Rare decay modes of fission fragments
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re Decay Modes of Fission Fragments Yu. P. Gangrskii Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia Abstract—The experimental data on rare modes of radioactive decay of fission fragments is reviewed. These decay modes are due to a large excess of neutrons and a high energy of β decay fragments. They appear in delayed emission of various particles after the β decay (several neutrons, α particles, or heavy clusters) and excitation of unusual states (giant multipole resonances and shape isomers). The β decay and internal conversion of γ radiation into bound states of the atomic electron shell and their influence on the probability of secondary particle emission are considered. The possibility is discussed of observing decays that have not yet been experimentally detected, but theoretically predicted, as well as information on the nuclear structure obtained by studying such decay modes. PACS numbers: 23.40.-s DOI: 10.1134/S1063779607060020
1. INTRODUCTION We consider the features of radioactive decay of fission fragments, i.e., highly neutron-rich nuclei in the range of atomic numbers Z = 30–60 and masses A = 70– 160 [1, 2]. Fission is one of the reactions occurring during the interaction of almost any particles with heavy nuclei. Since this reaction is exothermic and releases large energy, it can occur spontaneously, without external perturbation (spontaneous fission). In the heaviest nuclei, this decay mode is the main one, and namely it controls the upper boundary of nuclear stability. The high energy released during nuclei fission results in a wide variety of final fission products, i.e., a wide set of the numbers of protons and neutrons in the indicated Z and A ranges, including a large excess of neutrons and a large spread in excitation energies and angular momenta. Certainly, all these factors affect the fission process itself and the properties of formed fragments.Fission is a complex nuclear process in which the shape of the nucleus changes from spherical (or close to it) to strongly deformed at the instant of splitting into two fragments. In this case, the internal energy in the nucleus is multiply redistributed between both collective and single-particle degrees of freedom. As a result, nuclear fission exhibits properties that hardly ever appear in other processes. An example can be isomerism of the shape of the nucleus [3], which was discovered in the study of nuclear fission and which allowed determination of the complex shape of the potential surface of nuclei. The objects of the study of the nuclear fission are the mechanism of this process and properties of formed fragments, primarily the characteristics of their radioactive decay. Currently, radioactive decay is one of the main data sources on the atomic nucleus structure, the interaction forces between its nucleons, and the laws controlling these forces. The first data on atomic
nucleus structure were obtained namely by studying their radioactive decay. Although radioactive decay has certain limitations in comparison with ot
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