular measurable property of an isolated Physical system does not change as the system evolves In Particle physics, the baryon number is an approximate conserved Quantum number of a system In the example shown above, this leads to (assuming we would know only one particle to the right):

63Li  +  21H  →  42He  +  ?

To make the sums correct, the second nucleus to the right must have atomic number 2 and mass number 4; it is therefore also Helium-4. Naturally occurring Lithium ( Li) (standard atomic mass 6941(2 u) is composed of two stable Isotopes ( and, the latter being the more abundant Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical The complete equation therefore reads:

63Li  +  21H  →  42He  +  42He

or more simply:

63Li  +  21H  →  2 42He

Notation

Instead of using the full equations as shown in the previous section, in many situations a compact notation is used to describe nuclear reactions. Naturally occurring Lithium ( Li) (standard atomic mass 6941(2 u) is composed of two stable Isotopes ( and, the latter being the more abundant Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Naturally occurring Lithium ( Li) (standard atomic mass 6941(2 u) is composed of two stable Isotopes ( and, the latter being the more abundant Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical This is A(b,c)D, which is equivalent to A + b gives c + D. Common light particles are often abbreviated in this shorthand, typically p for proton, n for neutron, α representing an alpha particle or Helium-4, etc. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. Alpha particles (named after and denoted by the first letter in the Greek alphabet, α consist of two Protons and two Neutrons bound together into a The reaction above would be written as Li-6(d,α)α. Naturally occurring Lithium ( Li) (standard atomic mass 6941(2 u) is composed of two stable Isotopes ( and, the latter being the more abundant [1][2]

Energy conservation

Kinetic energy may be released during the course of a reaction (exothermic reaction) or kinetic energy may have to be supplied for the reaction to take place (endothermic reaction). A chemical reaction is a process that always results in the interconversion of Chemical substances The substance or substances initially involved in a chemical reaction are called A chemical reaction is a process that always results in the interconversion of Chemical substances The substance or substances initially involved in a chemical reaction are called This can be calculated by reference to a table of very accurate particle rest masses (see http://physics.nist.gov/PhysRefData/Compositions/index.html), as follows. According to the reference tables, the 63Li nucleus has a relative atomic mass of 6. The atomic mass (ma is the Mass of an atom most often expressed in unified atomic mass units The atomic mass may be considered to be the total mass 015 atomic mass units (abbreviated u), the deuteron has 2. The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express U is the twenty-first letter in the modern Latin alphabet. Its name in English is spelled u (juː 014 u, and the helium-4 nucleus has 4. 0026 u Thus:

In a nuclear reaction, the total (relativistic) energy is conserved. In Physics, the law of conservation of energy states that the total amount of Energy in an isolated system remains constant and cannot be created although it may The "missing" rest mass must therefore reappear as kinetic energy released in the reaction; its source is the nuclear binding energy. Binding energy is the Mechanical energy required to disassemble a whole into separate parts Using Einstein's mass-energy equivalence formula E = mc², the amount of energy released can be determined. In Physics, mass–energy equivalence is the concept that for particles slower than light any Mass has an associated Energy and vice versa. We first need the energy equivalent of one atomic mass unit:

1 u c2 = (1. 66054 × 10-27 kg) × (2. 99792 × 108 m/s)2 
= 1. 49242 × 10-10 kg (m/s)2 = 1. 49242 × 10-10 J (Joule)
× (1 MeV / 1. The joule (written in lower case ˈdʒuːl or /ˈdʒaʊl/ (symbol J) is the SI unit of Energy measuring heat, Electricity 60218 × 10-13 J)
= 931. 49 MeV,
so 1 u c2 = 931. 49 MeV.

Hence, the energy released is 0. 0238 × 931 MeV = 22. 4 MeV.

Expressed differently: the mass is reduced by 0. 3 %, corresponding to 0. 3 % of 90 PJ/kg is 300 TJ/kg.

