Reaction triples alpha

The reaction triples alpha is the process according to which three core X of Hélium (particles alpha) are transformed into a Atome of Carbone.

This reaction of nuclear Fusion can occur quickly with Température S higher than 100 000 000 K and in the star S with high abundance out of helium. It thus occurs in old stars where the helium produced by the Chaîne proton-proton and the Cycle carbon-nitrogenize-oxygen accumulated in the center of star. As initially helium does not produce energy, the star contracts until the moment when the temperature in the center reaches the point where the reaction triples alpha can take place.

⁴He + ⁴He ↔ ⁸Be + photon

⁸Be + ⁴He ↔ ¹² C + γ + 7.367 MeV

The energy assessment Net of this reaction is 7,275 MeV.

The ⁸Be produces at the time of the first stage is unstable east turns over in the form of two helium cores in 2,6×10^-16 seconds. Nevertheless, under the equilibrium conditions of the reaction, there exists always an minor amount of ⁸Be; the capture of another helium core produces carbon ¹²C then. This conversion of three helium cores or particles alpha in ¹²C is called reaction triple-alpha .

As the reaction triple-alpha is not very probable, it requires a very long time to produce carbon. A consequence of that is that no carbon was produced at the time of the Big Bang because the temperature dropped quickly in lower part of the temperature necessary to this fusion.

Normally, the probability that three cores can be thus amalgamated is extremely weak. Nevertheless, the fundamental state of beryllium-8 has same energy almost exactly as two particles alpha and, in the second stage, the couple ⁸Be + ⁴He has same energy almost exactly as one of the excited states of ¹²C. These resonances increase the probability considerably that a particle alpha combines with a beryllium-8 core to form a carbon atom. The fact that the existence of carbon depends as these energy levels have the good values exactly in a way very discussed by Fred Hoyle like a anthropic proof of the Principe was advanced.

Like side effect of the process, certain carbon cores can be amalgamated with additional helium cores by producing a stable Isotope of Oxygène and energy:

¹²C + ⁴He → ¹⁶O + γ

The following stage where oxygen also combines him with a particle alpha to form an atom of Néon is more difficult because of the rules concerning the nuclear Spin. This has as a consequence that the stellar Nucléosynthèse produces great quantity of carbon and of oxygen but a part only as of these elements are in their turn converted into neon and heavier elements. The nuclear fusion produced of energy only until the Iron; the heavier elements are created during the explosion of Supernova E with energy absorption.

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