A nuclear reaction is a physical process in which a nucleus by the collision with another atom or core particles changes its state or its composition. Often the elastic collisions of nuclei are not expected to because only changing the pulses of the two collision partners, sometimes not even the inelastic collisions, where in addition one of the collision partners is put into an excited state. In other cases - the nuclear reactions in the literal sense - to change the cores by delivering or receiving particles their composition. The total number of nucleons remains always obtained, in most cases considered, the neutron and the proton number may be used individually.
Not to the nuclear reactions among the radioactive decay, because here the nuclear transformation occurs spontaneously, not by a shock.
According to the conservation of momentum and conservation of energy there is in all nuclear reactions specific characteristics and limitations of the particle motions, see kinematics ( Teilchenprozesse ).
In the symbolic image shown on the right as an example to react a 6Li nucleus and a deuterium nucleus ( a deuteron ) and form the compound nucleus 8Be, which decays immediately into two alpha particles.
- 5.1 Between nuclear reactions
- 5.2 Direct reactions
Ernest Rutherford used alpha particles from 1911 in its natural scattering experiments in which he observed the elastic scattering on gold nuclei. The first observation of a nuclear reaction in the strict sense, also by Rutherford, dated from 1919: alpha particles were shot by nitrogen, what is behind on the zinc sulfide screen, which served as a scintillator signals from protons gave ( reaction: 14N α → 17O p).
The alpha particles used by Rutherford came from a radioactive substance. However, further exploration and use of nuclear reactions was mainly based on artificially accelerated Geschossteilchen and was therefore closely linked with the development of particle accelerators. John Cockcroft and Ernest Walton succeeded in 1930, the first evidence of a triggered by artificially accelerated particles nuclear reaction - then proudly referred to as nuclear disintegration. They irradiated lithium with protons of kinetic energy 300 keV; as reaction products of helium -4 nuclei ( alpha particles ) were observed. So this was the reaction 7Li p → 2 4He. The particular of them energy and mass balance for the first experimental evidence of the " conversion of mass into energy ," according to Einstein's equation E = mc2.
A nuclear reaction may be displayed and checked by an equation similar to a chemical equation. Decays can be represented similarly, but then there is only one core on the left.
Each particle that participates in the reaction is written with its chemical symbol, the mass number and atomic number on the top left bottom left. The neutron is written as n, the proton can be written as 1H or p.
Thus, the equation is correct, the sum of the mass numbers must match on both sides ( as required by the law of conservation of baryon number ), and also the sum of the atomic numbers ( as required by the law of conservation of electric charge ). In the example shown here (we assume that we know just the right one particle ):
So that the totals agree, the second core, the atomic number 2 and mass number 4 must have the right, so it is also a helium-4 nucleus. Therefore, the complete equation reads:
Some particles are found in so many reactions observed that they usually shortens. For example, the 4He nucleus is ( the alpha particle ) by the Greek letter α denotes. Deuterons (heavy hydrogen, 2H) are denoted by d. Furthermore, the ordinal numbers may be omitted after the verification of the equation, because they are uniquely determined by the chemical symbols. In addition, in many reactions of interest meets a relatively light particles ( nucleon or light core, the projectile ) to a relatively heavy core of a particle of the same " slight " class (which Ejektil ) is emitted, and the other core remains. In these cases, the reaction can be simplified can be written:
Projectile or Ejektil are usually: protons, deuterons, helium nuclei, tritons, neutrons, gamma rays, etc. The core output is often referred to as a target nucleus (of English target, target ').
With Teilchensymbolen the formula is then, for example,
With omitted ordinal numbers
Or in compressed form:
- 6Li (d, α ) α.
Other examples are:
- From a nucleus of silver -107 is produced under capture of a neutron and emitting a γ - quantum Silver 108:
- A lithium -6 nucleus absorbs a neutron and goes by in a Triton and a helium - 4 nucleus:
The reasons given in the Examples reaction types (no matter with which target nuclide ) is abbreviated as (d, α ) -, (n, γ ) -, (n, p) - and ( n, t ) reactions.
The compressed notation is used for scattering. 12C for example ( n, n), 12C or 12C, a short (n, n ) stands for a neutron elastic scattering on a carbon -12 nucleus. An inelastic scattering is indicated by a line on the precipitating particles, eg 12C (n, n ') or 12C ( α, α ').
A special case is the nuclear fission. A specific cleavage reaction, for example, the first known, was discovered by Otto Hahn and co-workers, can be used as 235U (n, 95Kr ) 140Ba write. However, if - as often in practice - not interested, which creates the many possible pairs of fission products, 235U (n, f) is simply written ( f for engl fission, fission '. ).
Another special case is the spallation, in which, triggered by a high-energy particles, a core is broken up into many fragments; here the notation in the above formula style is obviously little sense.