This is a large amount of energy for a nuclear reaction; the amount is so high because the binding energy per nucleon of the helium-4 nucleus is unusually high, because the He-4 nucleus is doubly magic. In Physics a nucleon is a collective name for two Baryons the Neutron and the Proton. In Nuclear physics, a magic number is a number of Nucleons (either Protons or Neutrons such that they are arranged into complete shells (The He-4 nucleus is unusually stable and tightly-bound for the same reason that the helium atom is inert: each pair of protons and neutrons in He-4 occupies a filled 1s nuclear orbital in the same way that the pair of electrons in the helium atom occupy a filled 1s electron orbital). Consequently, alpha particles appear frequently on the right hand side of nuclear reactions.

The energy released in a nuclear reaction can appear mainly in one of three ways:

When the product nucleus is metastable, this is indicated by placing an asterisk ("*") next to its atomic number. An asterisk ( *) (Latin asteriscum "little star" from Greek ἀστερίσκος) is a Typographical symbol or Glyph This energy is eventually released through nuclear decay.

A small amount of energy may also emerge in the form of X-rays. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Generally, the product nucleus has a different atomic number, and thus the configuration of its electron shells is wrong. An electron shell may be crudely thought of as an Orbit followed by Electrons around an Atom nucleus. As the electrons rearrange themselves and drop to lower energy levels, internal transition X-rays (X-rays with precisely defined emission lines) may be emitted. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared

Q-value and energy balance

In writing down the reaction equation, in a way analogous to a chemical equation, one may in addition give the reaction energy on the right side:

Target nucleus + projectile -> Final nucleus + ejectile + Q.

For the particular case discussed above, the reaction energy has already been calculated as Q = 22. 4 MeV. Hence:

63Li  +  21H  →  2 42He  +  22. Naturally occurring Lithium ( Li) (standard atomic mass 6941(2 u) is composed of two stable Isotopes ( and, the latter being the more abundant Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical 4 MeV

The reaction energy (the "Q-value") is positive for exothermal reactions and negative for endothermal reactions. On the one hand, it is the difference between the sums of kinetic energies on the final side and on the initial side. But on the other hand, it is also the difference between the nuclear rest masses on the initial side and on the final side (in this way, we have calculated the Q-value above).

Reaction rates

If the reaction equation is balanced, that does not mean that the reaction really occurs. The rate at which reactions occur depends on the particle energy, the particle flux and the reaction cross section. In the various subfields of Physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks In nuclear and Particle physics, the concept of a cross section is used to express the likelihood of interaction between particles

Neutrons vs ions

In the initial collision which begins the reaction, the particles must approach closely enough so that the short range strong force can affect them. In particle physics the strong interaction, or strong force, or color force, holds Quarks and Gluons together to form Protons and As most common nuclear particles are positively charged, this means they must overcome considerable electrostatic repulsion before the reaction can begin. Electrostatics is the branch of Science that deals with the Phenomena arising from what seems to be stationary Electric charges Since Classical 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. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny 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. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on

Neutrons, on the other hand, have no electric charge to cause repulsion, and are able to effect a nuclear reaction at very low energies. This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. In fact at extremely low particle energies (corresponding, say, to thermal equilibrium at room temperature), the neutron's de Broglie wavelength is greatly increased, possibly greatly increasing its capture cross section, at energies close to resonances of the nuclei involved. The neutron temperature, also called the neutron energy, indicates a free neutron's Kinetic energy, usually given in Electron volts The term In Physics, the de Broglie hypothesis (pronounced /brœj/ as French breuil close to "broy" is the statement that all Matter (any object has a Wave In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. Thus low energy neutrons may be even more reactive than high energy neutrons.

Notable types

While the number of possible nuclear reactions is immense, there are several types which are more common, or otherwise notable. Some examples include:

Direct reactions

An intermediate energy projectile transfers energy or picks up or loses nucleons to the nucleus in a single quick (10−21 second) event. Energy and momentum transfer are relatively small. These are particularly useful in experimental nuclear physics, because the reaction mechanisms are often simple enough to calculate with sufficient accuracy to probe the structure of the target nucleus.

Inelastic scattering

Only energy and momentum are transferred.

