To describe radioactivity and nuclear reactions such as fission, we can content ourselves with the image of nuclei composed of protons and neutrons. It is now known that protons and neutrons are composed of elementary corpuscles held together by a very intense attraction, the quarks. The existence of quarks has been confirmed by numerous experiments, but it has not been possible to extract them individually from nuclear matter, which gives an idea of the intensity of the attraction that binds them together.
What is the origin of the hidden attraction inside nucleons and nuclei?
The quarks carry both an electric charge and a strong charge. This charge is very special. A quark can exist in three states. To distinguish them, the physicists attributed to them a color, of course fictitious. For example, a quark-up will exist with three variants, a red quark-up, a blue quark-up, a green quark-up. Likewise a quark down. The strong charges borne by the red, blue and green variants have the same intensity. In antimatter, antiquarks charges will be called anti-red, anti-blue and anti-gren.
In a proton or neutron, the 3 quarks have a different "color". Because of this distinction, two quarks up (or two quarks down) are no longer completely identical. If they were identical, one of the fundamental principles of quantum mechanics, the Pauli exclusion principle , would forbid them to be in the same place with the same state of rotation, and thus a proton or neutron would not exist. On the other hand, if one is red, the other green or blue, they can coexist in the tiny proton or neutron volume.
Each kind of elementary interaction, electromagnetic or weak, is associated with a charge and a messenger wave exchanged between the particles carrying this charge. In the case of electromagnetic interaction, the waves exchanged between the electrons and the nucleus are familiar, photons. In the case of strong interactions, messenger waves are called gluons. Similar in some respects to photons, they are much less familiar, as they remain confined within the nuclear material like quarks. As their name suggests, their role is to glue quarks together and make compact protons, neutrons and nuclei.
Valence quarks and gluons are not the only constituents of protons and neutrons. In addition, there are further temporary quarks and antiquarks. In effect, a gluon has the possibility of coupling with a quark and an antiquark, and to materialize during very short times in these particles. For example, a red-antigreen gluon would temporarily give rise to a red up quark and a green up antiquark, which would recombine later (NB: This coupling is similar to that of a photon with an electron and positron pair).
Overall, valence quarks, gluons, transient quark and antiquark pairs, make a very complex object of a simple proton or neutron, long considered as an elementary brick of matter.
Busy with their exchanges, quarks and gluons remain confined inside a tiny sphere of about a fermi (millionth of a billionth of a meter) of radius. The situation changes when the tiny spheres of several nucleons come into contact. Exchange of quarks and gluons takes place between these closed confined
worlds which assemble to form a nucleus. The community of quarks and gluons is neutral of color, coming from nucleons themselves neutral of color. As a result, quarks and gluons are subjected to a global attraction that welds the nucleus.
Inside a nucleus quarks and gluons blend into a community. The boundaries between nucleons lose their meaning. The nucleus is more like a confederal Europe than a Europe of Nations. The balance between the up and down quarks varies with the size of the nuclei, which which goes up to the size of the heavy nuclei. Can there be a Brexit? When excess energy permits, groups of quarks can leave the nucleus. But they must exit by 3 or multiples of 3 to be neutral of color, to form stable states: for example the 3 quarks constituting a neutron, the 12 quarks of an alpha particle. The irruption of the 3 quarks of a neutron triggers the fission of the community of the 705 quarks of a nucleus of uranium-235 into two less populated communities of fission fragments.
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