轻稀土核132Nd、134Ce的B(E2，4+—>2+)/B(E2，2+—>0+)异常的研究

Absolute transition probabilities give particularly valuable insights into the nature of nuclear collectivity and its evolution with valence nucleons. In standard collective models, a universal feature is that the E2 transition strength of the 4+ state is greater than the 2+ state, accordingly B4/2≡B(E2，4+—>2+)/B(E2，2+—>0+)>1. Deviations from this expected behavior have come to serve as subtle hints that our understanding of the collective behavior of nuclei is somehow incomplete, and must be explored more closely. The values of B4/2<1 in nonmagic nuclei 132Nd and 134Ce are anomalously small, as it cannot be explained with collective approaches. With the aim of elucidating the structure of 132Nd and 134Ce and testing the validity of collective structure for these nuclei, the lifetimes of 132Nd and 134Ce are planning to be measured by the delayed coincidence technique and recoil distance Doppler-shift technique. Transition probabilities are extracted to get the new B4/2 values to determine the reason of these anomalies. Furthermore, the experimental systematics of transition probability has a deep impact on the understanding of nuclear structure and its evolution as a function of the valence nucleons. In addtion, more data of higher spin in the 132Nd and 134Ce are needed to study the critical-point symmetry and shape evolution.

电磁跃迁几率对于了解原子核的集体性特征及其随着价核子的演化具有重要意义。在非幻数核中，通常4+态的跃迁几率要大于2+态的跃迁几率，即B4/2≡B(E2，4+—>2+)/B(E2，2+—>0+)>1。然而，非幻核132Nd和134Ce显示异常低的B4/2值（<1），似乎提示我们对于原子核集体性的理解或集体模型的发展还不够完善，需要重新认识。因此，本基金项目利用重离子熔合蒸发反应布居132Nd 和134Ce 的激发态，通过延迟符合技术和反冲距离多普勒位移法精确测量其能级寿命，提取电磁跃迁几率，重新测定132Nd和134Ce核的实验B4/2比值，并进而探明这个比值异常低的原因。通过其与相应的同位素随中子数的变化关系相比较，深入了解核结构机制及其随价核子的演化规律。此外，本项目还将提取132Nd和134Ce更高自旋态的跃迁几率，探讨原子核的临界点对称性及原子核的形状随着角动量演化的过程。

文献报道的非幻偶-偶核134Ce的B4/2 ≡ B(E2; 4+→ 2+ )/B(E2; 2+→ 0+ )比值异常低，完全偏离集体模型的预测，从而对核结构理论提出新的挑战，似乎提示我们对于集体性的理解或集体模型的发展还不够完善，从而对核结构理论提出新的挑战。我们提取原子核的跃迁机率，不仅可以从根本上揭示核的集体性或单粒子性特征，并可以系统学研究某同位核素随着核子的变化规律。利用重离子熔合蒸发反应122Sn(16O, 4n)134Ce布居134Ce的激发态。采用延迟符合技术和反冲距离多普勒位移法（RDDS）进行寿命测量。最终测到的B4/2结果处于处于振动核的2.0和转动核的1.43之间。确认了这个核的集体性，通过与IBM理论模型比较，深入了解核结构机制及其随中子的演化规律。另外还测量了负宇称晕带5-、6-、7-、8-、9-和11六个能级的寿命，并提取跃迁四级矩Qt值，通过与TRS模型计算比较，确认了该负宇称带的组态。此外，还利用142Nd (28Si, 3n)167W反应对其低激发态的能级寿命，并对其形状演化进行了研究。得到的实验结果与CNSB理论模型比较，提出167W原子核的旋称反转主要是原子核的三轴形状导致。