近滴线核^{211}Th的高自旋态研究

High spin states of near the drip-line nucleus ^{211}Th are still unknown so far. It is hard to measure the excited states of extremely heavy neutron deficient nuclei by the traditional in-beam gamma-ray spectroscopic techniques. The difficulty is the consequence of rapidly decreasing production cross sections with proton number Z combined with a large fission probability. In this work, The recoil-decay tagging (RDT) technique coupled to the gas-filled recoil separator will be applied in order to detect the excited states of ^{211}Th at the Accelerator Laboratory of the Institute of Modern Physics, Lanzhou. Moreover, we will search for the new isomer state of ^{211}Th and measure its lifetime in the experiment. The highly selective RDT technique allows very weak reaction channels to be resolved from the large background due to fission and other competing processes. It has been successfully applied to study of the high spin states of heavy, exotic neutron deficient nuclei at present. In order to verify the prediction of the shape coexistence and shape evolution in Th isotopes, we concentrate on extracting the direct experimental evidence of the nuclear deformation and collective effects based on the level structure of ^{211}Th. Then, we will attempt to interpret the microscopic mechanism of the isomer states in this mass region. Also the information of the interplay between the particles and the corresponding core, and between the particles will be obtained by the theoretical calculation.

迄今为止，近滴线核^{211}Th高自旋态的能级信息尚属空白。传统的在束gama谱学技术受限于反应截面低、裂变截面大等因素，无法用于探索重核区近滴线核的高自旋态。本项目基于兰州重离子加速器，在充气反冲谱仪上采用国际上新兴的反冲衰变标记（RDT）实验技术测量近滴线核 ^{211}Th的高激发态谱学信息，搜寻同质异能态并提取相应的能级寿命。采用RDT实验技术可实现对高本底、低截面等极端不稳定核素的高自旋态研究，弥补了传统测量技术的缺陷。同时，根据^{211}Th的能级结构特征，着重寻找原子核形变和集体运动的实验证据，以验证理论预言的Th同位素链的形状演化和共存，研究其同质异能态的微观起源，提取该核区价核子之间，价核子与核芯之间相互作用的信息。

本项目基于中国科学院近代物理研究所充气反冲核谱仪（SHANS）实验装置，主要研究了锕系区极缺中子核素的奇异衰变性质。在本项目的资助下，我们研制了新的数字化电子学系统，实现了无死时间获取和脉冲信号全波形记录的数据获取方式，发展了数字化脉冲处理的信号分析技术。通过重离子熔合蒸发反应，课题组在SHANS实验终端，新合成了缺中子核素223,224Np，研究了Z=92附近质子壳层的结构性质，实验结果否定了Z=92质子亚满壳的存在。并通过数字化脉冲处理技术对 220Pa 的alpha 衰变性质重新做了测量，得到了更精确的衰变能量和半衰期，指认了220Pa基态的自旋宇称为1-。运用 gamma-alpha 符合测量技术，研究了缺中子核Th同位素的衰变性质，为进一步探索Z=90同位素中原子核壳结构的演化性质提供了帮助。此外，基于E-GOS曲线方法和推转壳等理论模型，研究了原子核的形状演化性质，并提出了新的原子核基态电四极矩的计算公式。