Collective excitations in the A = 130 region: Studies of iodine-127 and barium-136 with the (n,n'gamma) reaction

Nuclei consist of protons and neutrons and those with a certain number of nucleons (2, 8, 20, 28, 50, 82 and 126) are stable. The theory that best addresses this phenomenon is the nuclear shell model, which is based on the individual nucleons; however, this microscopic description becomes complicated when the number of valence nucleons increases. Some other approaches to understand nuclear structure are the geometric collective model (GCM), interacting boson model (IBM), and quasiparticle phonon model (QPM). These models predict levels arising from quadrupole vibrations, octupole vibrations and higher-order vibrations. The experimental signature for these vibrations is large reduced transition probabilities (RTPs). These decays take place predominantly by electric quadrupole (E2), magnetic: dipole (M1), electric dipole (E1) or electric octupole transitions (E3).;Nuclear vibrations about a spherical equilibrium shape are quantized and are called phonons. The common vibrations are symmetric vibrations, which arise from in-phase motion of protons and neutrons. Another kind of low-energy vibration arises from out-of-phase motion of protons and neutrons, these are the so called mixed-symmetry states (MSS), which have been of considerable recent interest.;Low-energy nuclear structure is studied using the inelastic neutron scatterring (INS) experiments at the University of Kentucky accelerator facility. Using the (n,n'gamma) reaction, we can measure the level lifetimes and cross sections from which we can infer the RTPs. This information helps us to characterize the levels and then compare with models in order to understand the nuclear structure.;In this work phonon states in both 127I and 136 Ba were studied. Excitation function and angular distribution experiments were performed for both nuclei. Coincidence experiments were performed for 127I. New gamma rays, level lifetimes and hence RTPs were obtained for a number of levels, as these nuclei were studied for the first time using the (n,n'gamma) reaction.;An odd-mass nucleus can frequently be described by the particle-core coupling model (PCCM). Experimental results for 127I are interpreted with this model. The detailed level scheme obtained for 127I in these experiments is compared with the core nucleus 126Te and reflects that the PCCM picture is valid here too. No levels were identified as MSS, but some of the large M1 strengths observed were attributed to spin-flip contributions.;The low-spin structure of 136Ba, as established from our data, helps to identify the phonon states. The lifetimes in the femtosecond region observed for some of the 0+ states give us additional useful information regarding the structure of 136Ba. It is also clear that this nucleus has an unfragmented MSS at 2129 keV. We have ruled out the possibility of the 2694-keV level as the 1+ MS state belonging to the MS two-phonon configuration. For the first time, the 3077-keV level is proposed as the 3+ MS state, arising from the coupling of the one-phonon MS state with the symmetric state.;Keywords: Spherical nuclei, Mixed-symmetry, Collective Excitations, Nuclear Models, Neutron Scattering, Gamma rays...