A~100核区E(5)临界点对称性和形状演化的研究

E(5) symmetry, introduced recently by Iachello from the Interacting Boson Model (IBM), describes atomic nuclei at the critical points of phase transitions from spherical to gamma-unstable shapes. To find nuclides close to E(5) symmetry has spurred intense interest in the nuclear-structure community. In this regard, 102Pd, 104Ru and 108Pd are three promising E(5) candidates according to theoretical expectations. While the excitation energies of their levels can be described satisfactorily by E(5) calculations, the hitherto known electromagnetic properties, albeit sensitive to possible symmetries under investigation, are, however, insufficient for stringent tests of the E(5) predictions. The present research project will make an endeavor to measure precisely the lifetimes of low-lying excited levels in above-mentioned three nuclei. In this way, transitional quadrupole moments (Qt), branching ratios (lamda) and absolute reduced strengths [B(E2)] of E2 transitions can be determined and then comprehensive comparisons with E(5) predictions will become possible. In addition, attempts will also be made to measure lifetimes of higher-lying levels, which allows for a study on the evolution of dynamical symmetries with variation of spin. As for the population of excited states in the nuclei to be investigated, Coulomb excitation (induced with 32S projectiles)as well as fusion-evaporation reaction will be employed as the major means, and for the lifetime measurements, Recoiling Distance Method(RDM), Doppler Shift Attenuation Method (DSAM) and Delayed-Coincidence technique will be employed.

关于原子核形状相变临界点对称性的研究是核结构领域的重要前沿课题。以相互作用玻色子模型为基石发展起来的理论计算，预期了E(5)对称性在A~100核区的存在，主要涉及104Ru、102Pd和108Pd 等偶偶核。然而单凭这些核已知的能谱信息以及少量电磁跃迁信息尚远不足以对它们是否为E(5)核做出严密可靠的判断。本项目拟通过在束谱学实验方法测量上述3个原子核的能级寿命，从而获得绝对电磁跃迁强度、跃迁分支比、跃迁四极矩等多方面的电磁信息，并结合基于相互作用玻色子模型的理论计算，阐述这些核的对称性。此外，本项目还将研究上述可能存在的E(5)对称性随角动量升高时的演化特征。

项目由中国原子能科学研究院提出，由吉林大学（第一单位，约定支配1/3的经费额度）和中国原子能院（第二单位，约定支配2/3的经费额度）联合承担。项目依托中国原子能院的加速器在原子核高自旋态领域开展研究，重点关注原子核的形状演化及形状相变，特别是E(5)临界点对称性的寻找和检验。项目着重开展了关于106Cd、138Nd、125Cs的高自旋态研究。基于反冲距离多普勒线移法对106Cd和138Nd的能级寿命进行了测量，获得了一些重要能级的寿命，进一步获取了约化跃迁几率B(E2)以及跃迁几率比 B4/2 等。上述实验结果反映了在低自旋区 106Cd符合E(5)临界点对称性的标准，138Nd 展示强的单粒子性。与此同时，对原子能院串列加速器伽玛终端上的多参数时间测量系统进行了优化建设，并提出了在有重叠峰干扰情况下原子核能级寿命的提取方法。另外，基于常规的在束核谱学实验及分析方法，拓展了125Cs核的能级结构，在高自旋区观测到了长椭与扁椭形状共存的实验证据。