# Cluster decay of superheavy nuclei

Article (PDF Available)inPhysical Review C 85(3):034615 · March 2012with 219 Reads
DOI: 10.1103/PhysRevC.85.034615
Abstract
Calculations of half-lives of superheavy (SH) nuclei show an unexpected result: for some of them cluster radioactivity (CR) dominates over α decay. We changed the concept of CR to allow emitted particles with Ze>28 from parents with Z>110 (daughter around 208Pb). We find a trend toward shorter half-lives and larger branching ratios relative to α decay for heavier SHs. A table of measured masses along with theoretical tables are used to determine Q values.
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During the last decade, six new superheavy elements were added into the seventh period of the periodic table, with the approval of their names and symbols. This milestone was followed by proclaiming 2019 the International Year of the Periodic Table of Chemical Elements by the United Nations General Assembly. According to theory, due to their large atomic numbers, the new arrivals are expected to be qualitatively and quantitatively different from lighter species. The questions pertaining to superheavy atoms and nuclei are in the forefront of research in nuclear and atomic physics and chemistry. This Colloquium offers a broad perspective on the field and outlines future challenges.
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Cluster radioactivity is an exotic nuclear decay observed in actinides where a light nucleus is emitted while the remaining heavy mass residue is the doubly magic Pb208 or a nucleus in its neighborhood. We have investigated this type of decay in heavier nuclei up to Lv (Z=116) within a microscopic theory. It has been found that the cluster radioactivity known in the light actinides may become the dominant decay channel in some superheavy nuclei. This superasymmetric fission channel is distinct from typical asymmetric fission in actinides. We predict a sharp fission fragment mass distribution with the heavy fragment close to Pb208.
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Cluster radioactivity is an exotic nuclear decay observed in actinides where a light nucleus is emitted while the remaining heavy mass residue is the doubly magic $^{208}$Pb or a nucleus in its neighborhood. We have investigated this type of decay in heavier nuclei up to Lv $(Z=116)$ within a microscopic theory. It has been found that super asymmetric fission with $^{208}$Pb as heavy fragment may be dominant decay channel in some super heavy nuclei. This reaction is closely related with cluster radioactivity.
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The probabilities of α decay and heavy-cluster decay in the superheavy region are investigated. The preformation probabilities for α and cluster emissions are evaluated with a new formula which depends on the Q value of the decay. The formula shows a better agreement with the experimentally extracted cluster preformation probabilities in the heavy region. The α half-lives using our model are compared with the experimental results and it is found that the present model can be employed successfully to predict α half-lives in the superheavy region. The probable heavy-cluster radioactivity from superheavy nuclei is also studied using the same model. A comparison of the heavy-cluster half-lives using the present model, the half-lives given by Poenaru et al. [Eur. Phys. J. A 54, 14 (2018)] (analytical superasymmetric fission) and Zhang et al. [Phys. Rev. C 97, 014318 (2018)] are performed. The agreement between these three formalisms is evident from the study. A further study on the competition between α decay and heavy-cluster radioactivity leading to Pb208 is performed. The study suggests that heavy-cluster radioactivity may be comparable to or even dominant over α decay for some of the isotopes with Z≥118. The study shows that the present model can be used effectively to obtain the α as well as the heavier cluster decay half-lives in the superheavy region.
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A short review of the studies of superheavy nuclei (SHN), done recently in our theoretical group of Warsaw, is presented. Main attention is given to description of the properties of SHN. The description is performed by macroscopic microscopic methods. Such properties as mass, a-decay energy and a-decay half-life are considered. Special attention is devoted to the analysis of the half-life. Although mainly treated in a phenomenological way, the role of the microscopic structure of a nucleus in this quantity is tested. It is found that this structure may significantly change the half-life of nuclei with the odd nucleon (or nucleons).
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The predictive power for masses of a recently published highly extrapolatable semiempirical shell model mass equation is compared to that of several current mass models and some differences are noted and addressed. The equation is shown to describe rather well the energies of several seemingly well-identified α-decay chains with known end product nuclei observed in superheavy elements research. The equation is also applied to the interpretation problem of some recent hot fusion-evaporation experiments with unknown end products and several conceivable reaction channels. Some plausible interpretations are indicated.
