<P> When alpha particle emitting isotopes are ingested, they are far more dangerous than their half - life or decay rate would suggest, due to the high relative biological effectiveness of alpha radiation to cause biological damage . Alpha radiation is an average of about 20 times more dangerous, and in experiments with inhaled alpha emitter up to 1000 times more dangerous, than an equivalent activity of beta emitting or gamma emitting radioisotopes . </P> <P> Some science authors use doubly ionized helium nuclei (He) and alpha particles as interchangeable terms . The nomenclature is not well defined, and thus not all high - velocity helium nuclei are considered by all authors to be alpha particles . As with beta and gamma particles / rays, the name used for the particle carries some mild connotations about its production process and energy, but these are not rigorously applied . Thus, alpha particles may be loosely used as a term when referring to stellar helium nuclei reactions (for example the alpha processes), and even when they occur as components of cosmic rays . A higher energy version of alphas than produced in alpha decay is a common product of an uncommon nuclear fission result called ternary fission . However, helium nuclei produced by particle accelerators (cyclotrons, synchrotrons, and the like) are less likely to be referred to as "alpha particles". </P> <P> The best - known source of alpha particles is alpha decay of heavier (> 106 u atomic weight) atoms . When an atom emits an alpha particle in alpha decay, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle . The atomic number of the atom goes down by exactly two, as a result of the loss of two protons--the atom becomes a new element . Examples of this sort of nuclear transmutation are when uranium becomes thorium, or radium becomes radon gas, due to alpha decay . </P> <P> Alpha particles are commonly emitted by all of the larger radioactive nuclei such as uranium, thorium, actinium, and radium, as well as the transuranic elements . Unlike other types of decay, alpha decay as a process must have a minimum - size atomic nucleus that can support it . The smallest nuclei that have to date been found to be capable of alpha emission are beryllium - 8 and the lightest nuclides of tellurium (element 52), with mass numbers between 106 and 110 . The process of alpha decay sometimes leaves the nucleus in an excited state, wherein the emission of a gamma ray then removes the excess energy . </P>

When a radioactive atom emits an alpha particle the original atom's mass number decreases by