Study Of High Spin Spectroscopy In Odd-A Nucleus ¹²⁵Cs

Nuclei in the mass region A~125 exhibit a richness of interesting phenomenon,such as the signature inversion,competing alignments between the h11/2 neutrons and the h11/2 protons,possible shape coexistence between the prolate and the oblate deformations,magnetic dipole band,band termination,breaking of chiral symmetry,and enhancement of octupole correlations.For clear understandings of various nuclear phenomena,adequate and reliable experimental data become particularly crucial.Therefore the 125 Cs nucleus was chosen in present paper as main object of research,in order to gain a better understanding of the systematic features of odd-A Cs isotopes.The Z =55,N=70 nucleus 125 Cs lying in the transitional region.In this region,the proton Fermi surface lies low within the h11/2 subshell,which drives the nucleus towards a prolate shape,whereas the neutron Fermi surface lies near the middle of the h11/2 subshell,which tries to drive the shape towards an oblate shape.These diverse shape-driving effects result in complex nuclear-structure features relevant to theγ-deformation and the dynamic competition between prolate and oblate shapes.Depending on specific configurations,the shapes of a given nucleus may be prolate,triaxial or even oblate.In addition,chiral twin bands associated with triaxiality in the A~130 region are receiving intense attentions and have been identified in several odd-odd Cs isotopes as well as in the odd-A case of 129 Cs.Excited states in 125 Cs were produced via the 116Cd(14N,5n)fusion-evaporation reaction.The experiment was carried out at the Niels Bohr.Institute(NBI),Denmark.The FN tandem accelerator system provided a 65-Me V beam.The γ rays emitted from the reaction were measured using the NORDBALL detector array.The main results from the present dissertation are summarized below:1.More than 50 gamma transitions have been identified and a more complete level scheme of 125 Cs has been established.A lot of transitions linking different bands have been established.Spin-parity assignments to the observed levels are based on the probable multipolarities inferred from the measured ADO ratios of the related transitions and on the previous knowledge about 125 Cs.Based on extraction of alignment angular momentum and routhian,CSM,the comparison between experimental and theoretical B(M1)/B(E2)value,the study of exciting energy of systematics,possible configurations of each observed band are discussed and assigned.2.Band 10 is known to be a=-1/2 signature built on the h11/2[550]1/2-configuration and its 11/2-state is an isomer with T1/2 ? 0.9 ms.Previous studies had ascertained the excitation energy of the 11/2-bandhead state to be 266 ke V.Based on the obsevations of several transitions linking this band with other bands with known excitation energies,the bandhead of band 10 is located at excitation energy of 294 ke V,significantly different from the value established in previous studies.3.The decay from pg9/2 bandhead to lower-lying states has been established.The out-band transitions to lower states are seriously contaminated,which made it very complex to locate the pg9/2 bandhead.After careful analysis,the observed coincidences can be accommodated by 125 Cs self-consistently,and the bandhead of pg9/2 is located at an excitation energy of 563 ke V4.TRS calculations were performed for the lowest(π.α)=(-,-1/2)and(π.α)=(-,+1/2)configuration in 125 Cs,namely for the α=-1/2 and +1/2 signatures of band 10.The TRS calculation results show that the α=-1/2 signature of band 10 has a prolate shape with(nh11/2)2 pair aligned at higher spins,and the α=+1/2 signature has a near-oblate shape with(nh11/2)2 alignment assumed.These may be the evidence for coexisting prolate and near-oblate shapes at high spins in the p h11/2configuration in 125 Cs.5.Two new bands are identified for the first time.Their configurations at low spins are assigned mainly based on observed spectroscopic features;one of the new bands is assigned the favored signature of the πd5/2[420]1/2+ configuration,and another is assigned the unfavored signature of the πg7/2[422]3/2+ configuration.This set of configuration assignments agrees with the prescription proposed for129 Cs by Sihotra et al.and calls for a reappraisal of the configuration assignments for similar bands in 127 Cs and even in odd-A iodine isotopes.The observed lowest 1/2+ and 3/2+ states in 125 Cs are proposed to be the bandheads of the d5/2[420]1/2+and g7/2[422]3/2+ configurations,respectively.It is demonstrated that a plain and consistent understanding about the energy spectra of the πg7/2 and πd5/2 levels in odd-A Cs isotopes is accessible.6.It is demonstrated that the πg7/2 band forks into two structures with increasing spin.One of the bands shows(35)I = 1 coupled structure and the another shows(35)I =2 decoupled structure.Experimental routhians and alignments,signature splittings and B(M1)/B(E2)ratios are extracted and then compared with the expectations from TRS,CSM and geometricle model.It is argued that the(35)I = 1 band is built on the(?g7/2/d5/2)?(?h11/2)2 configuration with near-oblate or even oblate shape whereas the(35)I = 2 band results from the(?h11/2)2 alignment at a ? ? 0? prolate shape followed by a subsequent(?h11/2)2 alignment.7.After careful analysis of all available experimental observations,we suggest that band 7 is built on the ?h11/2??h11/2??g7/2 configuration,which is composed of a?g7/2 spectator and the yrast ?h11/2??h11/2 configuration in doubly-odd nuclei in this region.Of particular interest is the observation of chiral twin bands built on the ?h11/2??h11/2 configuration in doubly-odd nuclei.Similar to the odd-odd case,Band 7 and band 8 in the odd-A nucleus 125 Cs are also showing many features consistent with the expectation of a pair of chiral bands,such as the similarities betwen bands 7 and 8 in observables of B(M1)/B(E2)ratis,energy staggering and alignments.The evidence for chiral bands in odd-A 125 Cs presents a novel case for the understanding of chiral breaking in nuclear systems.