In this symbolic representing of a nuclear reaction,lithium-6 (6 3Li) anddeuterium (2 1H) react to form the highly excited intermediate nucleus8 4Be which then decays immediately into twoalpha particles ofhelium-4 (4 2He).Protons are symbolically represented by red spheres, andneutrons by blue spheres.
Innuclear physics andnuclear chemistry, anuclear reaction is a process in which twonuclei, or a nucleus and an externalsubatomic particle, collide to produce one or more newnuclides. Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. If a nucleus interacts with another nucleus or particle, they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclearscattering, rather than a nuclear reaction.
In principle, a reaction can involve more than twoparticlescolliding, 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 (seetriple alpha process for an example very close to a three-body nuclear reaction). The term "nuclear reaction" may refer either to a change in a nuclideinduced by collision with another particle or to aspontaneous change of a nuclide without collision.
Natural nuclear reactions occur in the interaction betweencosmic rays and matter, and nuclear reactions can be employed artificially to obtain nuclear energy, at an adjustable rate, on-demand.Nuclear chain reactions infissionable materials produce inducednuclear fission. Variousnuclear fusion reactions of light elements power the energy production of theSun and stars. Most nuclear reactions (fusion and fission) results in transmutation of nuclei (called alsonuclear transmutation).
In 1919,Ernest Rutherford was able to accomplish transmutation of nitrogen into oxygen at the University of Manchester, using alpha particles directed at nitrogen14N + α →17O + p. This was the first observation of an induced nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. Eventually, in 1932 at Cambridge University, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleaguesJohn Cockcroft andErnest Walton, who used artificially accelerated protons against lithium-7, to split the nucleus into two alpha particles. The feat was popularly known as "splitting theatom", although it was not the modernnuclear fission reaction later (in 1938) discovered in heavy elements by the German scientistsOtto Hahn,Lise Meitner, andFritz Strassmann.[1]
Nuclear reactions may be shown in a form similar to chemical equations, for whichinvariant mass must balance for each side of the equation, and in which transformations of particles must follow certain conservation laws, such as conservation of charge and baryon number (total atomicmass number). An example of this notation follows:
To balance the equation above for mass, charge and mass number, the second nucleus to the right must have atomic number 2 and mass number 4; it is therefore also helium-4. The complete equation therefore reads:
Instead of using the full equations in the style above, in many situations a compact notation is used to describe nuclear reactions. This style of the form A(b,c)D is equivalent to A + b producing c + D. Common light particles are often abbreviated in this shorthand, typically p for proton, n for neutron, d fordeuteron, α representing analpha particle orhelium-4, β forbeta particle or electron, γ forgamma photon, etc. The reaction above would be written as6Li(d,α)α.[2][3]
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). This can be calculated by reference to a table of very accurate particle rest masses,[4] as follows: according to the reference tables, the6 3Li nucleus has astandard atomic weight of 6.015atomic mass units (abbreviatedu), the deuterium has 2.014 u, and the helium-4 nucleus has 4.0026 u. Thus:
the sum of the rest mass of the individual nuclei = 6.015 + 2.014 = 8.029 u;
the total rest mass on the two helium-nuclei = 2 × 4.0026 = 8.0052 u;
missing rest mass = 8.029 – 8.0052 = 0.0238 atomic mass units.
In a nuclear reaction, the total(relativistic) energy is conserved. The "missing" rest mass must therefore reappear as kinetic energy released in the reaction; its source is the nuclearbinding energy. Using Einstein'smass-energy equivalence formulaE = mc2, the amount of energy released can be determined. We first need the energy equivalent of oneatomic mass unit:
1 u c2 = (1.66054 × 10−27 kg) × (2.99792 × 108 m/s)2
Hence, the energy released is 0.0238 × 931 MeV = 22.2MeV.
Expressed differently: the mass is reduced by 0.3%, corresponding to 0.3% of 90 PJ/kg is 270 TJ/kg.
