【作者】 王守宇；

【导师】 刘运祚； 马英君；

【作者基本信息】 吉林大学 ， 粒子物理与原子核物理， 2005， 博士

【摘要】 利用融合蒸发反应¹¹⁶Cd(¹⁴N,4n)布居了¹²⁶Cs 的高自旋态,选取的束流能量为65MeV,123I 是其中一个较强的副反应道布居的。在¹²⁶Cs和¹²³I 中都取得了很多新的实验结果,分别叙述如下: 奇奇核¹²⁶Cs 高自旋态的实验研究结果:建立起了一个迄今为止内容最为丰富的¹²⁶Cs 能级纲图。能级纲图中加入了100 多条新的γ跃迁,原来已知的转动带都被推到了更高的自旋态,发现了新的转动带,并且还建立了转动带与原来已知的低激发态的连接和大部分转动带间的连接。通过对低激发态能级结构的分析,指定了各转动带最低观察态（带头）的自旋和宇称,其中基于πh₍11/2（?）νh11/2 组态晕带的带头自旋-宇称被指定为9⁺,支持由激发能系统学给出的结果。基于实验得到的各个转动带新的自旋值和带内跃迁的相对强度,对每个转动带进行了组态指定,并讨论了转动带的旋称劈裂、顺排角度量和约化跃迁几率。结合已有的实验数据对130 质量区旋称劈裂与反转的情况进行了系统性的分析和讨论。在原子核中寻找由于手征对称性破缺而形成的手征双重带是当前核结构研究的热点之一。本次工作不仅确认了¹²⁶Cs 之前的实验结果,而且观测到了多条晕带与伴带之间的连接跃迁,并将伴带推到了更高的自旋态。从本次工作中提取的双带激发能差和约化跃迁几率等实验信息都表明,¹²⁶Cs 中存在的双带结构很可能就是要寻找的手征双重带,从更多还原

