Beta decay (β) and electronic capture change the composition of protons and neutrons in a nucleus, the electric charge of the nucleus increasing or decreasing by one. This variation of charge is compensated by the emission of a charged particle - an electron or a positron - or, more rarely, by the capture of an electron. As another characteristic signature of these transformations, other particles that cannot be detected are emited: neutrinos or antineutrinos.
The main forces at work in the nucleus, those attractive that maintain its cohesion and those repulsive between electric charges of the same sign are unable to transform neutrons into protons and produce electrons, positrons, neutrinos and antineutrinos. Nature therefore uses a third type of interaction (this term is somehow more accurate than force) to allow and proceed beta decay or electron capture.
This third interaction is considered weak because beta decays that are the most visible manifestation are very slow transformations that happen rarely. The lifetimes of unstable nuclei are extremely variable (quarter of an hour for a free neutron, one week for iodine-131, thirty years for cesium-137, a billion years for potassium-40), but all these periods, including the quarter of an hour of the neutron, are very long for the nuclear clocks.
The first theory of beta decay was made in 1934 by the great Italian physicist Enrico Fermi, at a time when the existence of quarks was not suspected and the one of neutrinos only hypothetical. Since the 1970s, we know that when a nucleon changes its nature (proton or neutron), it is because one of the constituents (up or down quark) transformes itself from one species into another. It is at this elementary level that weak interaction steps in.
In the case of beta-minus decay mechanism is as follows. A down quark in a neutron, whose electric charge is -e/3, frequently emits a negative charge -e. Its charge is now +2e/3. It has become an up quark. In general, the up quark reabsorb immediately the negative charge and returns to the down quark state. The negative charge briefly emitted and immediately reabsorbed is carried by a particle called the "W-minus boson". However, this object is unstable and can decay into an electron and an antineutrino (*)
If the boson decays in the extraordinarily short time elapsing between its emission and its reabsorption, a beta-minus decay occurred.
This mechanism is explained in the framework of quantum mechanics. It is the Heisenberg uncertainty principle that allows a quark to emit and reabsorb an object much more massive than him, the W boson. The existence of this fugitive intermediate, whose properties had been predicted by theory in the late 1960s, has been confirmed experimentally in 1983.
Related topic : α decay : tunnel effect
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