This is a large amount of energy for a nuclear reaction; the amount is so high because the binding energy pernucleon of the helium-4 nucleus is unusually high because the He-4 nucleus is "doubly magic". (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 filled1snuclear orbital in the same way that the pair of electrons in the helium atom occupy a filled1selectron 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:
kinetic energy of the product particles (fraction of the kinetic energy of the charged nuclear reaction products can be directly converted into electrostatic energy);[5]
When the product nucleus is metastable, this is indicated by placing anasterisk ("*") next to its atomic number. This energy is eventually released throughnuclear decay.
A small amount of energy may also emerge in the form ofX-rays. Generally, the product nucleus has a different atomic number, and thus the configuration of itselectron shells is wrong. As the electrons rearrange themselves and drop to lower energy levels, internal transition X-rays (X-rays with precisely definedemission lines) may be emitted.
The reaction energy (the "Q-value") is positive for exothermal reactions and negative for endothermal reactions, opposite to thesimilar expression inchemistry. 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 theQ-value above).
If the reaction equation is balanced, that does not mean that the reaction really occurs. The rate at which reactions occur depends on the energy and theflux of the incident particles, and the reactioncross section. An example of a large repository of reaction rates is the REACLIB database, as maintained by theJoint Institute for Nuclear Astrophysics.
In the initial collision which begins the reaction, the particles must approach closely enough so that the short-rangestrong force can affect them. As most common nuclear particles are positively charged, this means they must overcome considerableelectrostatic repulsion before the reaction can begin. Even if the target nucleus is part of a neutralatom, the other particle must penetrate well beyond theelectron 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.
Neutrons, on the other hand, have no electric charge to cause repulsion, and are able to initiate a nuclear reaction at very low energies. In fact, at extremely low particle energies (corresponding, say, tothermal equilibrium at room temperature), the neutron'sde Broglie wavelength is greatly increased, possibly greatly increasing its capture cross-section, at energies close toresonances of the nuclei involved. Thus low-energy neutronsmay be even more reactive than high-energy neutrons.
While the number of possible nuclear reactions is immense, there are several types that are more common, or otherwise notable. Some examples include:
Fusion reactions – two light nuclei join to form a heavier one, with additional particles (usually protons or neutrons) emitted subsequently.
Spallation – a nucleus is hit by a particle with sufficient energy and momentum to knock out several small fragments or smash it into many fragments.
Induced gamma emission belongs to a class in which only photons were involved in creating and destroying states of nuclear excitation.
Fission reactions – a very heavy nucleus, after absorbing additional light particles (usually neutrons), splits into two or sometimes three pieces. This is an induced nuclear reaction.Spontaneous fission, which occurs without assistance of a neutron, is usually not considered a nuclear reaction. At most, it is not aninduced nuclear reaction.
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.
(α,α') measures nuclear surface shapes and sizes. Since α particles that hit the nucleus react more violently,elastic and shallow inelastic α scattering are sensitive to the shapes and sizes of the targets, likelight scattered from a small black object.
(e,e') is useful for probing the interior structure. Since electrons interact less strongly than do protons and neutrons, they reach to the centers of the targets and theirwave functions are less distorted by passing through the nucleus.
Usually at moderately low energy, one or more nucleons are transferred between the projectile and target. These are useful in studying outershell structure of nuclei. Transfer reactions can occur:
from the target to the projectile - pick-up reactions
Examples:
(α,n) and (α,p) reactions. Some of the earliest nuclear reactions studied involved an alpha particle produced byalpha decay, knocking a nucleon from a target nucleus.
(d,n) and (d,p) reactions. Adeuteronbeam impinges on a target; the target nuclei absorb either the neutron or proton from the deuteron. The deuteron is so loosely bound that this is almost the same as proton or neutron capture. A compound nucleus may be formed, leading to additional neutrons being emitted more slowly. (d,n) reactions are used to generate energetic neutrons.
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. The excited quasi-bound nucleus is called acompound nucleus.
