Collective Structures Of Odd-Z ¹³¹Cs Nucleus

The neutron-deficient nuclei in the A~130 mass region 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. For these nuclei, one characteristic is that the proton Fermi surface lies low within the orbitals driving the nucleus to prolate shape, whereas the neutron Fermi surface lies near the middle of the orbitals favoring an oblate shape; The other is that they are soft toγdeformation, triaxial deformation is likely to occur. Due to above mentioned characteristics, the nuclei in the A~130 mass region exhibit various intriguing phenomena, such as band termination, signature inversion, prolate and oblate shape coexistence, as well as breaking of chiral symmetry. As the , , , proton orbitals are near the Fermi surface of systematic study of rotational bands built on these configurations could assist us understand the way single particle motion coupled with collective motion of the even-even core. Previous study on 131Cs did not provide information regarding the rotational bands built on orbitals and theγ-vibration band built on the favored signature partner of band (a feature observed in the [ ]πg9/24049/2+h1 1/2123,125,127Cs isotopes)。Exploring the existence of these bands will provide us with an opportunity to comprehensively understand the nuclear structure properties of Cs isotopes, especially the evolution of collectivity and nuclear deformation with increasing mass. So, 131Cs was chosen as our research focus.The first part of the thesis gives a brief introduction of the development on high-spin populating and detecting techniques as well as the characteristics of the high-spin level structure of A~130 mass region nuclei.The second part of the thesis introduces concepts and experimental methods of the in-beamγ-ray spectroscopic techniques, such as the widely used methods to populate high spin states of atomic nucleus, the operating method of HI-13 tandem accelerator detector array, the electronic and data acquisition system, as well as the issues concerning on-line experimental value measuring and off-line data analysis.In the third part of the thesis, a systematic study of high spin level structure of 131Cs is conducted using the data obtained from 124Sn (11B, 4n) reaction at a beam energy of 57MeV. After unfolding, the 9. 4×107γ?γcoincidence events are sorted into a symmetric two dimensional Eγ? Eγmatrix together with an asymmetric DCO ratio matrix. In data pre-handling stage, excitation functions are calculated using cascade and pace2; Efficiency calibration for the detector array is carried out with standard radioactive source 133Ba and 152Eu, the two efficiency curves are normalized at the energy point of 400keV. Then, detailed description regarding analysis of coincidence data is made at both principle and exercise levels. A self-consistent"probability flow"idea is presented for the first time. Two new methods for level scheme construction are suggested: 1. Develop C programs to determine the placement of new transitions into a previous level scheme. 2. Set analogy between level scheme and phylogenetic tree to realize automatic construction of a decay path. Finally, elaborate illustration is given on the newly achieved experimental results, upon which relevant theoretical interpretation is made. The achievements for this work are outlined below:1. The level scheme from the present work is a significant extension with the addition of about 30 new transitions to that reported in the earlier works by Kumar: The yrast band is established up to 6 MeV excitation energy and spin. The previous reported three-quasiproton bands (B5 and B6) are considerably extended, their likely unfavored signature partners have been observed for the first time. A new rotational band labelled B9 is established from this work. 39/2h2. Relative intensities and DCO ratios are measured for the observedγ-rays. The spins and parities of the different band structures are assigned on the basis of the results of angular correlation. In addition, the B(M1)/B(E2) ratios are extracted for theΔI =1coupled bands. 3. Experimental Routhian and Alignments are extracted for various observed bands in rotating frame of reference, causes of observed upbend in those bands are discussed on the basis of Cranking Shell Model calculation. A small upbend is observed at the highest portion of B6, which indicates the alignment of a pair. So the five-quasiparticle orbital is a probable configuration for the upper portion of B6.4. Nilsson configuration assignments to various observed bands are carried out on the basis of band-crossing frequencies, signature splitting, additivity of alignments of the involved quasiparticles and the deduced B(M1)/B(E2) ratios. This work confirms the previous configuration assignments of B1~B8 by Kumar, the newly established band(B9) is believed to be built on nearly similar configurations as B4.5. The single-particle and collective motion competing and coexistence phenomena are observed in the three-quasiproton band (B6) and the band built on the configuration (B7). It is supposed that the band termination phenomenon which often occurs in [ ]πh 11/255012?bands is probable to be observed in the upper portion of B7.6. A systematic study concerning the excitations and spin-parity of bandheads for bands and bands is conducted on the Cs isotopes, the bands previously identified as the favored partner of the g 7/2d5/2πd 5/2bands in 127,129Cs are tentatively reassigned as the unfavored partner ofπg 7/2bands, while the previously unfavored partner ofπg 7/2bands are tentatively reassigned as the favored partner of theπd 5/2bands. Consequently, theπg 7/2bands andπd 5/2bands in Cs isotopes exhibit systematic features.The fourth part of the thesis summarizes the already achieved results and gives insight to future works.