中田 仁

Low-lying electromagnetic transitions have been studied by the method of nuclear resonance fluorescence (NRF) or nuclear photon scattering. Quasi-monochromatic linearly polarized photon beams from inverse Compton scattering of laser light are used to excite nuclei around shell closure. In this report, the results of NRF measurements on 208Pb are presented. The magnetic dipole resonance below the neutron separation energy is resolved in to several individual transitions. The experimental results are compared with theoretical predictions based on self-consistent RPA calculations. A role of the tensor force is discussed.

Spin-flip Ml strength in 208PB has been measured in photon scattering experiments with a quasi-monochromatic, linearly polarized photon beam. The data resolve an Ml giant resonance into at least seven, possibly eight, discrete transitions at energies between 7.1 and 7.4 MeV below the neutron separation energy. The measurd Ml snength in this region is found to be larger than the previous report. By compaiing the data with a self-consistent RPA estimation using a semi-realistic interaction, a role of the tensor force is reconflrmed. © 2009 American Institute of Physics.

Excited states in Ba-136, populated in deep inelastic collisions by the interaction of 450 MeV Se-82 ions with a La-139 target, have been studied by means of in-beam gamma-ray spectroscopy. A new isomer with I-pi = (10(+)) has been identified at an excitation energy E-x = 3.357 MeV. The half-life was determined as T-1/2 = 94 +/- 10 ns. The extracted B(E2) value is much smaller than those in Te-132 and Xe-134. This hindrance is investigated by a shell model calculation.

Under the assumption that isospin T is a good quantum number, isobaric analog states and various analogous transitions among these states are expected in isobars with mass number A. Strengths of analogous Gamow-Teller (GT) and M1 transitions have been compared for an A = 26 isobar triplet with T-z = + 1, 0, and - 1, where T-z is defined by T-z = (N - Z)/2. The T-z = 1 --> 0 GT transitions from the J(pi) = 0(+) ground state of Mg-26 to excited J(pi) = 1(+) states in Al-26 were studied up to the excitation energy (E-x) of 9 MeV by using a high energy-resolution (He-3,t) reaction. The distribution of observed GT strengths was well reproduced in a shell-model calculation. The isospin symmetric T-z = -1-->0 GT transitions can be studied in the Si-26 beta decay. The GT strengths from the (He-3,t) reaction were in good agreement with the beta-decay values evaluated up to E-x = 2.7-MeV states in 26Al. The GT strengths were further compared with the strengths of analogous M1 gamma transitions in Al-26, i.e., the M1 transitions from the excited 1(+) states to the isobaric analog state of the Mg-26 ground state in Al-26. Through this comparison, contributions of spin and orbital terms in these M1 transitions were studied. The GT and M1 strengths as well as spin and orbital contributions in M1 strengths are interpreted by both the shell model and the particle-rotor model assuming a correlated proton and neutron pair around a Mg-24 core. The isospin T of each excited 1(+) state in Al-26 was also studied by examining the existence or the nonexistence of the analog 1(+) state in the T-z = 1 Mg-26 nucleus.

By introducing a density-dependent contact term, M3Y-type interactions applicable to the Hartree-Fock calculations are developed. In order to view basic characters of the interactions, we carry out calculations on the uniform nuclear matter as well as on several doubly magic nuclei. It is shown that a parameter set called M3Y-P2 describes various properties similarly well to the Skyrme SLy5 and/or the Gogny D1S interactions. A remarkable difference from the SLy5 and D1S interactions is found in the spin-isospin properties in the nuclear matter, to which the one-pion-exchange potential gives a significant contribution. Affecting the single-particle energies, this difference may play a certain role in the new magic numbers in unstable nuclei. © 2003 The American Physical Society.

We develop a new method of implementing the Hartree-Fock calculations. A class of Gaussian bases is assumed, which includes the Kamimura-Gauss basis-set as well as the set equivalent to the harmonic-oscillator basis-set. By using the Fourier transformation to calculate the interaction matrix elements, we can treat various interactions in a unified manner, including finite-range ones. The present method is numerically applied to the spherically-symmetric Hartree-Fock calculations for the oxygen isotopes with the Skyrme and the Gogny interactions, by adopting the harmonic-oscillator, the Kamimura-Gauss and a hybrid basis-sets. The characters of the basis-sets are discussed. Adaptable to slowly decreasing density distribution, the Kamimura-Gauss set is suitable to describe unstable nuclei. A hybrid basis-set of the harmonic-oscillator and the Kamimura-Gauss ones is useful to accelerate the convergence, both for stable and unstable nuclei. © 2002 Elsevier Science B.V. All rights reserved.

An effective interaction for the mean-field calculations is explored from a semi-microscopic standpoint. The M3Y interaction is modified so as to reproduce the saturation density, energy and the LS splittings. This semi-microscopic interaction describes the properties of the symmetric nuclear matter reasnoably well. This interaction is also applied to the Hartree-Fock calculations for the stable and unstable oxygen isotopes by using the recently proposed numerical method.

A simple formula for the ratio of the number of odd- and even-parity states as a function of temperature is derived. This formula is used to calculate the ratio of level densities of opposite parities as a function of excitation energy. We test the formula with quantum Monte Carlo shell model calculations in the (Pf + g(9/2)) shell. The formula describes well the transition from low excitation energies where a single parity dominates to high excitations where the two densities are equal.

We introduce a particle-number reprojection method in the shell model Monte Carlo that enables the calculation of observables for a series of nuclei using a Monte Carlo sampling for a single nucleus. The method is applied to calculate nuclear level densities in the complete (pf + g(9/2))-shell using a good-sign Hamiltonian. Level densities of odd-A and odd-odd nuclei are reliably extracted despite an additional sign problem. Both the mass and the T-z dependence of the experimental level densities are well described without any adjustable parameters. The odd-even staggering observed in the calculated backshift parameter follows the experimental data more closely than do empirical formulas.

The asymmetry in the energy spectra between mirror nuclei (the Thomas-Ehrman shifts) around O-16 is investigated from a phenomenological viewpoint. The recent data on proton-rich nuclei indicates that the residual nuclear interaction is reduced for the loosely bound s-orbit by as much as 30%, which originates in the broad radial distribution of the proton single-particle wave function. (C) 1999 Elsevier Science B.V. All rights reserved.

We calculate microscopically total and parity-projected level densities for beta-stable even-even nuclei between Fe and Ge, using the shell model Monte Carlo methods in the complete (pf + 0 g(9/2))-shell. A single-particle level density parameter a and backshift parameter Delta are extracted by fitting the calculated densities to a backshifted Bethe formula, and their systematics are studied across the region. Shell effects are observed in Delta for nuclei with Z = 28 or N = 28 and in the behavior of A/a as a function of the number of neutrons. We find a significant parity dependence of the level densities for nuclei with A less than or similar to 60, which diminishes as A increases. (C) 1998 Elsevier Science B.V. All rights reserved.