Yutaka Utsuno

Magic Numbers in Exotic Nuclei and Spin-Isospin Properties of the NN Interaction

The magic numbers in exotic nuclei are discussed, and their novel origin is shown to be the spin-isospin dependent part of the nucleon-nucleon interaction in nuclei. The importance and robustness of this mechanism is shown in terms of meson exchange, G-matrix, and QCD theories. In neutron-rich exotic nuclei, magic numbers such as N = 8, 20, etc. can disappear, while N = 6, 16, etc. arise, affecting the structure of the lightest exotic nuclei to nucleosynthesis of heavy elements.

Novel features of nuclear forces and shell evolution in exotic nuclei

Novel simple properties of the monopole component of effective nucleon-nucleon interactions are presented, leading to the so-called monopole-based universal interaction. Shell structures are shown to change as functions of N and Z, consistent with experiments. Some key cases of this shell evolution are discussed, clarifying the effects of central and tensor forces. The validity of the present tensor force is examined in terms of the low-momentum interaction V(lowk) and the Q(box) formalism.

Monte Carlo shell model for atomic nuclei

Abstract The development, present status and future perspectives of the Monte Carlo shell model are discussed. The development and limitation of the conventional shell model calculations are shown, and stochastic approaches are introduced. As one of such approaches, the Quantum Monte Carlo Diagonalization (QMCD) method has been proposed. The formulation of the QMCD method is presented with an illustrative example. While the QMCD method is a general method for solving the quantum many-body interacting systems, its application to the nuclear shell model is referred to as the Monte Carlo Shell Model (MCSM). A test of the MCSM is presented, confirming the feasibility of the MCSM. The MCSM represents a breakthrough in shell model calculations: the level structure of low-lying states can be studied with realistic interactions for a wide, probably basically unlimited, variety of nuclei. The MCSM has two major characteristic features: the feasibility of including many single-particle orbits and the capability of handling many valence nucleons. We present some applications for which these features played essential roles. Such applications include the structure of exotic nuclei around the neutron number 20, a unified description of spherical to superdeformed states in nuclei around 56Ni, a new effective interaction for pf-shell nuclei and their unified description, and microscopic studies on the spherical-deformed shape phase transition and on the γ-unstable deformation.

Evidence for a new nuclear /`magic number/' from the level structure of 54Ca

Atomic nuclei are finite quantum systems composed of two distinct types of fermion—protons and neutrons. In a manner similar to that of electrons orbiting in an atom, protons and neutrons in a nucleus form shell structures. In the case of stable, naturally occurring nuclei, large energy gaps exist between shells that fill completely when the proton or neutron number is equal to 2, 8, 20, 28, 50, 82 or 126 (ref. 1). Away from stability, however, these so-called ‘magic numbers’ are known to evolve in systems with a large imbalance of protons and neutrons. Although some of the standard shell closures can disappear, new ones are known to appear. Studies aiming to identify and understand such behaviour are of major importance in the field of experimental and theoretical nuclear physics. Here we report a spectroscopic study of the neutron-rich nucleus 54Ca (a bound system composed of 20 protons and 34 neutrons) using proton knockout reactions involving fast radioactive projectiles. The results highlight the doubly magic nature of 54Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.

Onset of intruder ground state in exotic Na isotopes and evolution of the N=20 shell gap

The onset of intruder ground states in Na isotopes is investigated by comparing experimental data and shell-model calculations. This onset is one of the consequences of the disappearance of the N=20 magic structure, and the Na isotopes are shown to play a special role in clarifying the change of this magic structure. Both the electromagnetic moments and the energy levels clearly indicate an onset of ground state intruder configurations at neutron number N=19 already, which arises only with a narrow N=20 shell gap in Na isotopes resulting from the spin-isospin dependence of the nucleon-nucleon interaction (as compared to a wider gap in stable nuclei like {sup 40}Ca). It is shown why the previous report based on the mass led to a wrong conclusion.

