S. Tazaki

Application of Dyson boson mapping to the analysis of mode-mode coupling in Ge and Se isotopes

Abstract The Dyson boson mapping is applied to the analysis of the coupling between the quadrupole phonon mode and the pairing-vibrational modes. The original fermion (quasiparticle) space consisting of a product of multi-quadrupole-phonon space and multi-pairing-vibration space is transformed into a boson space by the Dyson boson mapping. In this boson space, numerical analyses are carried out for the cases of the Ge and Se isotopes. The results show that the so-called “mysterious” 0 + states in the Ge and Se isotopes are reasonably explained and that the Dyson boson mapping is quite applicable to analyses of the couplings between various kinds of collective modes.

Application of the extended P+QQ force model to N≈Z fp shell nuclei

Abstract To study collective motion, the extended pairing plus QQ force model proposed recently is applied to A=46 , 48 and 50 nuclei in the fp shell region. Exact shell model calculations in the truncated model space (f 7/2 ,p 3/2 ,p 1/2 ) prove the usefulness of the interaction. The simple model with the pairing plus quadrupole pairing plus QQ force and J -independent isoscalar proton–neutron force unexpectedly well reproduces observed binding energies, energy levels of collective (yrast) states and reduced E2 transition probabilities in 46 Ti, 46 V, 48 V, 48 Cr, 50 Cr and 50 Mn. The correspondence between theory and experiment is almost comparable to that attained by the full fp shell model calculations with realistic effective interactions. Some predictions are made for energy levels and variations of B (E2) in the yrast bands, in these nuclei. Characteristics of the interaction are investigated by comparing with the realistic effective interactions and by the backbending plot for the yrast band of 48 Cr.

E0 transition and coupling between quadrupole and pairing-vibrational modes

Abstract Using a theory of mode-mode coupling between the two-phonon 0 + mode and the pairing-vibrational mode, we have extensively investigated the first excited 0 + (0 2 2 ) states in spherical and transitional nuclei. The results tell us that the 0 2 + states in a wide range of nuclei are strongly mixed states of both the modes. By making use of these results, the matrix elements for the E0 transitions from the 0 2 + states to the ground states are calculated for the Zn, Ge, Se, Kr, Sr, Zr, Mo, Ru, Pd, Cd and Sn isotopes. For some of the Cd and Sn isotopes, the matrix elements between the 0 2 + and 0 2 + states are also obtained. These numerical calculations make a rather good fit to the E0 experiments.

Variation in displacement energies due to isospin-nonconserving forces.

For mirror nuclei with masses A=42-95, the effects of isospin-nonconserving nuclear forces are studied with the nuclear shell model using the Coulomb displacement energy and triplet displacement energy as probes. It is shown that the characteristic behavior of the displacement energies can be well reproduced if the isovector and isotensor nuclear interactions with J=0 and T=1 are introduced into the f(7/2) shell. These forces, with their strengths being found consistent with the nucleon-nucleon scattering data, tend to modify nuclear binding energies near the N=Z line. At present, no evidence is found that these forces are needed for the upper fp shell. Theoretical one- and two-proton separation energies are predicted accordingly, and locations of the proton drip line are thereby suggested.

Improvement of the extended P+QQ interaction by modifying the monopole field

Abstract The extended pairing plus QQ interaction with the J -independent isoscalar proton–neutron force as an average monopole field, which has succeeded in describing collective yrast states of N≈Z even- A nuclei, is improved. The improvement is accomplished by adding small monopole terms (relevant to spectroscopy) to the average monopole field (indispensable to the binding energy). This modification extends the applicability of the interaction to nuclei with N≈28 such as 48Ca, and moreover improves energy levels of nonyrast states. The modified interaction successfully describes not only even- A but also odd- A nuclei in the f 7/2 shell region. Results of exact shell model calculations in the model space ( f 7/2 , p 3/2 , p 1/2 ) (its usefulness has been demonstrated previously) are shown for A=47 , 48, 49, 50 and 51 nuclei.

Condensed structure of J = T = 0 α-like clusters in f72-shell even-even nuclei with N = Z

Abstract Microscopic calculations are carried out for investigating the α-like four-nucleon correlations in the core plus (f 7 2 ) 4n systems with N = Z. The results of the calculations lead us to the following conclusions. The α-like four-nucleon correlations form an α-like cluster with J = 0, T = 0 as a unit of the collective motion in the ground states of these systems. The ground states, in which each α-like cluster interacts with the others just like a boson, become the condensed structure of the α-like clusters. Furthermore, the calculation based on the condensed α-like cluster model in the fp-shell indicates that the nucleus 48Cr has the structure of the 40Ca core plus two α-like clusters.

Alpha-like four-nucleon correlations in the ground state of 48Cr

Abstract The ground-state structure of 48Cr is investigated by multiple α-like cluster model. The original model with only J = T = 0 α-like cluster is extended so as to deal with the contribution of J > 0 α-like units. The model reproduces the observed ground-state energy of 48Cr well and explains the characteristic variation of the ground-state energies from 44Ti to 48Cr caused by many-body correlations. The properties of the 48Cr ground state which originate in the α-like four-nucleon correlations are revealed by the model.

Isospin symmetry breaking at high spins in the mirror pair Se-67 and As-67

Recent experimental data have revealed large mirror energy differences (MED) between high-spin states in the mirror nuclei 67 Se and 67 As, the heaviest pair where MED have been determined so far. The MED are generally attributed to the isospin symmetry breaking caused by the Coulomb force and by the isospin-nonconserving part of the nucleon-nucleon residual interaction. The different contributions of the various terms have been extensively studied in the fp shell. By employing large-scale shell-model calculations, we show that the inclusion of the g 9/2 orbit causes interference between the electromagnetic spin-orbit and the Coulomb monopole radial terms at high spin. The large MED are attributed to the aligned proton pair excitations from the p 3/2 and f 5/2 orbits to the g 9/2 orbit. The relation of the MED to deformation is discussed.

Toward a unified realistic shell-model Hamiltonian with the monopole-based universal force

We propose a unified realistic shell-model Hamiltonian employing the pairing plus multipole Hamiltonian combined with the monopole interaction constructed starting from the monopole-based universal force by Otsuka it et al. (Phys. Rev. Lett. 104, 012501 (2010)). It is demonstrated that the proposed PMMU model can consistently describe a large amount of spectroscopic data as well as binding energies in the pf and pf5/2g9/2 shell spaces, and could serve as a practical shell model for even heavier mass regions.

Characteristics of the 21/2+ isomer in 93Mo: Toward the possibility of enhanced nuclear isomer decay

Abstract To discuss whether an enhanced isomer decay is a preferred process in a plasma environment it is required to know the structure of the isomer as well as the nearby states. The spin-21/2, 6.85-hour high-spin isomer in Mo 93 is investigated within a shell model which well describes nuclei in this mass region. By using the obtained wave-functions which correctly reproduce the observed B ( E 2 ) , B ( E 4 ) , and B ( M 1 ) transitions, characteristics of the isomer are shown in comparison with the isomeric states in neighboring nuclei. Calculations suggest that these high-spin isomers are formed with almost pure single-particle-like configurations. The 93 Mo 21 / 2 + isomer has the predominant configuration π ( g 9 / 2 ) 8 2 ⊗ ν d 5 / 2 lying below the 15 / 2 + , 17 / 2 + , and 19 / 2 + states due to neutron–proton interaction, which is the physical origin of its long lifetime. The key E 2 transition that connects the 21 / 2 + isomer to the upper 17 / 2 + level is predicted to be substantial (3.5 W.u.), and therefore there is a real prospect for observing induced isomer deexcitation.