A radionuclide radioactive nuclide , radioisotope or radioactive isotope is an atom that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation ; transferred to one of its electrons to release it as a conversion electron ; or used to create and emit a new particle alpha particle or beta particle from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay.
Different isotopes of the same element have the same number of protons in their atomic nuclei but differing numbers of neutrons. Radioisotopes are radioactive isotopes of an element. The unstable nucleus of a radioisotope can occur naturally, or as a result of artificially altering the atom. The best known example of a naturally-occurring radioisotope is uranium. All but 0.
About a decade ago, studies began to hint that many sharks have longer lifespans than previously suspected. Now, a new analysis that pulled together data from more than 50 studies suggests a "widespread" underestimation of lifespans among many sharks, rays, and cartilaginous fish. That's because newer methods of aging sharks—such as bomb carbon dating—are proving more accurate than the traditional method of counting the growth bands on vertebrae, says study author Alastair Harry , a fisheries scientist at Australia's James Cook University. The results, published recently in the journal Fish and Fisheries , suggest that many sharks—from great whites to sand tigers to dusty sharks—may roam the oceans for decades longer than believed. And just last year, scientists found that Greenland sharks, native to cold Arctic waters, might live for centuries.