Transfer reactions

Usually at moderately low energy, one or more nucleons are transferred between the projectile and target. These are useful in studying outer shell structure of nuclei. In Nuclear physics, the nuclear shell model is a model of the Atomic nucleus which uses the Pauli principle to describe the structure

Reactions with neutrons are important in nuclear reactors and nuclear weapons. This article is a subarticle of Nuclear power. A nuclear reactor is a device in which Nuclear chain reactions are initiated controlled A nuclear weapon is an explosive device that derives its destructive force from Nuclear reactions either fission or a combination of fission and fusion. While the best known neutron reactions are neutron scattering, neutron capture, and nuclear fission, for some light nuclei, the most probable reaction with a thermal neutron is a transfer reaction:

(n,α) 6Li + n → T + α 10B + n → 7Li + α 17O + n → 14C + α 21Ne + n → 18O + α 37Ar + n → 34S + α
(n,p) 3He + n → T + p 7Be + n → 7Li + p 14N + n → 14C + p 22Na + n → 22Ne + p

Some reactions are only possible with fast neutrons:

Compound nuclear reactions

Either a low energy projectile is absorbed or a higher energy particle transfers energy to the nucleus, leaving it with too much energy to be fully bound together. On a time scale of about 10−19 seconds, particles, usually neutrons, are "boiled" off. That is, it remains together until enough energy happens to be concentrated in one neutron to escape the mutual attraction. Charged particles rarely boil off because of the coulomb barrier. The Coulomb barrier, named after physicist Charles-Augustin de Coulomb (1736&ndash1806 is the energy barrier due to Electrostatic interaction that two nuclei need The excited quasi-bound nucleus is called a compound nucleus.

Further information: Spallation#Nuclear spallation

Calculation

Applying the methods of scattering by two potentials, the plane wave of each free charged particle is replaced by the exact solution for a charged particle moving in the presence of another point charge. In general spallation is a process in which fragments of material ( Spall) are ejected from a body due to impact or stress In Scattering theory and in particular in Quantum mechanics, the Born approximation consists of taking the incident field in place of the total field as the driving

Direct nuclear reactions are most often calculated by some form of distorted wave Born approximation. In Scattering theory and in particular in Quantum mechanics, the Born approximation consists of taking the incident field in place of the total field as the driving Applying again scattering by two potentials, the coulomb solutions and neutron plane waves are replaced by the optical model wave functions for the incident and outgoing particles moving in and near the nucleus. The nuclear force (or nucleon-nucleon interaction or residual strong force) is the force between two or more Nucleons It is responsible for These are obtained mostly from elastic scattering experiments, and from inelastic scattering to vibrational and rotational collective excitations. The reaction itself is then modeled by the Born approximation. In Scattering theory and in particular in Quantum mechanics, the Born approximation consists of taking the incident field in place of the total field as the driving That is, the excitation or transfer process is treated as a first order perturbation on elastic scattering. An early improvement on this was to exactly treat the coupling between a small number of excited states, known as coupled channels Born approximation.

See also

Sources

  1. ^ http://www.astrophysicsspectator.com/topics/stars/FusionHydrogenRate.html
  2. ^ http://books.google.com/books?id=ZVgOLCXNoMoC&pg=PA495&lpg=PA495&dq=%22nuclear+reaction%22+parentheses+shorthand&source=web&ots=hVze64yN1C&sig=WQVgiNcZDKFD8nj_i24NInMmGVI&hl=en

M. Nuclear Energy is released by the splitting (fission or merging together (fusion of the nuclei of Atom (s Nuclear physics is the field of Physics that studies the building blocks and interactions of Atomic nuclei. A nuclear chain reaction occurs when one Nuclear reaction causes an average of one or more nuclear reactions thus leading to a self-propagating number of these reactions The Oppenheimer-Phillips process or strip reaction is a special type of Nuclear reaction. The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom See also List of elements by atomic number In Chemistry and Physics, the atomic number (also known as the proton The atomic mass (ma is the Mass of an atom most often expressed in unified atomic mass units The atomic mass may be considered to be the total mass The CNO cycle (for Carbon - Nitrogen - Oxygen) or sometimes Bethe-Weizsäcker-cycle, is one of two sets of fusion reactions G. Bowler, Nuclear Physics, Pergamon Press 1973. ISBN 0-08-016983-X

Dictionary

nuclear reaction

-noun

  1. a process such as the fission of an atomic nucleus, or the fusion of one or more atomic nuclei and / or subatomic particles in which the number of protons and / or neutrons in a nucleus changes; the reaction products may contain a different element or a different isotope of the same element
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