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DOI:https://doi.org/10.1103/PhysRevC.66.049902
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A nuclidic mass formula composed of a gross term, an even-odd term and a shell term is presented as a revised version of the mass formula constructed by the present authors and published in 2000. The gross term has almost the same functional form as in the previous formula, but the parameter values in it are somewhat different. The even-odd term is treated more carefully, and a considerable improvement is realized. The shell term is exactly the same as the previous one; it was obtained using spherical single-particle potentials and by treating the deformed nucleus as a superposition of spherical nuclei. The new mass formula is applicable to nuclei with Z ≥ 2 and N ≥ 2. The root-mean-square deviation from experimental masses is 666.7 keV, which is less than that of the previous mass formula, 689.8 keV.
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Calculated partial half-lives and Q values of the most probable decays by spontaneous cluster emission are listed for nuclides with Z = 52-122. Superheavy nuclei far off the Î²-stability line are included; input mass values are from the mass tables published in 1988 in Atomic Data and Nuclear Data Tables. Parent nuclei listed here are selected to meet the following criteria: partial lifetime for cluster emission 10â»Â¹â¸. Î±-decay Q values and partial half-lives are given for all nuclei listed. A larger set of parent nuclei are presented in a T{sub E}X file, where half-lives 10â»Â³Â° are accepted provided the mass of the parent nucleus has been measured or estimated from systematics by Wapstra et al. The lifetimes for cluster emissions are calculated using the analytical superasymmetric fission model with even-odd effects taken into account; the lifetimes for Î± decay are experimental values or are deduced from a semiempirical formula.
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Spontaneous emission of some C, O, F, Ne, Mg, and Si isotopes, from heavy parent nuclei, have been experimentally observed since 1984, confirming earlier predictions. Experimental difficulties are mainly related to the low yield in the presence of a strong background of α particles. Until now, only some of the most favorable cases were investigated, leading to magic or almost magic proton and neutron numbers of daughter nuclei. We present a systematics of experimental results compared to calculations, clearly showing other possible candidates for future experiments. Universal curves may be used to estimate the expected half-lives.
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Background. In the “cold” fusion reactions based on the use of lead and bismuth targets, the proton-rich isotopes of superheavy (SH) elements up to Z=113 have been produced. More neutron-rich isotopes of SH elements (up to Z=118) have been synthesized in “hotter” fusion reactions of 48Ca with actinide targets. α-decay half-lives of different isotopes of the same SH elements (for example, 112) were found to vary by several orders of magnitude. This indicates strong shell effects in this area of the nuclear map. The understanding of these effects and other properties of SH nuclei is strongly impeded by the absence of experimental data on decay properties of the not-yet-synthesized isotopes of SH elements located between those produced in the “cold” fusion reactions and those produced in the “hot” fusion reactions and also by the yet missing neutron-enriched isotopes of these elements.Purpose. In this paper we search for the optimal fusion reactions which may be used to fill this gap of the nuclear map and significantly extend the area of known SH nuclei.Method. For the calculation of the cross sections we use the same approach which was employed earlier for successful predictions of all 48Ca induced fusion reactions.Results. Several fusion reactions of the stable projectiles 40Ar, 44Ca, and 48Ca with different isotopes of actinides (lighter and heavier than those that have been already utilized in the Dubna experiments) could be used for synthesis of new SH nuclei. Predicted cross sections for the production of new isotopes of SH nuclei were found to be quite large, and the corresponding experiments can be easily performed at existing facilities. For the first time a “narrow pathway” to the middle of the island of stability was found owing to possible β+ decay of SH nuclei 291115 and 291114 which could be formed in ordinary fusion reactions.
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The half-lives against α decay of transuranium nuclei including superheavies are calculated by three methods: a semiempirical formula taking into account the magic numbers of nucleons, the analytical superasymmetric fission model, and the universal curves. The calculations based on Q values determined by using the recently published compilations of atomic masses are compared to the experimental results.