| 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...
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