基于锕系核合成超铀元素研究的回顾与展望

Review and Prospect of Research on Synthesis of Transuranic Elements by Heavy Actinide Nuclei

  • 摘要: 合成新核素、拓展核素版图是核物理的前沿研究领域。随着重离子加速器的发展,目前已合成到118号元素,但探索更重的区域仍面临严峻挑战。熔合反应是合成超重新元素的传统方法,多核子转移反应是通往“超重稳定岛”的唯一途径,也是将来大规模合成超重新元素最可能的方法。在这两种反应中,锕系核作为靶核发挥着重要作用。本文介绍了合成锕系核的中子俘获反应;总结了国际上用于合成超重元素锕系靶的产生、性质和研究现状;回顾了基于锕系核合成超镄元素的研究以及119、120号新元素的最新进展;对目前国际上反应堆产生重锕系核的现状做了比较,并针对近年来合成原子序数Z>118超重元素面临的困难进行了评述,为将来的新元素合成提出建议。

     

    Abstract: Synthesizing new nuclides and expanding the chart of nuclides is one of the frontier research areas in nuclear physics. With the development of heavy-ion accelerators and radioactive ion beam facilities, significant achievements have been made in the synthesis of elements. Currently, Z=118 element has been successfully synthesized. However, exploring the heavier region faces significant challenges. Fusion reaction is the traditional method for the synthesis of superheavy elements. Multinucleon transfer reaction is the only way to the “island of stability”, and it is also the most likely method for large-scale synthesis of superheavy elements in the future. Actinide targets play a crucial role in these reactions. The neutron capture reactions for synthesizing actinide nuclei were reviewed, including both slow neutron capture process (s-process) and rapid neutron capture process (r-process). The elements from atomic number 93 to 100 synthesized through neutron capture reactions were listed. The heaviest element that can be synthesized through neutron capture reactions is Fm. The actinide targets used for synthesizing superheavy elements, such as 237Np, 244Pu, 243Am, 248Cm, 249Bk, and 249Cf. The synthesis status, properties, and applications of these targets were respectively summarized. The researches on the synthesis of Z>100 elements, as well as the latest progress on the discovery of the new elements 119 and 120, were reviewed. Currently, there are many reactors, which can provide high atomic number actinide nuclei, such as High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL), Advanced Test Reactor (ATR) in USA, and SM-3 reactor at the Research Institute of Advanced Reactors (RIAR) in Russia. The Jules Horowitz Reactor (JHR) in France is under construction, and this reactor is expected to be one of the most outstanding research reactors in Europe in terms of irradiation and technological capabilities. High Flux Engineering Test Reactor (HFETR) is the highest power operating high-flux engineering test reactor in China. The Tsinghua High Flux Reactor (THFR) in China is under construction, and this reactor will also provide heavy actinide nuclei in the future. The difficulties in synthesizing of Z=119 and 120 superheavy elements were outlined, and the suggestions of synthesizing these elements were given. Due to the extremely low cross sections for superheavy elements with Z>118, it is much difficult to synthesize new elements by using the existing actinide targets with 48Ca beam. Currently, the heaviest target which can be generated by powerful high-flux isotope reactor is 252Cf. Therefore, the beams heavier than 48Ca were required to produce new superheavy elements through fusion reactions. In addition, the large-scale scientific facilities can increase the yields of heavy actinide isotopes, such as 238Pu, 242Am, 244Cm, 246Cm, 249Bk, 252Cf, 253Es, and 257Fm. Additionally, the superconducting linear accelerator of the High Intensity Heavy-ion Accelerator Facility (HIAF), is capable of providing extremely intense heavy ion beams.

     

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