140质量区缺中子核同核异能态寿命测量

The proton and neutron number of the neutron-deficient nuclei in the 140 mass-region is near 64 and 82 respectively, which is at the deformed sub-shell. Since the valence nucleon may locate in the intruder orbital-1h11/2, many isomers had been observed in this region. 138Eu、138Pm and isotones with N=80 are our interested nuclide, which have rotational bands with unknown excitation energy. We will try to look for the isomers with half-life in microsecond magnitude in them by fusion-evaporation reaction 95Mo(46Ti,xpxn) and 120Sn(27Al,xpxn). The experiment will be carried out on the spectrometer for heavy atoms and nuclear structure (SHANS) in Lanzhou. The reaction product recoiled from the thin target will be deflected by SHANS, and only the ones having the same magnetic rigidity with the interested ones will be transported to the focal silicon detector. The de-excited γ rays from the isomers could be detected by the γ detectors surround the focal silicon detector in low background. All of the events’ energy and time will be recorded. In offline analysis, we try to find the isomers by choosing out the γ which is in coincident with the incident events in a certain time period. The level scheme between the ground state and the isomer will be published. The half-life of the isomer could be obtained by fitting the time spectra of the γ rays de-excited the isomer. The configuration will be studied by systematic analysis of the half-life, energy and transition strength of the interested levels in the isotones nearby.

140质量区缺中子核的Z和N分别在形变子壳64和82附近，价核子有可能处于闯入轨道1h11/2，核区内存在大量同核异能态。目标核138Eu、138Pm和N=80同中子素中的带结构还未能和低激发能级建立位置关系，需利用熔合蒸发反应95Mo(46Ti,xpxn)和120Sn(27Al,xpxn)生成目标核，寻找激发能级中微秒量级同核异能态。实验在中科院兰州充气反冲谱仪终端开展，反应产物从薄靶中反冲出来，经谱仪偏转分离出具有目标核磁刚度的粒子，注入到焦平面方硅探测器中，实现在低干扰下对同核异能态退激γ的测量。目标核退激的γ射线被周围的γ探测器俘获，在线记录所有能量和时间信息。离线处理时，挑选出和注入事件间隔在一定时间范围的退激γ事件，寻找同核异能态。构建同核异能态和基态间能级纲图，分析由长寿命能级退激的γ时间谱，测量能级半衰期。根据同中子素中能级寿命、能级高度和跃迁强度的系统性研究能级组态。

在兰州充气谱仪（SHANS）终端开展重离子熔合蒸发反应，利用注入核ER-母核α-子核α-孙子核α和α-γ关联测量，鉴别xpxn反应产物，合成新核素，研究衰变链中核素的α衰变精细结构和低激发态能级结构。根据核质量评估数据分析，222Pa基态到218Ac基态的α跃迁Qgs值推测为8890(50)keV，之前国外研究组观测到α衰变精细结构为8.16+8.18+8.21MeV(50%)，8.33MeV(20%)和 8.54MeV(30%)，三个分支均从基态到子核激发态，未观测到基态到基态的α衰变分支。本项目开展的实验成功观测到222Pa基态到子核激发态的衰变分支，衰变能与评估推测的数据一致，从而在实验上首次确定了222Pa核质量。另外，利用时间谱的双成分，修正了40Ar+186W反应1p3n和3p5n反应道的截面比，与理论值在误差范围内不相符，建议从熔合位垒和α形成因子的角度改进理论模型。在本基金资助下，合成了铀的目前最缺中子核214U，和百纳秒寿命超铀新核素222Np，测量了238U(9Be,5n)242Cm绝对激发函数曲线，理论上研究了利用转移反应生成超重核的反应机制和可能性。