【Abstract】 The neutron-deficient nuclei in the A~130 mass region have been of considerable interests due to the facts that they lie in a transitional region between the primarily spherical Sn(Z=50) nuclei and the well deformed La(Z=57) and Ce(Z=58) nuclei they are known to be soft with respect to γdeformation. In this mass region, the study of odd-odd nuclei has provided a fruitful ground for the discovery and discussion of a number of interesting nuclear structure phenomena such as signature inversion , prolate-oblate shape competition, highly deformed bands, and most recently the chiral twin bands. The Z=55 doubly odd Cs isotopes are expected to systematically display these characteristics. In the odd-odd cesium nuclei with mass number ranging from 120 to 132, the proton Fermi surface lies near the low-? h11/2 orbitals, which favor prolate nuclear shapes; the neutron Fermi surface lies in the mid-to high-? h11/2 orbitals, which favor oblate nuclear shapes. In view of such shape-driving effect, the doubly odd Cs nuclei have attracted much attention since they provide an opportunity to understand the possible interaction and competitive shape driving effects between proton and neutron quasiparticles. This thesis mainly focus on the in-beam γ-ray spectroscopic study of high-spin level structures of 126Cs, the chiral twin bands and the level structure of 123I. 1. Level structure of 126Cs The excited states of 126Cs were populated via the 116Cd (14N, 4n) reaction at a bombarding energy of 65 MeV. The 14N beam was provided by the tandemlinac accelerator complex at the Niels Bohr Instisute. The Cd target was a foil of thickness of 820 μg/cm2 with a 1mg/cm2 Au backing. The γ-ray spectra were taken with the Nordball detector system consisting of up to 20 Compton-suppressed HPGe detectors and a 55 element BaF2 inner ball. One of the HPGe detectors was replaced by LEPS (low energy photon spectrometer) detector to ensure sensitivity to important low-energy transitions at the bottom parts of γ-ray cascades. A total of 8×108 γ-γcoincidence events were accumulated in event-by-event mode. The level scheme of the odd-odd nucleus 126Cs has been studied in detail. All previously reported rotational bands have been extended to higher spins and related to the ground state through different decay paths. The spinof the lowest observed state for the yrast band has been determined to be 9η, which is consistent with the spin assignment proposed by a systematics study. The spin-parity of bandhead for bands 2-7 are assigned to (11)+, (5)?, (4)?, (7)?, (7)? and (8)?, respectively. On this basis, the spin-parity of the other states in bands 1-7 have been deduced from the measured ADO data. Configuration assignments were made for each of the rotational bands based on observed aligned angular momentum, band crossing frequencies, energy signature splitting and electromagnetic properties. Two nearly degenerate bands built on the πh11/2?νh11/2 configuration have been confirmed in 126Cs and they was interpreted as resulting from chiral symmetry breaking in the body-fixed intrinsic frame. We suggest that yrare bands 3, 4, 5, 6 and 7 are built predominantly on the πg7/2?νh11/2, πd5/2?νh11/2 , πh11/2?νg7/2, πh11/2?νd3/2 and πh11/2?νs1/2 configurations, respectively. Finally, a strongly coupled band, based on a high-K πg9/2?νh11/2 configuration was also established and is believed to be built on a high-K isomer. The signature inversion of the νh11/2 bands in odd-A is briefly discussed. We summarized the experimental trends of signature inversion of the odd-odd nuclei in the 130 mass region. First, the low-spin signature of the πh11/2?νh11/2 band is inverted. Second, in the Cs and La odd-odd isotopic chain, the spin at inversion point increase smoothly with neutron memberand the magnitude of the signature splitting also increase with the neutron number. The signature inversion of yrare bands in Cs isotopes have also been discussed in this paper. 2. Candidate chiral twin bands in the Odd-Odd Nucleus 126Cs In recent years there has been a surge of interest in searching for chiral twin bands in the A~130 region. Chiral twin bands have been identified in several N=73, 75 isotones of this region, revealing a small island of chiral rotation centered around 134Pr. All these bands are built on the πh11/2?νh11/2 configuration. Despite the recently experimental effort concentrated on the island of chirality in the A~130 region it is still not very clear about whether the chiral rotation persists for N=71 odd-odd nucleus 126Cs.Very recently, Li et al. have suggested that the positive-parity doublet bands in 126Cs is the candidate for chiral twin bands in 126Cs. Clearly more information is needed in order to verify the configuration of the side band (chiral partner band) of the yrare band. In the present work, we report on the significant extension of the possible chiral doublet bands in 126Cs. The additional information obtained in the present study allows for the detailed study of the rotational alignment and electromagnetic properties of the two bands, thus a configuration assignment can be made based upon characteristic bandproperties. The level scheme of the chiral doublet bands in 126Cs has been extended to higher spins, several new linking transitions between the two partner members of the chiral doublet bands were observed, and intensities of γ-transitions related to the chiral doublet bands were presented by analyzing the γ-γcoincidence data. B(M1)/B(E2) of the yrast band and B(M1)in/B(M1)out of the side band have been extracted. Both of them exhibit staggering as a function of spin and they stagger in phase, revealing a correlation between the two observables. The displacement between the E(I) plots of the side band and the yrast band is ~200 keV. Both features, namely the behavior of the reduced transition probability ratios and the displacement of ~200 keV observed in 126Cs, are very similar to those observed in 124,128,130Cs as reported by T.Koike et al. Particle-rotor model (PRM) calculations of the twin bands from the chiral solution were also performed and compared with experimental energy spectra. We found that the relative energy displacement, the alignments (before band-crossing) and the electromagnetic properties of the proposed chiral twin bands in 124-130Cs are quite different from those of the proposed chiral twin bands in 134Pr. These facts raise the questions that whether or not the proposed chiral twin bands in 124-130Cs and those in 134Pr can be understandwithin a single frame of the theory? If they have the same origin? Which of them is more close to the ideal chiral twin bands? We tend to believed that having the similar alignments (before bandcrossing) for the two partner bands is a necessary condition for them to be considered as the chiral twin bands. 3.High spin study of odd-A nucleus 123I Excited states of 123I were populated via the 116Cd(14N, 5n2p) reaction at 65 MeV. The resultant γrays were detected using standard γ-ray spectroscopic techniques with NORDBALL detector array. The results are summarized as follows: Two previously identified ?I=2 positive-parity bands ending on the first 5/2+ and 7/2+ states in 123I were extended up to spins above their first bandcrossings, along with interband ?I=1 transitions between levels of them were observed up to spin as high as 27/2+. Levels in these bands, excluding the first 5/2+ state, are interpreted as forming a ?I=1 band ending at the first 7/2+ state. This ?I=1 band is proposed to be built predominantly on the g7/2[404]7/2+ oblate configuration, based on the energy-level spectra, B(M1)/B(E2) ratios and the theoretical predictions from the particle-rotor model. The previously identified ?I=1 rotational band built on the prolate g9/2[404]9/2+ orbital has also been extended to higher spins. Another previously identified but weakly populated ?I=1 band is confirmed and is更多还原