缺中子核素228,230Pu的α衰变精细结构研究

αdecay is one of the most important decay modes for heavy and super-heavy nuclei, andα-γcoincidence measurements and α-spectroscopy are the most important tools to obtain information on the nuclear structure of these nuclei. The half-lives of α emitters provide information on the stability of nuclides, while the decay energies and branching ratios can provide important information on excitation energy, nuclear mass, deformation and evolution of shell closure. In this project we plan to study the α-decay fine structure of neutron-deficient nuclei 230Pu and 228Pu by using fusion reactions 26Mg+208Pb and 40Ar+192Os at the gas-filled recoil separator SHANS. Through this research we can get the answers to two interesting physical questions: The first one is why the branching ratios for 0+->2+α transitions in even-even Th, U and Pu nuclei achieve the maximum at the N=138, and the second one is why the branching ratio for 0+->2+αtransitions in 230Pu decreases rapidly, which results in a significant deviation from the systematic regularities.

α衰变是重核和超重核的主要衰变方式之一,α-γ符合测量方法和α衰变谱学方法是研究重核和超重核结构的重要实验手段。通过测定α衰变半衰期、衰变能及分支比等可以获得有关原子核稳定性、质量、能级、形变以及壳演化等重要物理信息。本项目计划在兰州充气谱仪装置上,通过熔合蒸发反应26Mg+208Pb和40Ar+192Os,开展缺中子偶偶核230Pu(N=136)和228Pu(N=134)的α衰变精细结构研究。通过本项目的成功实施，我们可以解答两个重要的物理问题:一是为什么偶质量数的Th、U和Pu同位素从基态0+到子核2+激发态α衰变的分支比在中子数N=138时取得极大值;二是为什么在中子数N=136时230Pu的0+->2+α衰变分支比出现迅速减少的反常现象。

为了进行228Pu和230Pu的alpha衰变精细结构研究，项目组在兰州充气谱仪上先后开展了两次40Ar+192Os实验, 并完成数据分析工作。为了能够准确提取228Pu和230Pu alpha衰变链中短寿命核素的衰变性质，发展了一套新的数字波形拟合方法, 利用该方法对于寿命在0.2-5μs的叠加信号，能量分辨可以达到65 keV, 这一分辨远好于有关文献报道的180 keV。为探索开展228Pu和230Pu的衰变精细结构研究的实验条件，开展了测试实验40Ar+182W并通过该实验发现了一个新的218Pa的同核异能态, alpha衰变能为9691(15)和9595(21) keV, 半衰期为 135(+62-32)μs。另外，项目成员还参与到了219Ualpha衰变精细结构和新核素220Np的研究工作中，为这些研究提供的气体探测器、异丁烷和酒精冷却系统和数据分析方法等。