Novel shape evolution in exotic Ni isotopes and configuration-dependent shell structure

model space.Experimental energy levels are reproduced well by a single fixed Hamiltonian. Intrinsic shapesare analyzed for MCSM eigenstates. Intriguing interplays among spherical, oblate, prolate and γ-unstable shapes are seen, including shape fluctuations, E(5)-like situation, the magicity of doubly-magic

Shape transitions in exotic Si and S isotopes and tensor-force-driven Jahn-Teller effect

We show how shape transitions in the neutron-rich exotic Si and S isotopes occur in terms of shell-model calculations with a newly constructed Hamiltonian based on VMU interaction. We first compare the calculated spectroscopic-strength distributions for the proton 0d5/2,3/2 and 1s1/2 orbitals with results extracted from a 48 Ca(e, ep) experiment to show the importance of the tensor-force component of the Hamiltonian. Detailed calculations for the excitation energies, B(E2), and two-neutron separation energies for the Si and S isotopes show excellent agreement with experimental data. The potential-energy surface exhibits rapid shape transitions along the isotopic chains towards N = 28 that are different for Si and S. We explain the results in terms of an intuitive picture by involving a Jahn-Teller-type effect that is sensitive to the tensor-force-driven shell evolution. The closed subshell nucleus 42 Si is a particularly good example of how the tensor-force-driven Jahn-Teller mechanism leads to a strong oblate rather than a spherical shape. a strong oblate shape (rather than spherical) due to the tensor- force-driven shell evolution. These PES results are interpreted in an intuitive picture which involves shell gaps and the Jahn- Teller-type effect. Two-neutron separation energies are also discussed. Weoutlinethepresentshell-modelcalculations.The sdand pf shells are taken as the valence shell with protons in sdand neutrons in pf .The interactions within each of these shells are based on existing interactions: USD (20) (GXPF1B (21) 2 )f or the sd (pf ) shell, except for the monopole interactions (4,23) V T =0,1 0d3/2,0d5/2 based on SDPF-M (24) due to a problem in USD as

Electromagnetic structure of 98Mo

Abstract The nucleus 98 Mo was multiply Coulomb excited using 20 Ne, 84 Kr and 136 Xe beams. Eighteen E2 and M1 reduced matrix elements connecting 7 low-lying levels have been determined using the least-squares code GOSIA. The results are compared with the predictions of an extended version of the IBM1 model. The quadrupole sum rules approach was used to determine the shape parameters in two 0 + (ground and first excited) states. The rotational invariants 〈Q 2 〉 and 〈 cos 3δ〉 obtained show the shape coexistence in 98 Mo nucleus: the triaxial ground state and the prolate first excited state.

Spins and Magnetic Moments of 49K and 51K: Establishing the 1/2+ and 3/2+ Level Ordering Beyond N=28

The ground-state spins and magnetic moments of (49,51)K have been measured using bunched-beam high-resolution collinear laser spectroscopy at ISOLDE CERN. For 49K a ground-state spin I = 1/2 was firmly established. The observed hyperfine structure of 51K requires a spin I > 1/2 and strongly suggests I = 3/2. From its magnetic moment μ(51K) = +0.5129(22)μ(N) a spin-parity I(π) = 3/2+ with a dominant π1d(3/2)(-1) hole configuration was deduced. This establishes for the first time the reinversion of the single-particle levels and illustrates the prominent role of the residual monopole interaction for single-particle levels and shell evolution.

Novel extrapolation method in the Monte Carlo shell model

We propose an extrapolation method utilizing energy variance in the Monte Carlo shell model to estimate the energy eigenvalue and observables accurately. We derive a formula for the energy variance with deformed Slater determinants, which enables us to calculate the energy variance efficiently. The feasibility of the method is demonstrated for the full pf-shell calculation of {sup 56}Ni, and the applicability of the method to a system beyond the current limit of exact diagonalization is shown for the pf+g{sub 9/2}-shell calculation of {sup 64}Ge.