Q- value and energy balance
Since the binding energy per nucleon is different in different cores, run some nuclear reactions exothermic, that is, they release energy in addition to the existing kinetic energy. Others are endothermic, ie with absorption of energy; this then has to as kinetic energy by means of one or both reaction partners " brought " to ( threshold energy ) to (ie, their cross section is different from zero ), the reaction is possible. Energies are usually expressed in nuclear physics in mega electron volts ( MeV).
The excess energy of exothermic reactions can be released as kinetic energy of the reaction products and / or gamma radiation.
With the same name charged reactants, the supply of activation energy is required to overcome the electrical repulsion for an exothermic reaction. Because the tunnel effect, this activation energy is not sharply defined. If a collision partners uncharged ( neutron or photon) is the electrical repulsion plays no role.
In the notation corresponding to a chemical equation for the energy gain or loss on Q can also be specified:
The last of the above examples, the breeding process in the blanket of a fusion reactor is so written:
This amount of energy ( in the nuclear physics commonly called only "Q- value " ) is positive in case of an exothermic reaction in endothermic negative. He is on the one hand, the difference between the sum of the kinetic energies on the end side and the top side. He also arises from the difference of total rest masses on the top side and the end side, converted using Einstein's equation E = mc2. The atomic mass unit u corresponds to the amount of energy:
Is the Q value in the above example
And converted to MeV:
, As indicated above.
Very convenient, you can identify Q -value calculations to perform, for example, with the Q Value Calculator.
Does a given, a constant stream of projectile particles on a given target, can be calculated from the cross section of the reaction of interest, the response rate (number of reactions per unit time). However, this is only a statistical average. The actually observed in a certain period of time the number of responses vary random fashion about the mean; the frequency with which the individual possible numbers occur follows the Poisson distribution.
Nuclear reactions are usually based on the strong and electromagnetic interactions, in certain cases, only the latter alone. The weak interaction plays practically in nuclear reactions in the terrestrial environment does not matter, however, is important in astrophysical processes; in such cases, it is also to transformations of neutrons, protons or vice versa.
The experimental observations suggest that nuclear reactions can proceed in very different ways depending on the involved nuclei / particles and depending on the impact energy. The types, intermediate nuclear reaction and direct reaction described below are only idealized limiting cases; actually observed reactions are usually mixtures of the types, that is, the resulting particles, radiation, etc. come in part from the different processes.
Between nuclear reactions
If the impact energy is small compared to the binding energy of a proton or neutron in the nucleus ( this is an average over all nuclides about 9 MeV ), the observations are often well explained by the intermediate core model: the two colliding particles / nuclei combine to form a new core which is then independent of the nature of its formation is again divided into two or more parts. Typical characteristics of a compound nucleus reaction are:
- Resonances in the excitation function
- Forward-backward symmetry, ie mirror symmetry about the 90 - degree direction of the observed angular distribution in the center-
At higher collision energies, however, occur in direct response mechanisms in the foreground. This includes, for example, the stripping reaction (german stripping, stripping '), which is particularly important for deuterons as projectiles. The relatively weakly bound deuteron ripping it into its two components neutron and proton; that of particles that the target nucleus is close enough is absorbed, while the other continues to fly. Thus, there is a (d, n ) - or ( d, p) - reaction. The (d, np ) reaction is in direct response (English deuteron breakup, breaking up of the deuteron ') possible if the deuteron tears in the force field of the target nucleus, but no absorption takes place. Strippingreaktionen also play, though less likely, for example, as (3He, d) -, (6Li, d ) reactions, etc. play a role.
Other direct reactions are the pick-up reaction ( engl. pick up, pick up ',' take '), in which the Geschossteilchen absorbs one nucleon from the target nucleus and brings with him, for example, a ( p, d) - or (d, 3He ) reaction, and the reaction kickoff ( engl. kick off, push away ' ) such as the type ( n, p). (n, p ) reactions contribute significantly to the material, for example, activation by fast neutrons in fusion reactors.
Typical of direct reactions
- A monotonically with impact energy over a larger area increasing excitation function
- A strong preference for small reaction angles, ie, a " forward peak " of the angular distribution.
All possible types of reaction are in direct response, as mentioned above, it is possible as an intermediate nuclear reaction. Often overlays both models are observed, such as an excitation function with a smooth, gradually increasing underground and individually seated thereon resonances.
Some special types of nuclear reactions
- Induced fission ( fission ) [( n, f ) ]
- Nuclear fusion [eg As T ( d, n) 4He ]
- Neutronenanlagerung [( n ) ]
- Of a proton [( p ) ]
With the study of nuclear reactions especially addressing the nuclear physics and particle physics. They also play an important role in the formation of the nuclides, see astrophysics, cosmochemistry. Applications, there is for example in energy technology, see nuclear reactor, fusion reactor, and medical (production of radionuclides for nuclear medicine and radiotherapy).