A nuclear reaction is a transformation of one or more atomic nuclei, it is distinguished from a Chemical reaction which relates to the electron S or the bonds between the atoms. A nuclear reaction, two atomic nuclei enter in collision and produce products different from the original particles. In theory, more than two particles could enter in collision, but that is much less probable. In the case of the Radioactivity, the transformation is spontaneous, but in the case of a nuclear reaction, it is produced by a moving particle. If the particles separate after the collision without being transformed, the process is not a reaction, but a elastic Collision. In the example of reaction shown on the right, ⁶Li and deuterium react by forming an intermediate atomic nucleus very excited ⁸Be which disintegrates very quickly by producing two particles alpha. Here, the Proton S are represented by red spheres, and the Neutron S, by blue spheres.

Representation

A nuclear reaction can be represented by an equation similar to that representing a Chemical reaction. nuclear disintegrations can be represented in a similar way, but with only one core on the left.

Each particle is written with its chemical symbol, its Atomic number on the left in bottom, and its Mass number in top. For the neutron, the symbol is N . The Proton can be written " H" (core of Hydrogen) or " p".

To check the equation, one must control that the sums of the atomic numbers are equal on the left and on the right (because of the Loi of conservation of the electric Charge, and that the sums of the mass numbers are also equal on the left and on the right (because of the law of conservation of the baryon Nombre. For example:

₃⁶Li + ₁²H → ₂⁴He + ₂⁴He

Obviously, the equation is correct. It could also be written

₃⁶Li + ₁²H → 2 ₂⁴He

Simplified representation

If some particles very often appear, abbreviations are used. For example, the core ⁴He (which is called also particle alpha) is shortened with the Greek letter " α". The deuterons (heavy hydrogen, ²H) are simply indicated " d". Also, as the atomic numbers are given implicitly by the chemical symbols, they can be removed when the equation was checked. Finally, in much of reactions, a relatively heavy core is struck by a light particle of an small group of common particles, emitting another common particle, and producing another core. For these reactions, the notation can be much simplified in the following way:

(particle of entry, particle of exit)

Consequently, one could périphraser the example preceding by introducing symbols:

₃⁶Li + D → α + α

then, removing the atomic numbers:

⁶Li + D → α + α

and finally, using the condensed form:

⁶Li (D, α) α

Conservation of energy

It is possible that kinetic energy is released during a reaction (exothermic Réaction), or that kinetic energy must be added to make possible the reaction (endothermic Réaction). To decide this question, http://physics.nist.gov/PhysRefData/Compositions/index.html) should a table of very exact mass of the particles (see. According to this table, the core ₃⁶Li has a Atomic mass of 6.015 units of atomic mass (abrévié U), the deuteron has 2.014 U, and the core ₂⁴He has 4.0026 U. Consequently:

• total Mass of rest on the left = 6.015 + 2.014 = 8.029 U

• total Mass of rest on the right = 2 × 4.0026 = 8.0052 U
• Loss of mass = 8.029 - 8.0052 = 0.0238 units of atomic mass.

In a nuclear reaction, total relativistic energy is preserved. Consequently, the lost mass must reappear like kinetic energy. Using the formula of Einstein '' E ''   =  '' mc '' ², one can determine the quantity of released energy.

But initially, it is necessary to calculate energy equivalent to a unit of atomic mass:

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  /  1.60218  ×  10^-13  J)

=  931.49  MeV,

Consequently, 1  u  c²  =  931.49  MeV.

Then, the kinetic quantity of produced energy is 0.0238 × 931 MeV = 22.4 MeV.

Or, expressed in a different way: the mass is reduced by 0.3%.

It is a great quantity of energy for a nuclear reaction; the quantity is so large because the energy binding by nucleon of the nuclide ⁴He is exceptionally broad, because the core of ⁴He is doubly magic. Consequently, the particles alpha often appear at the right-sided of the equation.

The energy released in a nuclear reaction can appear in three different manners:

• kinetic energy of the produced particles

• emission of the Photon S of very great energy, called gamma rays
• part of energy can remain in the core, like metastable level .

If the produced core is metastable, that is indicated by an asterisk (" *") nearly its atomic number. Possibly, this energy is released by nuclear transmutation.

