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The stability of helium-4 is the reason that hydrogen is converted to helium-4, and not deuterium (hydrogen-2) or helium-3 or other heavier elements during fusion reactions in the Sun. It is also partly responsible for the alpha particle being by far the most common type of baryonic particle to be ejected from an atomic nucleus; in other words ...
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 nuclear binding energy. Using Einstein's mass-energy equivalence formula E = mc 2, the amount of energy released can be determined.
A graphical representation of the semi-empirical binding energy formula. The binding energy per nucleon in MeV (highest numbers in yellow, in excess of 8.5 MeV per nucleon) is plotted for various nuclides as a function of Z, the atomic number (y-axis), vs. N, the number of neutrons (x-axis). The highest numbers are seen for Z = 26 (iron).
Reactions that release no neutrons are referred to as aneutronic. To be a useful energy source, a fusion reaction must satisfy several criteria. It must: Be exothermic This limits the reactants to the low Z (number of protons) side of the curve of binding energy. It also makes helium 4 He
The following apply for the nuclear reaction: a + b ↔ R → c. in the centre of mass frame, where a and b are the initial species about to collide, c is the final species, and R is the resonant state.
Some stable helium-3 (two protons and one neutron) is produced in fusion reactions from hydrogen, though its estimated abundance in the universe is about 10 −5 relative to helium-4. [92] Binding energy per nucleon of common isotopes. The binding energy per particle of helium-4 is significantly larger than all nearby nuclides.
Almost all neutrons that fused instead of decaying ended up combined into helium-4, due to the fact that helium-4 has the highest binding energy per nucleon among light elements. This predicts that about 8% of all atoms should be helium-4, leading to a mass fraction of helium-4 of about 25%, which is in line with observations.
Nuclear fusion reaction of two helium-4 nuclei produces beryllium-8, which is highly unstable, and decays back into smaller nuclei with a half-life of 8.19 × 10 −17 s, unless within that time a third alpha particle fuses with the beryllium-8 nucleus [3] to produce an excited resonance state of carbon-12, [4] called the Hoyle state, which ...