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Cluster decay, also named heavy particle radioactivity or heavy ion radioactivity, is a type of nuclear decay in which an atomic nucleus emits a small "cluster" of neutrons and protons, more than in an alpha particle, but less than a typical binary fission fragment. Ternary fission into three fragments also produces products in the cluster size. The loss of protons from the parent nucleus changes it to the nucleus of a different element, the daughter, with a mass number Ad = A - Ae and atomic number Zd = Z - Ze where Ae = Ne + Ze. For example: : → + This type of rare decay mode was observed in radioisotopes that decay predominantly by alpha emission, and it occurs only in a small percentage of the decays for all such isotopes. The branching ratio with respect to alpha decay : is rather small (see the Table below). Ta and Tc are the half-lives of the parent nucleus relative to alpha decay and cluster radioactivity, respectively. Cluster decay, like alpha decay, is a quantum tunneling process: in order to be emitted, the cluster must penetrate a potential barrier. This is a different process than the more random nuclear disintegration that precedes light fragment emission in ternary fission, which may be a result of a nuclear reaction, but can also be a type of spontaneous radioactive decay in certain nuclides, demonstrating that input energy is not necessarily needed for fission, which remains a fundamentally different process mechanistically. Theoretically any nucleus with Z > 40 for which the released energy (Q value) is a positive quantity, can be a cluster-emitter. In practice, observations are severely restricted to limitations imposed by currently available experimental techniques which require a sufficiently short half-life, Tc < 1032 s, and a sufficiently large branching ratio B > 10−17. In the absence of any energy loss for fragment deformation and excitation, as in cold fission phenomena or in alpha decay, the total kinetic energy is equal to the Q-value and is divided between the particles in inverse proportion with their masses, as required by conservation of linear momentum : where Ad is the mass number of the daughter, Ad = A – Ae. Cluster decay exists in an intermediate position between alpha decay (in which a nucleus spits out a He4 nucleus), and spontaneous fission, in which a heavy nucleus splits into two (or more) large fragments and an assorted number of neutrons. Spontaneous fission ends up with a probabilistic distribution of daughter products, which sets it apart from cluster decay. In cluster decay for a given radioisotope, the emitted particle is a light nucleus and the decay method always emits this same particle. For heavier emitted clusters there is otherwise practically no qualitative difference between cluster decay and spontaneous cold fission. == History == The first information about the atomic nucleus was obtained at the beginning of the 20th century by studying radioactivity. For a long period of time only three kinds of nuclear decay modes (alpha, beta, and gamma) were known. They illustrate three of the fundamental interactions in nature: strong, weak, and electromagnetic. Spontaneous fission became popular soon after its discovery in 1940 by K. Petrzhak and G. Flerov owing to both military and peaceful applications of neutron-induced fission discovered in 1939 by Otto Hahn, Lise Meitner, and Fritz Strassmann, employing the large amount of energy released during the process. There are many other kinds of radioactivity, e.g. cluster decay, proton decay, various beta-delayed decay modes (p, 2p, 3p, n, 2n, 3n, 4n, d, t, alpha, f), fission isomers, particle accompanied (ternary) fission, etc. The height of the potential barrier, mainly of Coulomb nature, for emission of the charged particles is much higher than the observed kinetic energy of the emitted particles. The spontaneous decay can only be explained by quantum tunneling in a similar way to the first application of the Quantum Mechanics to Nuclei given by G. Gamow for alpha decay. :''"In 1980 A. Sandulescu, D.N. Poenaru, and W. Greiner described calculations indicating the possibility of a new type of decay of heavy nuclei intermediate between alpha decay and spontaneous fission. The first observation of heavy-ion radioactivity was that of a 30-MeV, carbon-14 emission from radium-223 by H.J. Rose and G.A. Jones in 1984"''. Usually the theory explains an already experimentally observed phenomenon. Cluster decay is one of the rare examples of phenomena predicted before experimental discovery. Theoretical predictions were made in 1980, four years before experimental discovery. Four theoretical approaches were used: fragmentation theory by solving a Schrödinger equation with mass asymmetry as a variable to obtain the mass distributions of fragments; penetrability calculations similar to those used in traditional theory of alpha decay, and superasymmetric fission models, numerical (NuSAF) and analytical (ASAF). Superasymmetric fission models are based on the macroscopic-microscopic approach using the asymmetrical two-center shell model level energies as input data for the shell and pairing corrections. Either the liquid drop model or the Yukawa-plus-exponential model extended to different charge-to-mass ratios have been used to calculate the macroscopic deformation energy. Penetrability theory predicted eight decay modes: 14C, 24Ne, 28Mg, 32,34Si, 46Ar, and 48,50Ca from the following parent nuclei: 222,224Ra, 230,232Th, 236,238U, 244,246Pu, 248,250Cm, 250,252Cf, 252,254Fm, and 252,254No. The first experimental report was published in 1984, when physicists at Oxford University discovered that 223Ra emits one 14C nucleus out of every billion (109) alpha decays. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cluster decay」の詳細全文を読む スポンサード リンク
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