In general, the produced core has a different atomic number, and consequently, the configuration of its electron shells is not right. Then the electrons, while being arranged, emit also x-rays.

" Q-value"

By writing the equation for the nuclear reaction (in a way similar to an equation for a chemical reaction) one can add the energy of reaction on the right:

Noyau targets + projectile - > produced Noyau + éjectile + Q .

For the special case discussed in top, we already calculated the energy of reaction: Q = 22.4 MeV. Then:

₃⁶Li + ₁²H → ₂⁴He + ₂⁴He + 22.4 MeV

The energy of reaction (" Q-value" in English) is positive for the reactions exothermic and negative for the endothermic reactions. On the one hand, it is the difference between the sums of the kinetic energies on the right and on the left. But in addition, it is also the difference between the nuclear masses of rest on the left and on the right (and in this manner, we calculated the value in top).

Rate of reaction

If a reaction is checked as for the atomic numbers and mass numbers (as shown in top), that does not want to say that the reaction can take place. The rate of reaction depends on the energy of the particles, of the Flux of the particles and the cross Section of the reaction.

Comparison between neutrons and ions

In the initial collision, the particles must approach so close so that the strong Nuclear force (of a very reduced operating range) can enter concerned. As the nuclear particles have positive loads normally, they must overcome an electrostatic repulsion considerable. Even if the target nuclide belongs to a neutral atom, the other particle must approach the core of positive load. Consequently, it is initially necessary to accelerate the projectiles with high energy, for example, by:

• Particle accelerator
• a very large temperature, a few million degrees, producing reactions Thermonuclear S, as in the center of the sun (see low)
• cosmic rays

The Neutron S, in addition, do not have an electric charge, and they can carry out a nuclear reaction to very small energies. Frequently, the cross section grows even if energy decrease.

Sun

The Sun is an enormous thermonuclear engine self-sustained. For the moment, no drift is to be feared, this explosive reaction is contained by the gravitational force. Let us look at what this master key within the Sun to include/understand what is a nuclear reaction.

• There exist two types of reaction, the Fission and the fusion, the Fission consists in separating the core from the atom (to separate the Proton S and the Neutron S between them) and the fusion is the fact of associating two cores to form a new core. All the elements are formed as follows: at the origin of the Universe, there was only Hydrogène but the reactions in the middle of the star S form all the others element S to iron, the heavier element S are formed by another process.
• Will know that the fusion produces much more energy (the first nuclear bombs fissioned Atome S of Uranium, but today the nuclear bombs amalgamate atoms Hydrogène, H-bomb, these bombs are much more powerful and destroying).
• In our Sun, because of the very high temperatures which reign there, the particles are very agitated and have kinetic energy enormously (speed). Because of high speed of the Atom S, the Atome S cannot exist in normal form because the electron S refuse “to revolve” around. (Imagine that the Ground is the core and that the the Moon is a electron; if one heats the Earth (be imaginative) it will start to vibrate. The more one heats, the more it vibrates extremely, these Vibration S will become so strong that the the Moon will not be able to turn any more around, one will thus say that the Earth is ionized).
• Although they move the ones towards the others, the cores are not entrechoquent because the electromagnetic force pushes back them (the cores are all the two positive ones). But if the temperature is increased, the cores gain speed and at the time of the shocks, they approach always more and more, until the cores come into contact and which the strong nuclear force takes the relay, but as it is thousands of times more powerful than the electromagnetic force, the cores bind between them and form only one atom.
• the remarkable property of this reaction resides in the fact that the mass of the core is slightly lower than the sum of the masses of the two protons of the beginning of the reaction. The nuclear reaction of fusion is thus accompanied by a loss of mass.
• But, Einstein showed by the Theory of relativity that the mass can be transformed into energy and that energy can be transformed into mass according to the famous formula $E=MC^2$ which states that the energy is equal to the bulk product by the square of the Speed of light. The loss of mass of the reaction referred to above corresponds to a release of energy. Thus by transforming a fraction of its Masse that our Sun finds the resources which are necessary for him. This method is much more effective than the chemical reactions or the Contraction Kelvin-Helmholtz. It makes it possible a star like ours to shine during 10 billion years.

Simple: Nuclear reaction

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