T. Otsuka

NUCLEAR SHELL-MODEL AND INTERACTING BOSONS

In this workshop we have seen that the interacting boson model provides us with a unified phenomenological description of vibrational, rotational and transitional nuclei.

SHELL MODEL DESCRIPTION OF INTERACTING BOSONS

The interacting boson model, describing collective states of even-even nuclei, is introduced as a drastic truncation of large scale shell model calculations. The shell model hamiltonian can be diagonalized by using a correspondence, or mapping, of the nucleon states in the truncated space into states obtained by coupling proton and neutron s- and d-bosons. The equivalent boson hamiltonian in a simple case is obtained and diagonalized. Eigenstates with definite proton-neutron symmetry (good F-spin) emerge for certain values of proton and neutron numbers. In general the situation is more complex but the results obtained follow closely the experimental data.

Systematic study of the mixed ground-state and “intruder” bands in 110, 112, 114Cd☆

Abstract The ground-state and the “intruder” bands in 112 Cd were investigated by means of neutron capture and 110 Pd(α, 2n) reaction spectroscopy. Measurements of γ-rays, angular distributions and γγ-coincidences were performed using Compton-suppressed Ge detectors. Our experimental results, together with published data on 110 Cd and 114 Cd, were interpreted systematically in the framework of IBA-2 configuration-mixing calculations. The phenomenological model parameters are compared with microscopic predictions based on the OAI mapping procedure.

Spectroscopy of the 21+ state in 22O and shell structure near the neutron drip line

Abstract The energy and electromagnetic matrix element B(E2↑) for the 21+ state in 22O have been determined via inelastic scattering of a beam of these radioactive nuclei from a 197Au target. These results provide strong evidence for the existence of the N=14 subshell closure in 22O. This demonstrates that the shell structure in 22O is similar to that of stable oxygen isotopes, even though this nucleus is only two neutrons away from the neutron drip line. These results are reproduced in the standard sd shell model as well as with a an expanded space which also includes the f7/2 and p3/2 orbitals.

Tensor interaction contributions to single-particle energies

We calculate the contribution of the nucleon-nucleon tensor interaction to single-particle energies with finite-range G-matrix potentials and with zero-range Skyrme potentials. The Skx Skyrme parameters including the zero-range tensor terms with strengths calibrated to the finite-range results are refitted to nuclear properties. The fit allows the zero-range proton-neutron tensor interaction as calibrated to the finite-range potential results which gives the observed change in the single-particle gap {epsilon}(h{sub 11/2})-{epsilon}(g{sub 7/2}) going from {sup 114}Sn to {sup 132}Sn. However, the experimental l dependence of the spin-orbit splittings in {sup 132}Sn and {sup 208}Pb is not well described when the tensor is added, owing to a change in the radial dependence of the total spin-orbit potential. The gap shift and a good fit to the l dependence can be recovered when the like-particle tensor interaction is opposite in sign to that required for the G matrix.

A calculation of low-lying collective states in odd-even nuclei

Abstract We present results of a calculation of the low-lying collective quadrupole states in even-even nuclei within the framework of the proton-neutron interacting boson model.

Multiple quadrupole “phonon” excitations in 130Ba

Abstract The low spin structure of 130Ba was investigated via the emitted γ-radiation following the β-decay of 130La. New “K = 0”- and “K = 4”- bands were identified. A comparison with the O(6) symmetry of the IBM is shown. A discussion of the low lying levels of the O(6) symmetry in terms of multiple quadrupole “phonon” excitations built on the ground state is given.

A new seniority scheme for non-degenerate single particle orbits

Abstract A new method is proposed in the treatment of the seniority scheme. The method enables us to evaluate analytically the contribution from J =0 Cooper pairs in non-degenerate single-particle orbits to many-body matrix elements. It includes the SU (2) quasi-spin and the BCS approximation as two extreme limits. The effect of particle number conservation is properly taken into account.

Direct radiative capture of p -wave neutrons

The neutron direct radiative capture (DRC) process is investigated, highlighting the role of incident {ital p}-wave neutrons. A set of calculations is shown for the {sup 12}C({ital n},{gamma}) process at incoming neutron energies up to 500 keV, a crucial region for astrophysics. The cross section for neutron capture leading to loosely bound {ital s}, {ital p}, and {ital d} orbits of {sup 13}C is well reproduced by the DRC model demonstrating the feasibility of using this reaction channel to study the properties of nuclear wave functions on and outside the nuclear surface. A sensitivity analysis of the results on the neutron-nucleus interaction is performed for incident {ital s}- as well as {ital p}-waves. It turned out that the DRC cross section for {ital p}-wave neutrons is insensitive to this interaction, contrary to the case of incident {ital s}-wave neutrons.

Low spin states in 130Ba

Abstract The low spin structure of 130 Ba has been investigated following the β + -decay of ground and medium spin isomeric states of 130 La. The lanthanum nuclei were produced at the Cologne FN-tandem accelerator using the reaction 120 Sn( 14 N,4n) 130 La. γγ-coincidence data was taken by means of the OSIRIS 6-cube spectrometer. Many new low lying levels were established. Of special interest are new “ K = 0”- and “K = 4”- bands . We give the level and transition energies, spins and parities, multipole mixing ratios and B (E2)-ratios obtained from this measurement. The low lying states of positive parity are discussed in the framework of the IBA-1 model and compared with results for the F-spin partner 126 Xe. The description is given in terms of a consistent-Q formalism near the dynamical O(6)-symmetry. A new scheme which gives analytical expressions for the O(6)-wave functions is presented. The scheme is still valid approximately in vicinity of O(6).