Other attributes
Radionuclide - nuclides whose nuclei are unstable and experience radioactive decay. Most of the known nuclides are radioactive (only about 300 of the more than 3000 nuclides known to science are stable). Radioactive all nuclides, having charge number Z equal to 43 (technetium) or 61 (promethium) or more than 82 (lead); corresponding elements are called radioactive elements. Radionuclides (mostly beta unstable) exist in any element (i.e. for any charge number), and any element has significantly more radionuclides than stable nuclides.
Tritium fob
Since beta decay of any type does not change the mass number A of the nuclide, among nuclides with the same mass number (isobaric chain) there exists at least one beta-stable nuclide corresponding to a minimum on the dependence of atomic mass excess on the nuclear charge Z at a given A (isobaric chain); beta decays occur towards this minimum (β-decay with increasing Z, β+-decay and electron capture with decreasing Z); spontaneous transitions in the opposite direction are forbidden by the law of conservation of energy. For odd A this minimum is one, whereas for even A beta-stable nuclides there may be two or even three. Most of the light beta-stable nuclides are stable with respect to other forms of radioactive decay too[1], and thus are perfectly stable (except for the hitherto undetected proton decay that is predicted by many modern extensions of the Standard Model).
From A = 36 on even isobaric chains a second minimum appears. Beta-stable nuclei in local minima of isobaric chains are capable of double beta decay into a global chain minimum, though the half-lives in this channel are very long (1019 years and more) and in most cases where such a process is possible it has not been observed experimentally. Heavy beta-stable nuclei can experience alpha decay (starting with A ≈ 140), cluster decay and spontaneous fission.
Most of the radionuclides are produced artificially, but there are also naturally occurring radionuclides, which include:
- Radionuclides with long half-lives (>5-107 years, e.g. uranium-238, thorium-232, potassium-40), which have not had time to decay since nucleosynthesis during the Earth's existence, 4.5 billion years
- radiogenic radionuclides - products of decay of the above-mentioned long-lived radionuclides (for example, radon-222 and other radionuclides of the radium, thorium and actinium series);
- cosmogenic radionuclides resulting from cosmic radiation (tritium, carbon-14, beryllium-7, etc.).
