What happens in a nuclear reaction

A Nuclear reaction is a physical process in which at least one nucleus is converted into another nuclide and / or into free nucleons when an atomic nucleus collides with another nucleus or particle.

While the nuclear transformation occurs spontaneously in the case of radioactive decay, in the nuclear reaction it is triggered by an impacting particle. From the validity of the law of conservation of momentum and the law of conservation of energy for every collision process, certain properties and limitations of the particle movements result (see kinematics).


In the symbolic image shown as an example on the right, one reacts 6Li nucleus and a deuterium nucleus (a Deuteron) and form the intermediate core8Be, which immediately breaks up into two alpha particles.

Formula notation

A nuclear reaction can be represented and verified by an equation similar to a chemical equation. Nuclear decays can be represented similarly, but then only a Core on the left.

Every particle that takes part in the reaction is written with its chemical symbol, with the mass number at the top left and the atomic number at the bottom left. The neutron is written as "n", the proton can be written as "1H ”or“ p ”can be written.

For the equation to be correct, the sums of the mass numbers on both sides must agree (as the law of conservation of the baryon number requires), and so do the sums of the ordinal numbers (as the law of conservation of the electric charge requires). In the example shown here (let's assume we only know the one particle on the right):


In order for the sums to be correct, the second nucleus on the right must have the ordinal number 2 and the mass number 4, so it is also a helium-4 nucleus. The complete equation is therefore:


or simplified:


Simplified representation

Many particles appear in so many reactions that they are usually abbreviated. For example, the 4He core (the alpha particle) is designated with the Greek letter "α". Deuterons (heavy hydrogen, 21H) are designated with "d". In addition, the ordinal numbers can be omitted after checking the correctness of the equation, because they are clearly defined by the chemical symbols. In addition, in many common reactions, a light particle (the “projectile”, one of a small group of common particles) meets a relatively heavy core, a common light particle (the “ejectile”) is emitted, and another core is left behind. In these cases the reaction can be written as follows:

Output core (projectile, ejectile) end core

Projectile or ejectile can be: protons, deuterons, helium nuclei, tritons, neutrons, gamma quanta, etc. The starting nucleus is often referred to as the "target nucleus".

So we can simplify the previous equation by first introducing symbols:


then omit ordinal numbers:


and so we finally come to the compressed form:

  •         6Li (d, α) α.

Further examples:

107Ag (n, γ)108Ag
  • 14N is given by a neutron of cosmic radiation in 14C transforms (through this process becomes ongoing 14C generates: the basis of radiocarbon dating):
14N (n, p)14C.
  • A lithium-6 nucleus absorbs a neutron and is converted into a triton and a helium-4 nucleus:
6Li (n, t)4Hey

The types of reactions listed in these examples (regardless of the target nuclide) are referred to as (d, α), (n, γ), (n, p) and (n, t) reactions for short.

Nuclear fission is a special case. A certain cleavage reaction (e.g. the first known, discovered by Otto Hahn and co-workers) can be described as 235U.N, 95Kr)140Write Ba. However, if - as is often the case in practice - you are not interested in which of the many possible pairs of fission products is created, it becomes easy 235U (n, f) written (f for Engl. fission, Split).

Another special case is spallation, in which, triggered by a high-energy particle, a core is shattered into many fragments; here the notation in the formula style mentioned makes little sense.

Q value and energy balance

When notation according to a chemical equation, the reaction energy Q be given:

Initial core + projectile -> final core + ejectile + Q.

The last of the above examples, the breeding process in the blanket of a fusion reactor, is written like this:

The reaction energy (the "Q value") is positive with an exothermic reaction and negative with an endothermic reaction. On the one hand, it is the difference between the sum of the kinetic energies on the end side and the start side. On the other hand, it also results from the difference between the nuclear masses (measured in atomic mass units u) on the start side and the end side, converted using Einstein's equation E = m c2:

1 uc2 = (1.66054 × 10-27 kg) × (2.99792 × 108 m / s)2 
= 1.49242 × 10-10 kg (m / s)2 = 1.49242 × 10-10 J (joules)
× (1 MeV / 1.60218 × 10-13 J)
= 931.49 MeV,
so 1 uc2 = 931.49 MeV.

For the example given above, the Q value in atomic mass units u results:

M (6Li) + M (n) - M (4He) - M (3H)
= 6.015122 + 1.008665 - 4.002603 - 3.016029 = 0.005155 u
and converted into MeV:
Q = 0.005155 u × (931.49 MeV / u) = 4.802 MeV as given above.

Since the binding energy of different nuclei is different, nuclear reactions such as chemical reactions can take place endothermically or exothermically.

So that an endothermic reaction is possible, the missing energy has to be contributed as kinetic energy of one or both reaction partners (Threshold energy). The excess energy of exothermic reactions can be released as kinetic energy of the reaction products and / or as gamma radiation.

The strong interaction underlying nuclear reactions has only a short range. Therefore, with reactants charged with the same name, an activation energy is required for an exothermic reaction in order to overcome the electrical repulsion. Because of the tunnel effect, this activation energy is not sharply defined. If a collision partner is uncharged (neutron or photon), the electrical repulsion is irrelevant.


Some special types of Nuclear reactions:


With the exploration of nuclear reactions are mainly concerned with nuclear physics and particle physics. They play an important role in the formation of nuclides, see astrophysics, cosmochemistry. Applications there are e.g. in energy technology, see nuclear reactor, fusion reactor, and medical technology (production of radionuclides for nuclear medicine and radiation therapy).

Category: nuclear physics