Cs奇奇核同位素的高自旋态研究

This project aims to study the high spin states of the odd-odd nuclei 120Cs and 122Cs through the fusion evaporation reaction and inbeam gamma-ray spectroscopy technique. The experiment will be performed at CIAE. The 12C beam provided by the HI-13 tandem accelerator bombards the two-layer target of (112Sn+114Sn) to populate the high spin states of 120Cs and 122Cs at the same time.The two-layer target is rolled onto a lead back. The outer layer is 112Sn and the inner layer is 114Sn. The 12C beam with the energy of about 78 MeV bombards the 112Sn layer first and the 112Sn(12C, 1p3n)120Cs reaction happens with the max cross section of 116.4 mb at 75 MeV energy. The slowed 12C beam bombards the 114Sn layer and the 114Sn(12C, 1p3n)122Cs reaction happens with the cross section of 281.2 mb at 70 energy. The experiment will provide reliable experimental data of 120Cs and 122Cs to discuss the phenomena below: 1. the complex energy level spectra of the odd odd nuclei make the spin assignments very difficullt and the excitation energy systematics is a very helpful method. In 2000, C. B. Moon et al. assigned the spins to the πh11/2υh11/2 yrast band in 120Cs and 122Cs based on two pressumed isomers, which is 2 ? higher comparing the result of excitation energy systematics. Reinvesigation of the 120Cs and 122Cs will help to clarify this discrepency; 2. for the odd-odd nuclei with the mass around 130, the particle-type quasiproton lies in the bottom of the high-j?h11/2 subshell and the hole-type quasineutron sits near the top of the h11/2 subshell. The chiral geometry is believed to be sustained, and indeed, nearly degenerate doublet bands with the 2qp configuration of πh11/2υh11/2 have been systematically observed and proposed as the candidate chiral bands in the isotopes of Z=55~63. In particular, the cesium isotopes of N=69~75 provide valuable systematical experimental information for the study of this phenomenon's dependence on the valence neutron number. In 122Cs, the doublet bands have also been observed although only one signature sequence of four levels were established. It is interest to find the neutron fermi level in 124Cs and 126Cs is already close the middle of the νh11/2 subshell. Therefore it is very important to search for the occurrence of the chirality in 120Cs and 122Cs and examine the mechnics for the chirality in those nuclei. 3. In this mass region, due to the interaction between the Δj=Δl=3 orbitals with opposite parity, i.e. the d5/2 and the unique parity h11/2 intruder orbitals, octupole correlation is observed. More experiments for 120Cs and 122Cs will provide valuable systematical experimental information for the study of this phenomenon's dependence on the valence neutron number. Above all, this project will provide new reliable experimental results with confirmed spin assignment and by comparing with the theoretical model calculation will be great helpful to investigate the phenomena of the chiral bands and octupole correlation.

本项目拟利用融合蒸发反应和在束gamma谱测量技术研究奇奇核120Cs和122Cs的高自旋态结构。实验将在中国原子能科学研究院完成,采用12C束流轰击112Sn+114Sn双层实验靶，同时布居120Cs和122Cs的高自旋态。束流能量约为78 MeV，通过在束gamma-gamma符合测量。主要寻找并建立两目标核中基于πh11/2υh11/2组态的正宇称双重带、该双重带的带间跃迁、晕带与πh11/2υd5/2负宇称转动带之间的连接跃迁，以及转动带与低激发态之间的连接和高k转动带。根据实验结果重新确定转动带的绝对自旋，检验能级能量系统学的正确性，解决目前130区转动带自旋指定中部分实验结果与能级能量系统学之间的矛盾。本项目尝试进一步寻找130过渡区奇奇核中新的手征候选核。通过新的实验结果与理论模型计算之间的比较，能够系统性的研究手征双重带以及八极关联现象随中子费米面的变化情况。

本项目旨在进一步拓展130区手征候选核研究，为系统性研究基于πh11/2υh11/2内禀组态的正宇称双重带的性质提供更多的实验信息，方便理论模型进一步探索手征双重带的物理机制。同时，对该区八极关联和高k 转动带等现象进行实验研究。本项目目标核为Cs奇奇核同位素，实验利用中国原子能院的HI-13串列加速器及其探测终端通过融合蒸发反应对目标核高自旋态进行布居并对其衰变释放的射线进行在束符合测量。在本项目支持下，通过合作的方式完成了122Cs，130Pr和138Pm的实验。122Cs：利用19F轰击107Ag靶布居122Cs的高自旋态。通过该实验观测到基于πh11/2υh11/2组态的晕带和πh11/2υ(d5/2/g7/2)的负宇称转动带以及二者之间的E1连接跃迁。提取的这些射线B(E1)实验值达到了10E-4 W.u.量级，说明在该核中观测到了八极关联现象。122Cs中的约化跃迁几率比124Cs有所增加，表明Cs同位素系列中八极相互作用随中子费米面增加而减小。130Pr：布居方式为96Ru(35Cl, 2n2p)130Pr。建立了包括基态在内的正宇称双重带。根据顺排角动量和B(M1)/B(E2)值确定了πh11/2υh11/2的内禀组态。正宇称双重带能量差达到750-800 keV，双重带B(M1)/B(E2)随自旋的奇偶振动是相反的相位，否定了130Pr为手征候选核的可能。通过推转壳模型(CSM)计算确定该双重带是由四个旋称分支组成。该实验的结果表明对于130区奇奇核的正宇称双重带，随着质子费米面的增加和中子费米面的降低，由角动量构成的手征三轴形变图象不再成立，而是形成非三轴图象即旋称劈裂。138Pm：布居方式为124Te(19F, 5n)138Pm。实验在低能区未能观测到该核基于πh11/2υh11/2组态的正宇称双重带，因而138Pm很可能不是一个手征候选核。融合蒸发反应释放的中子在晶体中的散射在符合谱中600 keV能量附近产生一段连续的本底谱，对这一能区的符合分析造成极大困难。本项目组尝试利用MCNP-4C模拟对能谱进行了模拟修正。同时，尝试了在中子输运模拟这一方向进行了研究。通过修正元素测量中样品对中子场的影响，较大提高了元素含量测量精度。