Radioactive Decay: Alpha, Beta, Electron Capture
Alpha Decay
Radioactive nuclides with a very large atomic number (> 82) often decay with the emission of an alpha particle. By increasing the number of protons, Coulomb repulsion forces exceed the nuclear forces. Thus, the unstable nucleus emits a particle composed of two protons and two neutrons. The atomic number (Z = number of protons) decreases by two, and the mass number decreases by four. Q is the energy released in the process, the decay energy. Q is equivalent to the difference in mass between the parent nucleus and the produced nucleus. Q appears as kinetic energy of the alpha particle and the produced nucleus. Alpha particles have energies ranging from 5-10 MeV. Given their high mass and that they are emitted at high speed (+/- 107 m/s), when they hit matter, they gradually lose their energy, ionizing atoms, and slow down very quickly. They are stopped in a few centimeters, a few thousandths of air or water. In the human body, they do not penetrate the skin. They are completely absorbed by a 0.1 mm aluminum sheet or a single sheet of paper.
Beta-Minus (β-) Decay
Radionuclides with too many neutrons, or a high N/P ratio (neutron-to-proton ratio), reduce the high N/P ratio to reach stability. Q comes from the difference in mass between the nucleus and the products. Q is shared between the emitted particles (including gamma rays emitted by the daughter nucleus), but the energy of the produced nucleus is very small and is considered negligible. The neutrino has no charge and practically no mass; it is difficult to detect. Its existence is verified only by the energy difference.
Beta-Plus (β+) Decay
This occurs when the N/P ratio is smaller than that of stable nuclei with the same atomic number or number of neutrons. Stability is achieved by increasing the value of N/P. As in β- decay, Q is carried by the emitted particles. Both β+ and β- particles are emitted as a spectrum of energies. β+ decay occurs when the energy difference between the parent and daughter nuclei is greater than 1.02 MeV. A β+ particle is unstable and eventually combines with an electron. Annihilation occurs, which results in two gamma photons of 0.51 MeV each; the mass of two electrons becomes energy. Beta particles have less mass than alpha particles and penetrate further. They are absorbed by a 0.5 mm aluminum foil and are stopped by several feet of air or 1 cm of water. In the human body, they pierce the skin but not the subcutaneous tissue.
Electron Capture
An orbital electron is captured by the nucleus, transforming a proton into a neutron. It is an alternative process to β+ decay. The capture of the electron is usually from the K-shell, but also from the L and M shells. An empty space is created, which is filled by an outer electron, producing characteristic X-rays. There is also Auger electron emission, which are monoenergetic electrons produced by the absorption of characteristic X-rays and the re-emission of energy, ejecting orbital electrons from the atom (like an internal photoelectric effect).
Isomeric Transition
For some nuclides, the excited state of the nucleus persists for a considerable time. In this case, we say that the nucleus exists in a metastable state. The metastable nucleus is an isomer of the final nucleus; they have the same mass number and atomic number but a different energy state.